| Message ID | 20230913152238.905247-3-mszeredi@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | quering mount attributes | expand |
On Wed, Sep 13, 2023 at 6:22 PM Miklos Szeredi <mszeredi@redhat.com> wrote: > > Add a way to query attributes of a single mount instead of having to parse > the complete /proc/$PID/mountinfo, which might be huge. > > Lookup the mount by the old (32bit) or new (64bit) mount ID. If a mount > needs to be queried based on path, then statx(2) can be used to first query > the mount ID belonging to the path. > > Design is based on a suggestion by Linus: > > "So I'd suggest something that is very much like "statfsat()", which gets > a buffer and a length, and returns an extended "struct statfs" *AND* > just a string description at the end." > > The interface closely mimics that of statx. > > Handle ASCII attributes by appending after the end of the structure (as per > above suggestion). Allow querying multiple string attributes with > individual offset/length for each. String are nul terminated (termination > isn't counted in length). > > Mount options are also delimited with nul characters. Unlike proc, special > characters are not quoted. > > Link: https://lore.kernel.org/all/CAHk-=wh5YifP7hzKSbwJj94+DZ2czjrZsczy6GBimiogZws=rg@mail.gmail.com/ > Signed-off-by: Miklos Szeredi <mszeredi@redhat.com> > --- > arch/x86/entry/syscalls/syscall_64.tbl | 1 + > fs/internal.h | 5 + > fs/namespace.c | 312 ++++++++++++++++++++++++- > fs/proc_namespace.c | 19 +- > fs/statfs.c | 1 + > include/linux/syscalls.h | 3 + > include/uapi/asm-generic/unistd.h | 5 +- > include/uapi/linux/mount.h | 36 +++ > 8 files changed, 373 insertions(+), 9 deletions(-) > > diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl > index 1d6eee30eceb..6d807c30cd16 100644 > --- a/arch/x86/entry/syscalls/syscall_64.tbl > +++ b/arch/x86/entry/syscalls/syscall_64.tbl > @@ -375,6 +375,7 @@ > 451 common cachestat sys_cachestat > 452 common fchmodat2 sys_fchmodat2 > 453 64 map_shadow_stack sys_map_shadow_stack > +454 common statmnt sys_statmnt > > # > # Due to a historical design error, certain syscalls are numbered differently > diff --git a/fs/internal.h b/fs/internal.h > index d64ae03998cc..8f75271428aa 100644 > --- a/fs/internal.h > +++ b/fs/internal.h > @@ -83,6 +83,11 @@ int path_mount(const char *dev_name, struct path *path, > const char *type_page, unsigned long flags, void *data_page); > int path_umount(struct path *path, int flags); > > +/* > + * proc_namespace.c > + */ > +int show_path(struct seq_file *m, struct dentry *root); > + > /* > * fs_struct.c > */ > diff --git a/fs/namespace.c b/fs/namespace.c > index de47c5f66e17..088a52043bba 100644 > --- a/fs/namespace.c > +++ b/fs/namespace.c > @@ -69,7 +69,8 @@ static DEFINE_IDA(mnt_id_ida); > static DEFINE_IDA(mnt_group_ida); > > /* Don't allow confusion with mount ID allocated wit IDA */ > -static atomic64_t mnt_id_ctr = ATOMIC64_INIT(1ULL << 32); > +#define OLD_MNT_ID_MAX UINT_MAX > +static atomic64_t mnt_id_ctr = ATOMIC64_INIT(OLD_MNT_ID_MAX); > > static struct hlist_head *mount_hashtable __read_mostly; > static struct hlist_head *mountpoint_hashtable __read_mostly; > @@ -4678,6 +4679,315 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path, > return err; > } > > +static bool mnt_id_match(struct mount *mnt, u64 id) > +{ > + if (id <= OLD_MNT_ID_MAX) > + return id == mnt->mnt_id; > + else > + return id == mnt->mnt_id_unique; > +} > + > +struct vfsmount *lookup_mnt_in_ns(u64 id, struct mnt_namespace *ns) > +{ > + struct mount *mnt; > + struct vfsmount *res = NULL; > + > + lock_ns_list(ns); > + list_for_each_entry(mnt, &ns->list, mnt_list) { > + if (!mnt_is_cursor(mnt) && mnt_id_match(mnt, id)) { > + res = &mnt->mnt; > + break; > + } > + } > + unlock_ns_list(ns); > + return res; > +} > + > +struct stmt_state { > + void __user *const buf; > + size_t const bufsize; > + struct vfsmount *const mnt; > + u64 const mask; > + struct seq_file seq; > + struct path root; > + struct statmnt sm; > + size_t pos; > + int err; > +}; > + > +typedef int (*stmt_func_t)(struct stmt_state *); > + > +static int stmt_string_seq(struct stmt_state *s, stmt_func_t func) > +{ > + struct seq_file *seq = &s->seq; > + int ret; > + > + seq->count = 0; > + seq->size = min_t(size_t, seq->size, s->bufsize - s->pos); > + seq->buf = kvmalloc(seq->size, GFP_KERNEL_ACCOUNT); > + if (!seq->buf) > + return -ENOMEM; > + > + ret = func(s); > + if (ret) > + return ret; > + > + if (seq_has_overflowed(seq)) { > + if (seq->size == s->bufsize - s->pos) > + return -EOVERFLOW; > + seq->size *= 2; > + if (seq->size > MAX_RW_COUNT) > + return -ENOMEM; > + kvfree(seq->buf); > + return 0; > + } > + > + /* Done */ > + return 1; > +} > + > +static void stmt_string(struct stmt_state *s, u64 mask, stmt_func_t func, > + stmt_str_t *str) > +{ > + int ret = s->pos >= s->bufsize ? -EOVERFLOW : 0; > + struct statmnt *sm = &s->sm; > + struct seq_file *seq = &s->seq; > + > + if (s->err || !(s->mask & mask)) > + return; > + > + seq->size = PAGE_SIZE; > + while (!ret) > + ret = stmt_string_seq(s, func); > + > + if (ret < 0) { > + s->err = ret; > + } else { > + seq->buf[seq->count++] = '\0'; > + if (copy_to_user(s->buf + s->pos, seq->buf, seq->count)) { > + s->err = -EFAULT; > + } else { > + str->off = s->pos; > + str->len = seq->count - 1; > + s->pos += seq->count; > + } > + } > + kvfree(seq->buf); > + sm->mask |= mask; > +} > + > +static void stmt_numeric(struct stmt_state *s, u64 mask, stmt_func_t func) > +{ > + if (s->err || !(s->mask & mask)) > + return; > + > + s->err = func(s); > + s->sm.mask |= mask; > +} > + > +static u64 mnt_to_attr_flags(struct vfsmount *mnt) > +{ > + unsigned int mnt_flags = READ_ONCE(mnt->mnt_flags); > + u64 attr_flags = 0; > + > + if (mnt_flags & MNT_READONLY) > + attr_flags |= MOUNT_ATTR_RDONLY; > + if (mnt_flags & MNT_NOSUID) > + attr_flags |= MOUNT_ATTR_NOSUID; > + if (mnt_flags & MNT_NODEV) > + attr_flags |= MOUNT_ATTR_NODEV; > + if (mnt_flags & MNT_NOEXEC) > + attr_flags |= MOUNT_ATTR_NOEXEC; > + if (mnt_flags & MNT_NODIRATIME) > + attr_flags |= MOUNT_ATTR_NODIRATIME; > + if (mnt_flags & MNT_NOSYMFOLLOW) > + attr_flags |= MOUNT_ATTR_NOSYMFOLLOW; > + > + if (mnt_flags & MNT_NOATIME) > + attr_flags |= MOUNT_ATTR_NOATIME; > + else if (mnt_flags & MNT_RELATIME) > + attr_flags |= MOUNT_ATTR_RELATIME; > + else > + attr_flags |= MOUNT_ATTR_STRICTATIME; > + > + if (is_idmapped_mnt(mnt)) > + attr_flags |= MOUNT_ATTR_IDMAP; > + > + return attr_flags; > +} > + > +static u64 mnt_to_propagation_flags(struct mount *m) > +{ > + u64 propagation = 0; > + > + if (IS_MNT_SHARED(m)) > + propagation |= MS_SHARED; > + if (IS_MNT_SLAVE(m)) > + propagation |= MS_SLAVE; > + if (IS_MNT_UNBINDABLE(m)) > + propagation |= MS_UNBINDABLE; > + if (!propagation) > + propagation |= MS_PRIVATE; > + > + return propagation; > +} > + > +static int stmt_sb_basic(struct stmt_state *s) > +{ > + struct super_block *sb = s->mnt->mnt_sb; > + > + s->sm.sb_dev_major = MAJOR(sb->s_dev); > + s->sm.sb_dev_minor = MINOR(sb->s_dev); > + s->sm.sb_magic = sb->s_magic; > + s->sm.sb_flags = sb->s_flags & (SB_RDONLY|SB_SYNCHRONOUS|SB_DIRSYNC|SB_LAZYTIME); > + > + return 0; > +} > + > +static int stmt_mnt_basic(struct stmt_state *s) > +{ > + struct mount *m = real_mount(s->mnt); > + > + s->sm.mnt_id = m->mnt_id_unique; > + s->sm.mnt_parent_id = m->mnt_parent->mnt_id_unique; > + s->sm.mnt_id_old = m->mnt_id; > + s->sm.mnt_parent_id_old = m->mnt_parent->mnt_id; > + s->sm.mnt_attr = mnt_to_attr_flags(&m->mnt); > + s->sm.mnt_propagation = mnt_to_propagation_flags(m); > + s->sm.mnt_peer_group = IS_MNT_SHARED(m) ? m->mnt_group_id : 0; > + s->sm.mnt_master = IS_MNT_SLAVE(m) ? m->mnt_master->mnt_group_id : 0; > + > + return 0; > +} > + > +static int stmt_propagate_from(struct stmt_state *s) > +{ > + struct mount *m = real_mount(s->mnt); > + > + if (!IS_MNT_SLAVE(m)) > + return 0; > + > + s->sm.propagate_from = get_dominating_id(m, ¤t->fs->root); > + > + return 0; > +} > + > +static int stmt_mnt_root(struct stmt_state *s) > +{ > + struct seq_file *seq = &s->seq; > + int err = show_path(seq, s->mnt->mnt_root); > + > + if (!err && !seq_has_overflowed(seq)) { > + seq->buf[seq->count] = '\0'; > + seq->count = string_unescape_inplace(seq->buf, UNESCAPE_OCTAL); > + } > + return err; > +} > + > +static int stmt_mountpoint(struct stmt_state *s) > +{ > + struct vfsmount *mnt = s->mnt; > + struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt }; > + int err = seq_path_root(&s->seq, &mnt_path, &s->root, ""); > + > + return err == SEQ_SKIP ? 0 : err; > +} > + > +static int stmt_fs_type(struct stmt_state *s) > +{ > + struct seq_file *seq = &s->seq; > + struct super_block *sb = s->mnt->mnt_sb; > + > + seq_puts(seq, sb->s_type->name); > + if (sb->s_subtype) { > + seq_putc(seq, '.'); > + seq_puts(seq, sb->s_subtype); > + } > + return 0; > +} > + > +static int stmt_sb_opts(struct stmt_state *s) > +{ > + struct seq_file *seq = &s->seq; > + struct super_block *sb = s->mnt->mnt_sb; > + char *p, *end, *next, *u = seq->buf; > + int err; > + > + if (!sb->s_op->show_options) > + return 0; > + > + err = sb->s_op->show_options(seq, s->mnt->mnt_root); > + if (err || seq_has_overflowed(seq) || !seq->count) > + return err; > + > + end = seq->buf + seq->count; > + *end = '\0'; > + for (p = seq->buf + 1; p < end; p = next + 1) { > + next = strchrnul(p, ','); > + *next = '\0'; > + u += string_unescape(p, u, 0, UNESCAPE_OCTAL) + 1; > + } > + seq->count = u - 1 - seq->buf; > + return 0; > +} > + > +static int do_statmnt(struct stmt_state *s) > +{ > + struct statmnt *sm = &s->sm; > + struct mount *m = real_mount(s->mnt); > + > + if (!capable(CAP_SYS_ADMIN) && > + !is_path_reachable(m, m->mnt.mnt_root, &s->root)) > + return -EPERM; > + > + stmt_numeric(s, STMT_SB_BASIC, stmt_sb_basic); > + stmt_numeric(s, STMT_MNT_BASIC, stmt_mnt_basic); > + stmt_numeric(s, STMT_PROPAGATE_FROM, stmt_propagate_from); > + stmt_string(s, STMT_MNT_ROOT, stmt_mnt_root, &sm->mnt_root); > + stmt_string(s, STMT_MOUNTPOINT, stmt_mountpoint, &sm->mountpoint); > + stmt_string(s, STMT_FS_TYPE, stmt_fs_type, &sm->fs_type); > + stmt_string(s, STMT_SB_OPTS, stmt_sb_opts, &sm->sb_opts); > + > + if (s->err) > + return s->err; > + > + if (copy_to_user(s->buf, sm, min_t(size_t, s->bufsize, sizeof(*sm)))) > + return -EFAULT; > + > + return 0; Similar concern as with listmnt, I think that users would want to have a way to get the fixed size statmnt part that fits in the buffer, even if the variable length string values do not fit and be able to query the required buffer size to get the strings. The API could be either to explicitly request STMT_MNT_ROOT_LEN | STMT_MOUNTPOINT_LEN ... without allowing mixing of no-value and value requests, or to out-out from any string values using a single flag, which is probably more simple for API and implementation. Thanks, Amir. > +} > + > +SYSCALL_DEFINE5(statmnt, u64, mnt_id, > + u64, mask, struct statmnt __user *, buf, > + size_t, bufsize, unsigned int, flags) > +{ > + struct vfsmount *mnt; > + int err; > + > + if (flags) > + return -EINVAL; > + > + down_read(&namespace_sem); > + mnt = lookup_mnt_in_ns(mnt_id, current->nsproxy->mnt_ns); > + err = -ENOENT; > + if (mnt) { > + struct stmt_state s = { > + .mask = mask, > + .buf = buf, > + .bufsize = bufsize, > + .mnt = mnt, > + .pos = sizeof(*buf), > + }; > + > + get_fs_root(current->fs, &s.root); > + err = do_statmnt(&s); > + path_put(&s.root); > + } > + up_read(&namespace_sem); > + > + return err; > +} > + > static void __init init_mount_tree(void) > { > struct vfsmount *mnt; > diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c > index 250eb5bf7b52..20681d1f6798 100644 > --- a/fs/proc_namespace.c > +++ b/fs/proc_namespace.c > @@ -132,6 +132,15 @@ static int show_vfsmnt(struct seq_file *m, struct vfsmount *mnt) > return err; > } > > +int show_path(struct seq_file *m, struct dentry *root) > +{ > + if (root->d_sb->s_op->show_path) > + return root->d_sb->s_op->show_path(m, root); > + > + seq_dentry(m, root, " \t\n\\"); > + return 0; > +} > + > static int show_mountinfo(struct seq_file *m, struct vfsmount *mnt) > { > struct proc_mounts *p = m->private; > @@ -142,13 +151,9 @@ static int show_mountinfo(struct seq_file *m, struct vfsmount *mnt) > > seq_printf(m, "%i %i %u:%u ", r->mnt_id, r->mnt_parent->mnt_id, > MAJOR(sb->s_dev), MINOR(sb->s_dev)); > - if (sb->s_op->show_path) { > - err = sb->s_op->show_path(m, mnt->mnt_root); > - if (err) > - goto out; > - } else { > - seq_dentry(m, mnt->mnt_root, " \t\n\\"); > - } > + err = show_path(m, mnt->mnt_root); > + if (err) > + goto out; > seq_putc(m, ' '); > > /* mountpoints outside of chroot jail will give SEQ_SKIP on this */ > diff --git a/fs/statfs.c b/fs/statfs.c > index 96d1c3edf289..cc774c2e2c9a 100644 > --- a/fs/statfs.c > +++ b/fs/statfs.c > @@ -9,6 +9,7 @@ > #include <linux/security.h> > #include <linux/uaccess.h> > #include <linux/compat.h> > +#include <uapi/linux/mount.h> > #include "internal.h" > > static int flags_by_mnt(int mnt_flags) > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h > index 22bc6bc147f8..1099bd307fa7 100644 > --- a/include/linux/syscalls.h > +++ b/include/linux/syscalls.h > @@ -408,6 +408,9 @@ asmlinkage long sys_statfs64(const char __user *path, size_t sz, > asmlinkage long sys_fstatfs(unsigned int fd, struct statfs __user *buf); > asmlinkage long sys_fstatfs64(unsigned int fd, size_t sz, > struct statfs64 __user *buf); > +asmlinkage long sys_statmnt(u64 mnt_id, u64 mask, > + struct statmnt __user *buf, size_t bufsize, > + unsigned int flags); > asmlinkage long sys_truncate(const char __user *path, long length); > asmlinkage long sys_ftruncate(unsigned int fd, unsigned long length); > #if BITS_PER_LONG == 32 > diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h > index abe087c53b4b..640997231ff6 100644 > --- a/include/uapi/asm-generic/unistd.h > +++ b/include/uapi/asm-generic/unistd.h > @@ -823,8 +823,11 @@ __SYSCALL(__NR_cachestat, sys_cachestat) > #define __NR_fchmodat2 452 > __SYSCALL(__NR_fchmodat2, sys_fchmodat2) > > +#define __NR_statmnt 454 > +__SYSCALL(__NR_statmnt, sys_statmnt) > + > #undef __NR_syscalls > -#define __NR_syscalls 453 > +#define __NR_syscalls 455 > > /* > * 32 bit systems traditionally used different > diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h > index bb242fdcfe6b..4ec7308a9259 100644 > --- a/include/uapi/linux/mount.h > +++ b/include/uapi/linux/mount.h > @@ -138,4 +138,40 @@ struct mount_attr { > /* List of all mount_attr versions. */ > #define MOUNT_ATTR_SIZE_VER0 32 /* sizeof first published struct */ > > +struct stmt_str { > + __u32 off; > + __u32 len; > +}; > + > +struct statmnt { > + __u64 mask; /* What results were written [uncond] */ > + __u32 sb_dev_major; /* Device ID */ > + __u32 sb_dev_minor; > + __u64 sb_magic; /* ..._SUPER_MAGIC */ > + __u32 sb_flags; /* MS_{RDONLY,SYNCHRONOUS,DIRSYNC,LAZYTIME} */ > + __u32 __spare1; > + __u64 mnt_id; /* Unique ID of mount */ > + __u64 mnt_parent_id; /* Unique ID of parent (for root == mnt_id) */ > + __u32 mnt_id_old; /* Reused IDs used in proc/.../mountinfo */ > + __u32 mnt_parent_id_old; > + __u64 mnt_attr; /* MOUNT_ATTR_... */ > + __u64 mnt_propagation; /* MS_{SHARED,SLAVE,PRIVATE,UNBINDABLE} */ > + __u64 mnt_peer_group; /* ID of shared peer group */ > + __u64 mnt_master; /* Mount receives propagation from this ID */ > + __u64 propagate_from; /* Propagation from in current namespace */ > + __u64 __spare[20]; > + struct stmt_str mnt_root; /* Root of mount relative to root of fs */ > + struct stmt_str mountpoint; /* Mountpoint relative to root of process */ > + struct stmt_str fs_type; /* Filesystem type[.subtype] */ > + struct stmt_str sb_opts; /* Super block string options (nul delimted) */ > +}; > + > +#define STMT_SB_BASIC 0x00000001U /* Want/got sb_... */ > +#define STMT_MNT_BASIC 0x00000002U /* Want/got mnt_... */ > +#define STMT_PROPAGATE_FROM 0x00000004U /* Want/got propagate_from */ > +#define STMT_MNT_ROOT 0x00000008U /* Want/got mnt_root */ > +#define STMT_MOUNTPOINT 0x00000010U /* Want/got mountpoint */ > +#define STMT_FS_TYPE 0x00000020U /* Want/got fs_type */ > +#define STMT_SB_OPTS 0x00000040U /* Want/got sb_opts */ > + > #endif /* _UAPI_LINUX_MOUNT_H */ > -- > 2.41.0 >
On Wed, Sep 13, 2023 at 05:22:35PM +0200, Miklos Szeredi wrote: > Add a way to query attributes of a single mount instead of having to parse > the complete /proc/$PID/mountinfo, which might be huge. > > Lookup the mount by the old (32bit) or new (64bit) mount ID. If a mount > needs to be queried based on path, then statx(2) can be used to first query > the mount ID belonging to the path. > > Design is based on a suggestion by Linus: > > "So I'd suggest something that is very much like "statfsat()", which gets > a buffer and a length, and returns an extended "struct statfs" *AND* > just a string description at the end." So what we agreed to at LSFMM was that we split filesystem option retrieval into a separate system call and just have a very focused statx() for mounts with just binary and non-variable sized information. We even gave David a hard time about this. :) I would really love if we could stick to that. Linus, I realize this was your suggestion a long time ago but I would really like us to avoid structs with variable sized fields at the end of a struct. That's just so painful for userspace and universally disliked. If you care I can even find the LSFMM video where we have users of that api requesting that we please don't do this. So it'd be great if you wouldn't insist on it. This will also allow us to turn statmnt() into an extensible argument system call versioned by size just like we do any new system calls with struct arguments (e.g., mount_setattr(), clone3(), openat2() and so on). Which is how we should do things like that. Other than that I really think this is on track for what we ultimately want. > +struct stmt_str { > + __u32 off; > + __u32 len; > +}; > + > +struct statmnt { > + __u64 mask; /* What results were written [uncond] */ > + __u32 sb_dev_major; /* Device ID */ > + __u32 sb_dev_minor; > + __u64 sb_magic; /* ..._SUPER_MAGIC */ > + __u32 sb_flags; /* MS_{RDONLY,SYNCHRONOUS,DIRSYNC,LAZYTIME} */ > + __u32 __spare1; > + __u64 mnt_id; /* Unique ID of mount */ > + __u64 mnt_parent_id; /* Unique ID of parent (for root == mnt_id) */ > + __u32 mnt_id_old; /* Reused IDs used in proc/.../mountinfo */ > + __u32 mnt_parent_id_old; > + __u64 mnt_attr; /* MOUNT_ATTR_... */ > + __u64 mnt_propagation; /* MS_{SHARED,SLAVE,PRIVATE,UNBINDABLE} */ > + __u64 mnt_peer_group; /* ID of shared peer group */ > + __u64 mnt_master; /* Mount receives propagation from this ID */ > + __u64 propagate_from; /* Propagation from in current namespace */ > + __u64 __spare[20]; > + struct stmt_str mnt_root; /* Root of mount relative to root of fs */ > + struct stmt_str mountpoint; /* Mountpoint relative to root of process */ > + struct stmt_str fs_type; /* Filesystem type[.subtype] */ I think if we want to do this here we should add: __u64 fs_type __u64 fs_subtype fs_type can just be our filesystem magic number and we introduce magic numbers for sub types as well. So we don't need to use strings here. Userspace can trivially map the magic numbers to filesystem names. We don't need to do this for them. > + struct stmt_str sb_opts; /* Super block string options (nul delimted) */ > +}; > > The interface closely mimics that of statx. > > Handle ASCII attributes by appending after the end of the structure (as per > above suggestion). Allow querying multiple string attributes with > individual offset/length for each. String are nul terminated (termination > isn't counted in length). > > Mount options are also delimited with nul characters. Unlike proc, special > characters are not quoted. > > Link: https://lore.kernel.org/all/CAHk-=wh5YifP7hzKSbwJj94+DZ2czjrZsczy6GBimiogZws=rg@mail.gmail.com/ > Signed-off-by: Miklos Szeredi <mszeredi@redhat.com> > --- > arch/x86/entry/syscalls/syscall_64.tbl | 1 + > fs/internal.h | 5 + > fs/namespace.c | 312 ++++++++++++++++++++++++- > fs/proc_namespace.c | 19 +- > fs/statfs.c | 1 + > include/linux/syscalls.h | 3 + > include/uapi/asm-generic/unistd.h | 5 +- > include/uapi/linux/mount.h | 36 +++ > 8 files changed, 373 insertions(+), 9 deletions(-) > > diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl > index 1d6eee30eceb..6d807c30cd16 100644 > --- a/arch/x86/entry/syscalls/syscall_64.tbl > +++ b/arch/x86/entry/syscalls/syscall_64.tbl > @@ -375,6 +375,7 @@ > 451 common cachestat sys_cachestat > 452 common fchmodat2 sys_fchmodat2 > 453 64 map_shadow_stack sys_map_shadow_stack > +454 common statmnt sys_statmnt > > # > # Due to a historical design error, certain syscalls are numbered differently > diff --git a/fs/internal.h b/fs/internal.h > index d64ae03998cc..8f75271428aa 100644 > --- a/fs/internal.h > +++ b/fs/internal.h > @@ -83,6 +83,11 @@ int path_mount(const char *dev_name, struct path *path, > const char *type_page, unsigned long flags, void *data_page); > int path_umount(struct path *path, int flags); > > +/* > + * proc_namespace.c > + */ > +int show_path(struct seq_file *m, struct dentry *root); > + > /* > * fs_struct.c > */ > diff --git a/fs/namespace.c b/fs/namespace.c > index de47c5f66e17..088a52043bba 100644 > --- a/fs/namespace.c > +++ b/fs/namespace.c > @@ -69,7 +69,8 @@ static DEFINE_IDA(mnt_id_ida); > static DEFINE_IDA(mnt_group_ida); > > /* Don't allow confusion with mount ID allocated wit IDA */ > -static atomic64_t mnt_id_ctr = ATOMIC64_INIT(1ULL << 32); > +#define OLD_MNT_ID_MAX UINT_MAX > +static atomic64_t mnt_id_ctr = ATOMIC64_INIT(OLD_MNT_ID_MAX); > > static struct hlist_head *mount_hashtable __read_mostly; > static struct hlist_head *mountpoint_hashtable __read_mostly; > @@ -4678,6 +4679,315 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path, > return err; > } > > +static bool mnt_id_match(struct mount *mnt, u64 id) > +{ > + if (id <= OLD_MNT_ID_MAX) > + return id == mnt->mnt_id; > + else > + return id == mnt->mnt_id_unique; > +} > + > +struct vfsmount *lookup_mnt_in_ns(u64 id, struct mnt_namespace *ns) > +{ > + struct mount *mnt; > + struct vfsmount *res = NULL; > + > + lock_ns_list(ns); > + list_for_each_entry(mnt, &ns->list, mnt_list) { > + if (!mnt_is_cursor(mnt) && mnt_id_match(mnt, id)) { > + res = &mnt->mnt; > + break; > + } > + } > + unlock_ns_list(ns); > + return res; > +} > + > +struct stmt_state { > + void __user *const buf; > + size_t const bufsize; > + struct vfsmount *const mnt; > + u64 const mask; > + struct seq_file seq; > + struct path root; > + struct statmnt sm; > + size_t pos; > + int err; > +}; > + > +typedef int (*stmt_func_t)(struct stmt_state *); > + > +static int stmt_string_seq(struct stmt_state *s, stmt_func_t func) > +{ > + struct seq_file *seq = &s->seq; > + int ret; > + > + seq->count = 0; > + seq->size = min_t(size_t, seq->size, s->bufsize - s->pos); > + seq->buf = kvmalloc(seq->size, GFP_KERNEL_ACCOUNT); > + if (!seq->buf) > + return -ENOMEM; > + > + ret = func(s); > + if (ret) > + return ret; > + > + if (seq_has_overflowed(seq)) { > + if (seq->size == s->bufsize - s->pos) > + return -EOVERFLOW; > + seq->size *= 2; > + if (seq->size > MAX_RW_COUNT) > + return -ENOMEM; > + kvfree(seq->buf); > + return 0; > + } > + > + /* Done */ > + return 1; > +} > + > +static void stmt_string(struct stmt_state *s, u64 mask, stmt_func_t func, > + stmt_str_t *str) > +{ > + int ret = s->pos >= s->bufsize ? -EOVERFLOW : 0; > + struct statmnt *sm = &s->sm; > + struct seq_file *seq = &s->seq; > + > + if (s->err || !(s->mask & mask)) > + return; > + > + seq->size = PAGE_SIZE; > + while (!ret) > + ret = stmt_string_seq(s, func); > + > + if (ret < 0) { > + s->err = ret; > + } else { > + seq->buf[seq->count++] = '\0'; > + if (copy_to_user(s->buf + s->pos, seq->buf, seq->count)) { > + s->err = -EFAULT; > + } else { > + str->off = s->pos; > + str->len = seq->count - 1; > + s->pos += seq->count; > + } > + } > + kvfree(seq->buf); > + sm->mask |= mask; > +} > + > +static void stmt_numeric(struct stmt_state *s, u64 mask, stmt_func_t func) > +{ > + if (s->err || !(s->mask & mask)) > + return; > + > + s->err = func(s); > + s->sm.mask |= mask; > +} > + > +static u64 mnt_to_attr_flags(struct vfsmount *mnt) > +{ > + unsigned int mnt_flags = READ_ONCE(mnt->mnt_flags); > + u64 attr_flags = 0; > + > + if (mnt_flags & MNT_READONLY) > + attr_flags |= MOUNT_ATTR_RDONLY; > + if (mnt_flags & MNT_NOSUID) > + attr_flags |= MOUNT_ATTR_NOSUID; > + if (mnt_flags & MNT_NODEV) > + attr_flags |= MOUNT_ATTR_NODEV; > + if (mnt_flags & MNT_NOEXEC) > + attr_flags |= MOUNT_ATTR_NOEXEC; > + if (mnt_flags & MNT_NODIRATIME) > + attr_flags |= MOUNT_ATTR_NODIRATIME; > + if (mnt_flags & MNT_NOSYMFOLLOW) > + attr_flags |= MOUNT_ATTR_NOSYMFOLLOW; > + > + if (mnt_flags & MNT_NOATIME) > + attr_flags |= MOUNT_ATTR_NOATIME; > + else if (mnt_flags & MNT_RELATIME) > + attr_flags |= MOUNT_ATTR_RELATIME; > + else > + attr_flags |= MOUNT_ATTR_STRICTATIME; > + > + if (is_idmapped_mnt(mnt)) > + attr_flags |= MOUNT_ATTR_IDMAP; > + > + return attr_flags; > +} > + > +static u64 mnt_to_propagation_flags(struct mount *m) > +{ > + u64 propagation = 0; > + > + if (IS_MNT_SHARED(m)) > + propagation |= MS_SHARED; > + if (IS_MNT_SLAVE(m)) > + propagation |= MS_SLAVE; > + if (IS_MNT_UNBINDABLE(m)) > + propagation |= MS_UNBINDABLE; > + if (!propagation) > + propagation |= MS_PRIVATE; > + > + return propagation; > +} > + > +static int stmt_sb_basic(struct stmt_state *s) > +{ > + struct super_block *sb = s->mnt->mnt_sb; > + > + s->sm.sb_dev_major = MAJOR(sb->s_dev); > + s->sm.sb_dev_minor = MINOR(sb->s_dev); > + s->sm.sb_magic = sb->s_magic; > + s->sm.sb_flags = sb->s_flags & (SB_RDONLY|SB_SYNCHRONOUS|SB_DIRSYNC|SB_LAZYTIME); > + > + return 0; > +} > + > +static int stmt_mnt_basic(struct stmt_state *s) > +{ > + struct mount *m = real_mount(s->mnt); > + > + s->sm.mnt_id = m->mnt_id_unique; > + s->sm.mnt_parent_id = m->mnt_parent->mnt_id_unique; > + s->sm.mnt_id_old = m->mnt_id; > + s->sm.mnt_parent_id_old = m->mnt_parent->mnt_id; > + s->sm.mnt_attr = mnt_to_attr_flags(&m->mnt); > + s->sm.mnt_propagation = mnt_to_propagation_flags(m); > + s->sm.mnt_peer_group = IS_MNT_SHARED(m) ? m->mnt_group_id : 0; > + s->sm.mnt_master = IS_MNT_SLAVE(m) ? m->mnt_master->mnt_group_id : 0; > + > + return 0; > +} > + > +static int stmt_propagate_from(struct stmt_state *s) > +{ > + struct mount *m = real_mount(s->mnt); > + > + if (!IS_MNT_SLAVE(m)) > + return 0; > + > + s->sm.propagate_from = get_dominating_id(m, ¤t->fs->root); > + > + return 0; > +} > + > +static int stmt_mnt_root(struct stmt_state *s) > +{ > + struct seq_file *seq = &s->seq; > + int err = show_path(seq, s->mnt->mnt_root); > + > + if (!err && !seq_has_overflowed(seq)) { > + seq->buf[seq->count] = '\0'; > + seq->count = string_unescape_inplace(seq->buf, UNESCAPE_OCTAL); > + } > + return err; > +} > + > +static int stmt_mountpoint(struct stmt_state *s) > +{ > + struct vfsmount *mnt = s->mnt; > + struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt }; > + int err = seq_path_root(&s->seq, &mnt_path, &s->root, ""); > + > + return err == SEQ_SKIP ? 0 : err; > +} > + > +static int stmt_fs_type(struct stmt_state *s) > +{ > + struct seq_file *seq = &s->seq; > + struct super_block *sb = s->mnt->mnt_sb; > + > + seq_puts(seq, sb->s_type->name); > + if (sb->s_subtype) { > + seq_putc(seq, '.'); > + seq_puts(seq, sb->s_subtype); > + } > + return 0; > +} > + > +static int stmt_sb_opts(struct stmt_state *s) > +{ > + struct seq_file *seq = &s->seq; > + struct super_block *sb = s->mnt->mnt_sb; > + char *p, *end, *next, *u = seq->buf; > + int err; > + > + if (!sb->s_op->show_options) > + return 0; > + > + err = sb->s_op->show_options(seq, s->mnt->mnt_root); > + if (err || seq_has_overflowed(seq) || !seq->count) > + return err; > + > + end = seq->buf + seq->count; > + *end = '\0'; > + for (p = seq->buf + 1; p < end; p = next + 1) { > + next = strchrnul(p, ','); > + *next = '\0'; > + u += string_unescape(p, u, 0, UNESCAPE_OCTAL) + 1; > + } > + seq->count = u - 1 - seq->buf; > + return 0; > +} > + > +static int do_statmnt(struct stmt_state *s) > +{ > + struct statmnt *sm = &s->sm; > + struct mount *m = real_mount(s->mnt); > + > + if (!capable(CAP_SYS_ADMIN) && > + !is_path_reachable(m, m->mnt.mnt_root, &s->root)) > + return -EPERM; > + > + stmt_numeric(s, STMT_SB_BASIC, stmt_sb_basic); > + stmt_numeric(s, STMT_MNT_BASIC, stmt_mnt_basic); > + stmt_numeric(s, STMT_PROPAGATE_FROM, stmt_propagate_from); > + stmt_string(s, STMT_MNT_ROOT, stmt_mnt_root, &sm->mnt_root); > + stmt_string(s, STMT_MOUNTPOINT, stmt_mountpoint, &sm->mountpoint); > + stmt_string(s, STMT_FS_TYPE, stmt_fs_type, &sm->fs_type); > + stmt_string(s, STMT_SB_OPTS, stmt_sb_opts, &sm->sb_opts); > + > + if (s->err) > + return s->err; > + > + if (copy_to_user(s->buf, sm, min_t(size_t, s->bufsize, sizeof(*sm)))) > + return -EFAULT; > + > + return 0; > +} > + > +SYSCALL_DEFINE5(statmnt, u64, mnt_id, > + u64, mask, struct statmnt __user *, buf, > + size_t, bufsize, unsigned int, flags) > +{ > + struct vfsmount *mnt; > + int err; > + > + if (flags) > + return -EINVAL; > + > + down_read(&namespace_sem); > + mnt = lookup_mnt_in_ns(mnt_id, current->nsproxy->mnt_ns); > + err = -ENOENT; > + if (mnt) { > + struct stmt_state s = { > + .mask = mask, > + .buf = buf, > + .bufsize = bufsize, > + .mnt = mnt, > + .pos = sizeof(*buf), > + }; > + > + get_fs_root(current->fs, &s.root); > + err = do_statmnt(&s); > + path_put(&s.root); > + } > + up_read(&namespace_sem); > + > + return err; > +} > + > static void __init init_mount_tree(void) > { > struct vfsmount *mnt; > diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c > index 250eb5bf7b52..20681d1f6798 100644 > --- a/fs/proc_namespace.c > +++ b/fs/proc_namespace.c > @@ -132,6 +132,15 @@ static int show_vfsmnt(struct seq_file *m, struct vfsmount *mnt) > return err; > } > > +int show_path(struct seq_file *m, struct dentry *root) > +{ > + if (root->d_sb->s_op->show_path) > + return root->d_sb->s_op->show_path(m, root); > + > + seq_dentry(m, root, " \t\n\\"); > + return 0; > +} > + > static int show_mountinfo(struct seq_file *m, struct vfsmount *mnt) > { > struct proc_mounts *p = m->private; > @@ -142,13 +151,9 @@ static int show_mountinfo(struct seq_file *m, struct vfsmount *mnt) > > seq_printf(m, "%i %i %u:%u ", r->mnt_id, r->mnt_parent->mnt_id, > MAJOR(sb->s_dev), MINOR(sb->s_dev)); > - if (sb->s_op->show_path) { > - err = sb->s_op->show_path(m, mnt->mnt_root); > - if (err) > - goto out; > - } else { > - seq_dentry(m, mnt->mnt_root, " \t\n\\"); > - } > + err = show_path(m, mnt->mnt_root); > + if (err) > + goto out; > seq_putc(m, ' '); > > /* mountpoints outside of chroot jail will give SEQ_SKIP on this */ > diff --git a/fs/statfs.c b/fs/statfs.c > index 96d1c3edf289..cc774c2e2c9a 100644 > --- a/fs/statfs.c > +++ b/fs/statfs.c > @@ -9,6 +9,7 @@ > #include <linux/security.h> > #include <linux/uaccess.h> > #include <linux/compat.h> > +#include <uapi/linux/mount.h> > #include "internal.h" > > static int flags_by_mnt(int mnt_flags) > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h > index 22bc6bc147f8..1099bd307fa7 100644 > --- a/include/linux/syscalls.h > +++ b/include/linux/syscalls.h > @@ -408,6 +408,9 @@ asmlinkage long sys_statfs64(const char __user *path, size_t sz, > asmlinkage long sys_fstatfs(unsigned int fd, struct statfs __user *buf); > asmlinkage long sys_fstatfs64(unsigned int fd, size_t sz, > struct statfs64 __user *buf); > +asmlinkage long sys_statmnt(u64 mnt_id, u64 mask, > + struct statmnt __user *buf, size_t bufsize, > + unsigned int flags); > asmlinkage long sys_truncate(const char __user *path, long length); > asmlinkage long sys_ftruncate(unsigned int fd, unsigned long length); > #if BITS_PER_LONG == 32 > diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h > index abe087c53b4b..640997231ff6 100644 > --- a/include/uapi/asm-generic/unistd.h > +++ b/include/uapi/asm-generic/unistd.h > @@ -823,8 +823,11 @@ __SYSCALL(__NR_cachestat, sys_cachestat) > #define __NR_fchmodat2 452 > __SYSCALL(__NR_fchmodat2, sys_fchmodat2) > > +#define __NR_statmnt 454 > +__SYSCALL(__NR_statmnt, sys_statmnt) > + > #undef __NR_syscalls > -#define __NR_syscalls 453 > +#define __NR_syscalls 455 > > /* > * 32 bit systems traditionally used different > diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h > index bb242fdcfe6b..4ec7308a9259 100644 > --- a/include/uapi/linux/mount.h > +++ b/include/uapi/linux/mount.h > @@ -138,4 +138,40 @@ struct mount_attr { > /* List of all mount_attr versions. */ > #define MOUNT_ATTR_SIZE_VER0 32 /* sizeof first published struct */ > > +struct stmt_str { > + __u32 off; > + __u32 len; > +}; > + > +struct statmnt { > + __u64 mask; /* What results were written [uncond] */ > + __u32 sb_dev_major; /* Device ID */ > + __u32 sb_dev_minor; > + __u64 sb_magic; /* ..._SUPER_MAGIC */ > + __u32 sb_flags; /* MS_{RDONLY,SYNCHRONOUS,DIRSYNC,LAZYTIME} */ > + __u32 __spare1; > + __u64 mnt_id; /* Unique ID of mount */ > + __u64 mnt_parent_id; /* Unique ID of parent (for root == mnt_id) */ > + __u32 mnt_id_old; /* Reused IDs used in proc/.../mountinfo */ > + __u32 mnt_parent_id_old; > + __u64 mnt_attr; /* MOUNT_ATTR_... */ > + __u64 mnt_propagation; /* MS_{SHARED,SLAVE,PRIVATE,UNBINDABLE} */ > + __u64 mnt_peer_group; /* ID of shared peer group */ > + __u64 mnt_master; /* Mount receives propagation from this ID */ > + __u64 propagate_from; /* Propagation from in current namespace */ > + __u64 __spare[20]; > + struct stmt_str mnt_root; /* Root of mount relative to root of fs */ > + struct stmt_str mountpoint; /* Mountpoint relative to root of process */ > + struct stmt_str fs_type; /* Filesystem type[.subtype] */ > + struct stmt_str sb_opts; /* Super block string options (nul delimted) */ > +}; > + > +#define STMT_SB_BASIC 0x00000001U /* Want/got sb_... */ > +#define STMT_MNT_BASIC 0x00000002U /* Want/got mnt_... */ > +#define STMT_PROPAGATE_FROM 0x00000004U /* Want/got propagate_from */ > +#define STMT_MNT_ROOT 0x00000008U /* Want/got mnt_root */ > +#define STMT_MOUNTPOINT 0x00000010U /* Want/got mountpoint */ > +#define STMT_FS_TYPE 0x00000020U /* Want/got fs_type */ > +#define STMT_SB_OPTS 0x00000040U /* Want/got sb_opts */ > + > #endif /* _UAPI_LINUX_MOUNT_H */ > -- > 2.41.0 >
On Thu, 14 Sept 2023 at 11:28, Christian Brauner <brauner@kernel.org> wrote: > > On Wed, Sep 13, 2023 at 05:22:35PM +0200, Miklos Szeredi wrote: > > Add a way to query attributes of a single mount instead of having to parse > > the complete /proc/$PID/mountinfo, which might be huge. > > > > Lookup the mount by the old (32bit) or new (64bit) mount ID. If a mount > > needs to be queried based on path, then statx(2) can be used to first query > > the mount ID belonging to the path. > > > > Design is based on a suggestion by Linus: > > > > "So I'd suggest something that is very much like "statfsat()", which gets > > a buffer and a length, and returns an extended "struct statfs" *AND* > > just a string description at the end." > > So what we agreed to at LSFMM was that we split filesystem option > retrieval into a separate system call and just have a very focused > statx() for mounts with just binary and non-variable sized information. > We even gave David a hard time about this. :) I would really love if we > could stick to that. > > Linus, I realize this was your suggestion a long time ago but I would > really like us to avoid structs with variable sized fields at the end of > a struct. That's just so painful for userspace and universally disliked. > If you care I can even find the LSFMM video where we have users of that > api requesting that we please don't do this. So it'd be great if you > wouldn't insist on it. I completely missed that. What I'm thinking is making it even simpler for userspace: struct statmnt { ... char *mnt_root; char *mountpoint; char *fs_type; u32 num_opts; char *opts; }; I'd still just keep options nul delimited. Is there a good reason not to return pointers (pointing to within the supplied buffer obviously) to userspace? > > This will also allow us to turn statmnt() into an extensible argument > system call versioned by size just like we do any new system calls with > struct arguments (e.g., mount_setattr(), clone3(), openat2() and so on). > Which is how we should do things like that. The mask mechanism also allow versioning of the struct. > > Other than that I really think this is on track for what we ultimately > want. > > > +struct stmt_str { > > + __u32 off; > > + __u32 len; > > +}; > > + > > +struct statmnt { > > + __u64 mask; /* What results were written [uncond] */ > > + __u32 sb_dev_major; /* Device ID */ > > + __u32 sb_dev_minor; > > + __u64 sb_magic; /* ..._SUPER_MAGIC */ > > + __u32 sb_flags; /* MS_{RDONLY,SYNCHRONOUS,DIRSYNC,LAZYTIME} */ > > + __u32 __spare1; > > + __u64 mnt_id; /* Unique ID of mount */ > > + __u64 mnt_parent_id; /* Unique ID of parent (for root == mnt_id) */ > > + __u32 mnt_id_old; /* Reused IDs used in proc/.../mountinfo */ > > + __u32 mnt_parent_id_old; > > + __u64 mnt_attr; /* MOUNT_ATTR_... */ > > + __u64 mnt_propagation; /* MS_{SHARED,SLAVE,PRIVATE,UNBINDABLE} */ > > + __u64 mnt_peer_group; /* ID of shared peer group */ > > + __u64 mnt_master; /* Mount receives propagation from this ID */ > > + __u64 propagate_from; /* Propagation from in current namespace */ > > + __u64 __spare[20]; > > + struct stmt_str mnt_root; /* Root of mount relative to root of fs */ > > + struct stmt_str mountpoint; /* Mountpoint relative to root of process */ > > + struct stmt_str fs_type; /* Filesystem type[.subtype] */ > > I think if we want to do this here we should add: > > __u64 fs_type > __u64 fs_subtype > > fs_type can just be our filesystem magic number and we introduce magic It's already there: sb_magic. However it's not a 1:1 mapping (ext* only has one magic). > numbers for sub types as well. So we don't need to use strings here. Ugh. Thanks, Miklos
On Thu, Sep 14, 2023 at 12:13:54PM +0200, Miklos Szeredi wrote: > On Thu, 14 Sept 2023 at 11:28, Christian Brauner <brauner@kernel.org> wrote: > > > > On Wed, Sep 13, 2023 at 05:22:35PM +0200, Miklos Szeredi wrote: > > > Add a way to query attributes of a single mount instead of having to parse > > > the complete /proc/$PID/mountinfo, which might be huge. > > > > > > Lookup the mount by the old (32bit) or new (64bit) mount ID. If a mount > > > needs to be queried based on path, then statx(2) can be used to first query > > > the mount ID belonging to the path. > > > > > > Design is based on a suggestion by Linus: > > > > > > "So I'd suggest something that is very much like "statfsat()", which gets > > > a buffer and a length, and returns an extended "struct statfs" *AND* > > > just a string description at the end." > > > > So what we agreed to at LSFMM was that we split filesystem option > > retrieval into a separate system call and just have a very focused > > statx() for mounts with just binary and non-variable sized information. > > We even gave David a hard time about this. :) I would really love if we > > could stick to that. > > > > Linus, I realize this was your suggestion a long time ago but I would > > really like us to avoid structs with variable sized fields at the end of > > a struct. That's just so painful for userspace and universally disliked. > > If you care I can even find the LSFMM video where we have users of that > > api requesting that we please don't do this. So it'd be great if you > > wouldn't insist on it. > > I completely missed that. No worries, I think the discussion touching on this starts at: https://youtu.be/j3fp2MtRr2I?si=f-YBg6uWq80dV3VC&t=1603 (with David talking quietly without a microphone for some parts unfortunately...) > What I'm thinking is making it even simpler for userspace: > > struct statmnt { > ... > char *mnt_root; > char *mountpoint; > char *fs_type; > u32 num_opts; > char *opts; > }; > > I'd still just keep options nul delimited. > > Is there a good reason not to return pointers (pointing to within the > supplied buffer obviously) to userspace? It's really unpleasant to program with. Yes, I think you pointed out before that it often doesn't matter much as long as the system call is really only relevant to some special purpose userspace. But statmount() will be used pretty extensively pretty quickly for the purpose of finding out mount options on a mount (Querying a whole sequences of mounts via repeated listmount() + statmount() calls on the other hand will be rarer.). And there's just so many tools that need this: libmount, systemd, all kinds of container runtimes, path lookup libraries such as libpathrs, languages like go and rust that expose and wrap these calls and so on. Most of these tools don't need to know about filesystem mount options and if they do they can just query that through an extra system call. No harm in doing that. The agreement we came to to split out listing submounts into a separate system call was exactly to avoid having to have a variable sized pointer at the end of the struct statmnt (That's also part of the video above btw.) and to make it as simple as possible. Plus, the format for how to return arbitrary filesystem mount options warrants a separate discussion imho as that's not really vfs level information. > > This will also allow us to turn statmnt() into an extensible argument > > system call versioned by size just like we do any new system calls with > > struct arguments (e.g., mount_setattr(), clone3(), openat2() and so on). > > Which is how we should do things like that. > > The mask mechanism also allow versioning of the struct. Yes, but this is done with reserved space which just pushes away the problem and bloats the struct for the sake of an unknown future. If we were to use an extensible argument struct we would just version by size. The only requirement is that you extend by 64 bit (see struct clone_args) which had been extended. > > > > > Other than that I really think this is on track for what we ultimately > > want. > > > > > +struct stmt_str { > > > + __u32 off; > > > + __u32 len; > > > +}; > > > + > > > +struct statmnt { > > > + __u64 mask; /* What results were written [uncond] */ > > > + __u32 sb_dev_major; /* Device ID */ > > > + __u32 sb_dev_minor; > > > + __u64 sb_magic; /* ..._SUPER_MAGIC */ > > > + __u32 sb_flags; /* MS_{RDONLY,SYNCHRONOUS,DIRSYNC,LAZYTIME} */ > > > + __u32 __spare1; > > > + __u64 mnt_id; /* Unique ID of mount */ > > > + __u64 mnt_parent_id; /* Unique ID of parent (for root == mnt_id) */ > > > + __u32 mnt_id_old; /* Reused IDs used in proc/.../mountinfo */ > > > + __u32 mnt_parent_id_old; > > > + __u64 mnt_attr; /* MOUNT_ATTR_... */ > > > + __u64 mnt_propagation; /* MS_{SHARED,SLAVE,PRIVATE,UNBINDABLE} */ > > > + __u64 mnt_peer_group; /* ID of shared peer group */ > > > + __u64 mnt_master; /* Mount receives propagation from this ID */ > > > + __u64 propagate_from; /* Propagation from in current namespace */ > > > + __u64 __spare[20]; > > > + struct stmt_str mnt_root; /* Root of mount relative to root of fs */ > > > + struct stmt_str mountpoint; /* Mountpoint relative to root of process */ > > > + struct stmt_str fs_type; /* Filesystem type[.subtype] */ > > > > I think if we want to do this here we should add: > > > > __u64 fs_type > > __u64 fs_subtype > > > > fs_type can just be our filesystem magic number and we introduce magic > > It's already there: sb_magic. > > However it's not a 1:1 mapping (ext* only has one magic). That's a very odd choice but probably fixable by giving it a subtype. > > > numbers for sub types as well. So we don't need to use strings here. > > Ugh. Hm, idk. It's not that bad imho. We'll have to make some ugly tradeoffs.
On Wed, Sep 13, 2023 at 11:23 AM Miklos Szeredi <mszeredi@redhat.com> wrote: > > Add a way to query attributes of a single mount instead of having to parse > the complete /proc/$PID/mountinfo, which might be huge. > > Lookup the mount by the old (32bit) or new (64bit) mount ID. If a mount > needs to be queried based on path, then statx(2) can be used to first query > the mount ID belonging to the path. > > Design is based on a suggestion by Linus: > > "So I'd suggest something that is very much like "statfsat()", which gets > a buffer and a length, and returns an extended "struct statfs" *AND* > just a string description at the end." > > The interface closely mimics that of statx. > > Handle ASCII attributes by appending after the end of the structure (as per > above suggestion). Allow querying multiple string attributes with > individual offset/length for each. String are nul terminated (termination > isn't counted in length). > > Mount options are also delimited with nul characters. Unlike proc, special > characters are not quoted. > > Link: https://lore.kernel.org/all/CAHk-=wh5YifP7hzKSbwJj94+DZ2czjrZsczy6GBimiogZws=rg@mail.gmail.com/ > Signed-off-by: Miklos Szeredi <mszeredi@redhat.com> > --- > arch/x86/entry/syscalls/syscall_64.tbl | 1 + > fs/internal.h | 5 + > fs/namespace.c | 312 ++++++++++++++++++++++++- > fs/proc_namespace.c | 19 +- > fs/statfs.c | 1 + > include/linux/syscalls.h | 3 + > include/uapi/asm-generic/unistd.h | 5 +- > include/uapi/linux/mount.h | 36 +++ > 8 files changed, 373 insertions(+), 9 deletions(-) ... > diff --git a/fs/namespace.c b/fs/namespace.c > index de47c5f66e17..088a52043bba 100644 > --- a/fs/namespace.c > +++ b/fs/namespace.c ... > +static int do_statmnt(struct stmt_state *s) > +{ > + struct statmnt *sm = &s->sm; > + struct mount *m = real_mount(s->mnt); > + > + if (!capable(CAP_SYS_ADMIN) && > + !is_path_reachable(m, m->mnt.mnt_root, &s->root)) > + return -EPERM; I realize statmnt() is different from fstatfs(), but from an access control perspective they look a lot alike to me which is why I think we should probably have a security_sb_statfs() call here. Same thing for the listmnt() syscall in patch 3/3. > + stmt_numeric(s, STMT_SB_BASIC, stmt_sb_basic); > + stmt_numeric(s, STMT_MNT_BASIC, stmt_mnt_basic); > + stmt_numeric(s, STMT_PROPAGATE_FROM, stmt_propagate_from); > + stmt_string(s, STMT_MNT_ROOT, stmt_mnt_root, &sm->mnt_root); > + stmt_string(s, STMT_MOUNTPOINT, stmt_mountpoint, &sm->mountpoint); > + stmt_string(s, STMT_FS_TYPE, stmt_fs_type, &sm->fs_type); > + stmt_string(s, STMT_SB_OPTS, stmt_sb_opts, &sm->sb_opts); > + > + if (s->err) > + return s->err; > + > + if (copy_to_user(s->buf, sm, min_t(size_t, s->bufsize, sizeof(*sm)))) > + return -EFAULT; > + > + return 0; > +}
On 14/9/23 14:11, Amir Goldstein wrote: > On Wed, Sep 13, 2023 at 6:22 PM Miklos Szeredi <mszeredi@redhat.com> wrote: >> Add a way to query attributes of a single mount instead of having to parse >> the complete /proc/$PID/mountinfo, which might be huge. >> >> Lookup the mount by the old (32bit) or new (64bit) mount ID. If a mount >> needs to be queried based on path, then statx(2) can be used to first query >> the mount ID belonging to the path. >> >> Design is based on a suggestion by Linus: >> >> "So I'd suggest something that is very much like "statfsat()", which gets >> a buffer and a length, and returns an extended "struct statfs" *AND* >> just a string description at the end." >> >> The interface closely mimics that of statx. >> >> Handle ASCII attributes by appending after the end of the structure (as per >> above suggestion). Allow querying multiple string attributes with >> individual offset/length for each. String are nul terminated (termination >> isn't counted in length). >> >> Mount options are also delimited with nul characters. Unlike proc, special >> characters are not quoted. >> >> Link: https://lore.kernel.org/all/CAHk-=wh5YifP7hzKSbwJj94+DZ2czjrZsczy6GBimiogZws=rg@mail.gmail.com/ >> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com> >> --- >> arch/x86/entry/syscalls/syscall_64.tbl | 1 + >> fs/internal.h | 5 + >> fs/namespace.c | 312 ++++++++++++++++++++++++- >> fs/proc_namespace.c | 19 +- >> fs/statfs.c | 1 + >> include/linux/syscalls.h | 3 + >> include/uapi/asm-generic/unistd.h | 5 +- >> include/uapi/linux/mount.h | 36 +++ >> 8 files changed, 373 insertions(+), 9 deletions(-) >> >> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl >> index 1d6eee30eceb..6d807c30cd16 100644 >> --- a/arch/x86/entry/syscalls/syscall_64.tbl >> +++ b/arch/x86/entry/syscalls/syscall_64.tbl >> @@ -375,6 +375,7 @@ >> 451 common cachestat sys_cachestat >> 452 common fchmodat2 sys_fchmodat2 >> 453 64 map_shadow_stack sys_map_shadow_stack >> +454 common statmnt sys_statmnt >> >> # >> # Due to a historical design error, certain syscalls are numbered differently >> diff --git a/fs/internal.h b/fs/internal.h >> index d64ae03998cc..8f75271428aa 100644 >> --- a/fs/internal.h >> +++ b/fs/internal.h >> @@ -83,6 +83,11 @@ int path_mount(const char *dev_name, struct path *path, >> const char *type_page, unsigned long flags, void *data_page); >> int path_umount(struct path *path, int flags); >> >> +/* >> + * proc_namespace.c >> + */ >> +int show_path(struct seq_file *m, struct dentry *root); >> + >> /* >> * fs_struct.c >> */ >> diff --git a/fs/namespace.c b/fs/namespace.c >> index de47c5f66e17..088a52043bba 100644 >> --- a/fs/namespace.c >> +++ b/fs/namespace.c >> @@ -69,7 +69,8 @@ static DEFINE_IDA(mnt_id_ida); >> static DEFINE_IDA(mnt_group_ida); >> >> /* Don't allow confusion with mount ID allocated wit IDA */ >> -static atomic64_t mnt_id_ctr = ATOMIC64_INIT(1ULL << 32); >> +#define OLD_MNT_ID_MAX UINT_MAX >> +static atomic64_t mnt_id_ctr = ATOMIC64_INIT(OLD_MNT_ID_MAX); >> >> static struct hlist_head *mount_hashtable __read_mostly; >> static struct hlist_head *mountpoint_hashtable __read_mostly; >> @@ -4678,6 +4679,315 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path, >> return err; >> } >> >> +static bool mnt_id_match(struct mount *mnt, u64 id) >> +{ >> + if (id <= OLD_MNT_ID_MAX) >> + return id == mnt->mnt_id; >> + else >> + return id == mnt->mnt_id_unique; >> +} >> + >> +struct vfsmount *lookup_mnt_in_ns(u64 id, struct mnt_namespace *ns) >> +{ >> + struct mount *mnt; >> + struct vfsmount *res = NULL; >> + >> + lock_ns_list(ns); >> + list_for_each_entry(mnt, &ns->list, mnt_list) { >> + if (!mnt_is_cursor(mnt) && mnt_id_match(mnt, id)) { >> + res = &mnt->mnt; >> + break; >> + } >> + } >> + unlock_ns_list(ns); >> + return res; >> +} >> + >> +struct stmt_state { >> + void __user *const buf; >> + size_t const bufsize; >> + struct vfsmount *const mnt; >> + u64 const mask; >> + struct seq_file seq; >> + struct path root; >> + struct statmnt sm; >> + size_t pos; >> + int err; >> +}; >> + >> +typedef int (*stmt_func_t)(struct stmt_state *); >> + >> +static int stmt_string_seq(struct stmt_state *s, stmt_func_t func) >> +{ >> + struct seq_file *seq = &s->seq; >> + int ret; >> + >> + seq->count = 0; >> + seq->size = min_t(size_t, seq->size, s->bufsize - s->pos); >> + seq->buf = kvmalloc(seq->size, GFP_KERNEL_ACCOUNT); >> + if (!seq->buf) >> + return -ENOMEM; >> + >> + ret = func(s); >> + if (ret) >> + return ret; >> + >> + if (seq_has_overflowed(seq)) { >> + if (seq->size == s->bufsize - s->pos) >> + return -EOVERFLOW; >> + seq->size *= 2; >> + if (seq->size > MAX_RW_COUNT) >> + return -ENOMEM; >> + kvfree(seq->buf); >> + return 0; >> + } >> + >> + /* Done */ >> + return 1; >> +} >> + >> +static void stmt_string(struct stmt_state *s, u64 mask, stmt_func_t func, >> + stmt_str_t *str) >> +{ >> + int ret = s->pos >= s->bufsize ? -EOVERFLOW : 0; >> + struct statmnt *sm = &s->sm; >> + struct seq_file *seq = &s->seq; >> + >> + if (s->err || !(s->mask & mask)) >> + return; >> + >> + seq->size = PAGE_SIZE; >> + while (!ret) >> + ret = stmt_string_seq(s, func); >> + >> + if (ret < 0) { >> + s->err = ret; >> + } else { >> + seq->buf[seq->count++] = '\0'; >> + if (copy_to_user(s->buf + s->pos, seq->buf, seq->count)) { >> + s->err = -EFAULT; >> + } else { >> + str->off = s->pos; >> + str->len = seq->count - 1; >> + s->pos += seq->count; >> + } >> + } >> + kvfree(seq->buf); >> + sm->mask |= mask; >> +} >> + >> +static void stmt_numeric(struct stmt_state *s, u64 mask, stmt_func_t func) >> +{ >> + if (s->err || !(s->mask & mask)) >> + return; >> + >> + s->err = func(s); >> + s->sm.mask |= mask; >> +} >> + >> +static u64 mnt_to_attr_flags(struct vfsmount *mnt) >> +{ >> + unsigned int mnt_flags = READ_ONCE(mnt->mnt_flags); >> + u64 attr_flags = 0; >> + >> + if (mnt_flags & MNT_READONLY) >> + attr_flags |= MOUNT_ATTR_RDONLY; >> + if (mnt_flags & MNT_NOSUID) >> + attr_flags |= MOUNT_ATTR_NOSUID; >> + if (mnt_flags & MNT_NODEV) >> + attr_flags |= MOUNT_ATTR_NODEV; >> + if (mnt_flags & MNT_NOEXEC) >> + attr_flags |= MOUNT_ATTR_NOEXEC; >> + if (mnt_flags & MNT_NODIRATIME) >> + attr_flags |= MOUNT_ATTR_NODIRATIME; >> + if (mnt_flags & MNT_NOSYMFOLLOW) >> + attr_flags |= MOUNT_ATTR_NOSYMFOLLOW; >> + >> + if (mnt_flags & MNT_NOATIME) >> + attr_flags |= MOUNT_ATTR_NOATIME; >> + else if (mnt_flags & MNT_RELATIME) >> + attr_flags |= MOUNT_ATTR_RELATIME; >> + else >> + attr_flags |= MOUNT_ATTR_STRICTATIME; >> + >> + if (is_idmapped_mnt(mnt)) >> + attr_flags |= MOUNT_ATTR_IDMAP; >> + >> + return attr_flags; >> +} >> + >> +static u64 mnt_to_propagation_flags(struct mount *m) >> +{ >> + u64 propagation = 0; >> + >> + if (IS_MNT_SHARED(m)) >> + propagation |= MS_SHARED; >> + if (IS_MNT_SLAVE(m)) >> + propagation |= MS_SLAVE; >> + if (IS_MNT_UNBINDABLE(m)) >> + propagation |= MS_UNBINDABLE; >> + if (!propagation) >> + propagation |= MS_PRIVATE; >> + >> + return propagation; >> +} >> + >> +static int stmt_sb_basic(struct stmt_state *s) >> +{ >> + struct super_block *sb = s->mnt->mnt_sb; >> + >> + s->sm.sb_dev_major = MAJOR(sb->s_dev); >> + s->sm.sb_dev_minor = MINOR(sb->s_dev); >> + s->sm.sb_magic = sb->s_magic; >> + s->sm.sb_flags = sb->s_flags & (SB_RDONLY|SB_SYNCHRONOUS|SB_DIRSYNC|SB_LAZYTIME); >> + >> + return 0; >> +} >> + >> +static int stmt_mnt_basic(struct stmt_state *s) >> +{ >> + struct mount *m = real_mount(s->mnt); >> + >> + s->sm.mnt_id = m->mnt_id_unique; >> + s->sm.mnt_parent_id = m->mnt_parent->mnt_id_unique; >> + s->sm.mnt_id_old = m->mnt_id; >> + s->sm.mnt_parent_id_old = m->mnt_parent->mnt_id; >> + s->sm.mnt_attr = mnt_to_attr_flags(&m->mnt); >> + s->sm.mnt_propagation = mnt_to_propagation_flags(m); >> + s->sm.mnt_peer_group = IS_MNT_SHARED(m) ? m->mnt_group_id : 0; >> + s->sm.mnt_master = IS_MNT_SLAVE(m) ? m->mnt_master->mnt_group_id : 0; >> + >> + return 0; >> +} >> + >> +static int stmt_propagate_from(struct stmt_state *s) >> +{ >> + struct mount *m = real_mount(s->mnt); >> + >> + if (!IS_MNT_SLAVE(m)) >> + return 0; >> + >> + s->sm.propagate_from = get_dominating_id(m, ¤t->fs->root); >> + >> + return 0; >> +} >> + >> +static int stmt_mnt_root(struct stmt_state *s) >> +{ >> + struct seq_file *seq = &s->seq; >> + int err = show_path(seq, s->mnt->mnt_root); >> + >> + if (!err && !seq_has_overflowed(seq)) { >> + seq->buf[seq->count] = '\0'; >> + seq->count = string_unescape_inplace(seq->buf, UNESCAPE_OCTAL); >> + } >> + return err; >> +} >> + >> +static int stmt_mountpoint(struct stmt_state *s) >> +{ >> + struct vfsmount *mnt = s->mnt; >> + struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt }; >> + int err = seq_path_root(&s->seq, &mnt_path, &s->root, ""); >> + >> + return err == SEQ_SKIP ? 0 : err; >> +} >> + >> +static int stmt_fs_type(struct stmt_state *s) >> +{ >> + struct seq_file *seq = &s->seq; >> + struct super_block *sb = s->mnt->mnt_sb; >> + >> + seq_puts(seq, sb->s_type->name); >> + if (sb->s_subtype) { >> + seq_putc(seq, '.'); >> + seq_puts(seq, sb->s_subtype); >> + } >> + return 0; >> +} >> + >> +static int stmt_sb_opts(struct stmt_state *s) >> +{ >> + struct seq_file *seq = &s->seq; >> + struct super_block *sb = s->mnt->mnt_sb; >> + char *p, *end, *next, *u = seq->buf; >> + int err; >> + >> + if (!sb->s_op->show_options) >> + return 0; >> + >> + err = sb->s_op->show_options(seq, s->mnt->mnt_root); >> + if (err || seq_has_overflowed(seq) || !seq->count) >> + return err; >> + >> + end = seq->buf + seq->count; >> + *end = '\0'; >> + for (p = seq->buf + 1; p < end; p = next + 1) { >> + next = strchrnul(p, ','); >> + *next = '\0'; >> + u += string_unescape(p, u, 0, UNESCAPE_OCTAL) + 1; >> + } >> + seq->count = u - 1 - seq->buf; >> + return 0; >> +} >> + >> +static int do_statmnt(struct stmt_state *s) >> +{ >> + struct statmnt *sm = &s->sm; >> + struct mount *m = real_mount(s->mnt); >> + >> + if (!capable(CAP_SYS_ADMIN) && >> + !is_path_reachable(m, m->mnt.mnt_root, &s->root)) >> + return -EPERM; >> + >> + stmt_numeric(s, STMT_SB_BASIC, stmt_sb_basic); >> + stmt_numeric(s, STMT_MNT_BASIC, stmt_mnt_basic); >> + stmt_numeric(s, STMT_PROPAGATE_FROM, stmt_propagate_from); >> + stmt_string(s, STMT_MNT_ROOT, stmt_mnt_root, &sm->mnt_root); >> + stmt_string(s, STMT_MOUNTPOINT, stmt_mountpoint, &sm->mountpoint); >> + stmt_string(s, STMT_FS_TYPE, stmt_fs_type, &sm->fs_type); >> + stmt_string(s, STMT_SB_OPTS, stmt_sb_opts, &sm->sb_opts); >> + >> + if (s->err) >> + return s->err; >> + >> + if (copy_to_user(s->buf, sm, min_t(size_t, s->bufsize, sizeof(*sm)))) >> + return -EFAULT; >> + >> + return 0; > Similar concern as with listmnt, I think that users would > want to have a way to get the fixed size statmnt part that fits > in the buffer, even if the variable length string values do not fit > and be able to query the required buffer size to get the strings. > > The API could be either to explicitly request > STMT_MNT_ROOT_LEN | STMT_MOUNTPOINT_LEN ... > without allowing mixing of no-value and value requests, > or to out-out from any string values using a single flag, > which is probably more simple for API and implementation. There is also the possibility that the size needed to satisfy the request will change between request and call, not sure how to deal with that, but the size estimate is needed ... Ian
On Thu, 14 Sept 2023 at 17:27, Christian Brauner <brauner@kernel.org> wrote: > > On Thu, Sep 14, 2023 at 12:13:54PM +0200, Miklos Szeredi wrote: > No worries, I think the discussion touching on this starts at: > https://youtu.be/j3fp2MtRr2I?si=f-YBg6uWq80dV3VC&t=1603 > (with David talking quietly without a microphone for some parts > unfortunately...) (Thanks for digging that out.) That discussion touched on two aspects of using a single call vs. multiple calls: - atomicity - marshalling Atomicity of getting a snapshot of the current mount tree with all of its attributes was never guaranteed, although reading /proc/self/mountinfo into a sufficiently large buffer would work that way. However, I don't see why mount trees would require stronger guarantees than dentry trees (for which we have basically none). Marshalling/demashalling of arbitrary structures is indeed ugly. I think what Linus suggested, and what this interface was based on is much less than that. Also see my suggestion below: it doesn't need demashalling at all due to the fact that the kernel can fill in the pointers. And yes, this could be used for arbitrary structures without compromising type safety, but at the cost of adding more complexity to the kernel (at least ascii strings are just one type). Even more type clean interface: struct statmnt *statmnt(u64 mnt_id, u64 mask, void *buf, size_t bufsize, unsigned int flags); Kernel would return a fully initialized struct with the numeric as well as string fields filled. That part is trivial for userspace to deal with. For sizing the buffer and versioning the struct see discussion below. > > What I'm thinking is making it even simpler for userspace: > > > > struct statmnt { > > ... > > char *mnt_root; > > char *mountpoint; > > char *fs_type; > > u32 num_opts; > > char *opts; > > }; > > > > I'd still just keep options nul delimited. > > > > Is there a good reason not to return pointers (pointing to within the > > supplied buffer obviously) to userspace? > > It's really unpleasant to program with. Yes, I think you pointed out > before that it often doesn't matter much as long as the system call is > really only relevant to some special purpose userspace. > > But statmount() will be used pretty extensively pretty quickly for the > purpose of finding out mount options on a mount (Querying a whole > sequences of mounts via repeated listmount() + statmount() calls on the > other hand will be rarer.). > > And there's just so many tools that need this: libmount, systemd, all > kinds of container runtimes, path lookup libraries such as libpathrs, > languages like go and rust that expose and wrap these calls and so on. > > Most of these tools don't need to know about filesystem mount options > and if they do they can just query that through an extra system call. No > harm in doing that. Just pass sizeof(struct statmnt) as the buffer size, and it will work that way. > The agreement we came to to split out listing submounts into a separate > system call was exactly to avoid having to have a variable sized pointer > at the end of the struct statmnt (That's also part of the video above > btw.) and to make it as simple as possible. > > Plus, the format for how to return arbitrary filesystem mount options > warrants a separate discussion imho as that's not really vfs level > information. Okay. Let's take fs options out of this. That leaves: - fs type and optionally subtype - root of mount within fs - mountpoint path The type and subtype are naturally limited to sane sizes, those are not an issue. For paths the evolution of the relevant system/library calls was: char *getwd(char buf[PATH_MAX]); char *getcwd(char *buf, size_t size); char *get_current_dir_name(void); It started out using a fixed size buffer, then a variable sized buffer, then an automatically allocated buffer by the library, hiding the need to resize on overflow. The latest style is suitable for the statmnt() call as well, if we worry about pleasantness of the API. > > > > This will also allow us to turn statmnt() into an extensible argument > > > system call versioned by size just like we do any new system calls with > > > struct arguments (e.g., mount_setattr(), clone3(), openat2() and so on). > > > Which is how we should do things like that. > > > > The mask mechanism also allow versioning of the struct. > > Yes, but this is done with reserved space which just pushes away the > problem and bloats the struct for the sake of an unknown future. If we > were to use an extensible argument struct we would just version by size. > The only requirement is that you extend by 64 bit (see struct > clone_args) which had been extended. No need for reserved space in fact. Versioning would still work, as long as userspace is strictly checking the return mask. I.e. newly added fields will come after the old buffer, as assumed by the kernel. But the kernel will never set the mask bits for these fields, so userspace should not ever look at them. Note: the interface does have a bufsize parameter, so no possibility of memory corruption in any event. I added the reserved space so that userspace would be protected from rubbish at the end of the struct if the kernel was older. A library wrapper could work around that issue (move the variable part beyond the end of the new struct), but it would require code update in the wrapper, not just updating the struct. But in fact it's much simpler to just add ample reserved space and be done with it forever, no need to worry about versioning at all. > > > numbers for sub types as well. So we don't need to use strings here. > > > > Ugh. > > Hm, idk. It's not that bad imho. We'll have to make some ugly tradeoffs. Subtype is a fuse thing (e.g. sshfs would show up as fuse.sshfs /proc/self/mountinfo. Forcing each fuse filesystem to invent a magic number... please no. Thanks, Miklos
On Thu, 14 Sept 2023 at 22:40, Paul Moore <paul@paul-moore.com> wrote: > > On Wed, Sep 13, 2023 at 11:23 AM Miklos Szeredi <mszeredi@redhat.com> wrote: > ... > > > +static int do_statmnt(struct stmt_state *s) > > +{ > > + struct statmnt *sm = &s->sm; > > + struct mount *m = real_mount(s->mnt); > > + > > + if (!capable(CAP_SYS_ADMIN) && > > + !is_path_reachable(m, m->mnt.mnt_root, &s->root)) > > + return -EPERM; > > I realize statmnt() is different from fstatfs(), but from an access > control perspective they look a lot alike to me which is why I think > we should probably have a security_sb_statfs() call here. Same thing > for the listmnt() syscall in patch 3/3. Okay, makes sense. Thanks, Miklos
On Wed, Sep 13, 2023 at 9:25 AM Miklos Szeredi <mszeredi@redhat.com> wrote: > > Add a way to query attributes of a single mount instead of having to parse > the complete /proc/$PID/mountinfo, which might be huge. > > Lookup the mount by the old (32bit) or new (64bit) mount ID. If a mount > needs to be queried based on path, then statx(2) can be used to first query > the mount ID belonging to the path. > > Design is based on a suggestion by Linus: > > "So I'd suggest something that is very much like "statfsat()", which gets > a buffer and a length, and returns an extended "struct statfs" *AND* > just a string description at the end." > > The interface closely mimics that of statx. > > Handle ASCII attributes by appending after the end of the structure (as per > above suggestion). Allow querying multiple string attributes with > individual offset/length for each. String are nul terminated (termination > isn't counted in length). > > Mount options are also delimited with nul characters. Unlike proc, special > characters are not quoted. > Thank you for writing this patch. I wish that this had existed the many times I've written parsers for mounts files in my life. What do you think about exposing the locked flags, a la what happens on propagation of mount across user namespaces?
On 18/9/23 02:18, Sargun Dhillon wrote: > On Wed, Sep 13, 2023 at 9:25 AM Miklos Szeredi <mszeredi@redhat.com> wrote: >> Add a way to query attributes of a single mount instead of having to parse >> the complete /proc/$PID/mountinfo, which might be huge. >> >> Lookup the mount by the old (32bit) or new (64bit) mount ID. If a mount >> needs to be queried based on path, then statx(2) can be used to first query >> the mount ID belonging to the path. >> >> Design is based on a suggestion by Linus: >> >> "So I'd suggest something that is very much like "statfsat()", which gets >> a buffer and a length, and returns an extended "struct statfs" *AND* >> just a string description at the end." >> >> The interface closely mimics that of statx. >> >> Handle ASCII attributes by appending after the end of the structure (as per >> above suggestion). Allow querying multiple string attributes with >> individual offset/length for each. String are nul terminated (termination >> isn't counted in length). >> >> Mount options are also delimited with nul characters. Unlike proc, special >> characters are not quoted. >> > Thank you for writing this patch. I wish that this had existed the many times > I've written parsers for mounts files in my life. > > What do you think about exposing the locked flags, a la what happens > on propagation of mount across user namespaces? Which flags do you mean? If you mean shared, slave and I think there's a group id as well, etc. then yes they were available in the original fsinfo() implementation as they were requested. So, yes, it would be good to also include those too. Ian
On Mon, Sep 18, 2023 at 07:36:39AM +0800, Ian Kent wrote: > > On 18/9/23 02:18, Sargun Dhillon wrote: > > On Wed, Sep 13, 2023 at 9:25 AM Miklos Szeredi <mszeredi@redhat.com> wrote: > > > Add a way to query attributes of a single mount instead of having to parse > > > the complete /proc/$PID/mountinfo, which might be huge. > > > > > > Lookup the mount by the old (32bit) or new (64bit) mount ID. If a mount > > > needs to be queried based on path, then statx(2) can be used to first query > > > the mount ID belonging to the path. > > > > > > Design is based on a suggestion by Linus: > > > > > > "So I'd suggest something that is very much like "statfsat()", which gets > > > a buffer and a length, and returns an extended "struct statfs" *AND* > > > just a string description at the end." > > > > > > The interface closely mimics that of statx. > > > > > > Handle ASCII attributes by appending after the end of the structure (as per > > > above suggestion). Allow querying multiple string attributes with > > > individual offset/length for each. String are nul terminated (termination > > > isn't counted in length). > > > > > > Mount options are also delimited with nul characters. Unlike proc, special > > > characters are not quoted. > > > > > Thank you for writing this patch. I wish that this had existed the many times > > I've written parsers for mounts files in my life. > > > > What do you think about exposing the locked flags, a la what happens > > on propagation of mount across user namespaces? > > Which flags do you mean? When you propagate mounts across mount+user namespaces a subset of (security sensitive) mount attributes become locked. This information is currently only available via internal flags but not in any way explicitly exposed to userspace. There's a proposal to extend mount_setattr(2) to explicitly allow locking flags but that would mean a new set of mount attr flags. So until the format of that is determined and settled this should be kept out of statmount().
> Atomicity of getting a snapshot of the current mount tree with all of > its attributes was never guaranteed, although reading > /proc/self/mountinfo into a sufficiently large buffer would work that > way. However, I don't see why mount trees would require stronger > guarantees than dentry trees (for which we have basically none). So atomicity was never put forward as a requirement. In that session/recording I explicitly state that we won't guarantee atomicity. And systemd agreed with this. So I think we're all on the same page. > Even more type clean interface: > > struct statmnt *statmnt(u64 mnt_id, u64 mask, void *buf, size_t > bufsize, unsigned int flags); > > Kernel would return a fully initialized struct with the numeric as > well as string fields filled. That part is trivial for userspace to > deal with. I really would prefer a properly typed struct and that's what everyone was happy with in the session as well. So I would not like to change the main parameters. > > Plus, the format for how to return arbitrary filesystem mount options > > warrants a separate discussion imho as that's not really vfs level > > information. > > Okay. Let's take fs options out of this. Thanks. > > That leaves: > > - fs type and optionally subtype So since subtype is FUSE specific it might be better to move this to filesystem specific options imho. > - root of mount within fs > - mountpoint path > > The type and subtype are naturally limited to sane sizes, those are > not an issue. What's the limit for fstype actually? I don't think there is one. There's one by chance but not by design afaict? Maybe crazy idea: That magic number thing that we do in include/uapi/linux/magic.h is there a good reason for this or why don't we just add a proper, simple enum: enum { FS_TYPE_ADFS 1 FS_TYPE_AFFS 2 FS_TYPE_AFS 3 FS_TYPE_AUTOFS 4 FS_TYPE_EXT2 5 FS_TYPE_EXT3 6 FS_TYPE_EXT4 7 . . . FS_TYPE_MAX } that we start returning from statmount(). We can still return both the old and the new fstype? It always felt a bit odd that fs developers to just select a magic number. > > For paths the evolution of the relevant system/library calls was: > > char *getwd(char buf[PATH_MAX]); > char *getcwd(char *buf, size_t size); > char *get_current_dir_name(void); > > It started out using a fixed size buffer, then a variable sized > buffer, then an automatically allocated buffer by the library, hiding > the need to resize on overflow. > > The latest style is suitable for the statmnt() call as well, if we > worry about pleasantness of the API. So, can we then do the following struct: struct statmnt { __u64 mask; /* What results were written [uncond] */ __u32 sb_dev_major; /* Device ID */ __u32 sb_dev_minor; __u64 sb_magic; /* ..._SUPER_MAGIC */ __u32 sb_flags; /* MS_{RDONLY,SYNCHRONOUS,DIRSYNC,LAZYTIME} */ __u32 __spare1; __u64 mnt_id; /* Unique ID of mount */ __u64 mnt_parent_id; /* Unique ID of parent (for root == mnt_id) */ __u32 mnt_id_old; /* Reused IDs used in proc/.../mountinfo */ __u32 mnt_parent_id_old; __u64 mnt_attr; /* MOUNT_ATTR_... */ __u64 mnt_propagation; /* MS_{SHARED,SLAVE,PRIVATE,UNBINDABLE} */ __u64 mnt_peer_group; /* ID of shared peer group */ __u64 mnt_master; /* Mount receives propagation from this ID */ __u64 propagate_from; /* Propagation from in current namespace */ __aligned_u64 mountpoint; __u32 mountpoint_len; __aligned_u64 mountroot; __u32 mountroot_len; __u64 __spare[20]; }; Userspace knows already how to deal with that because of bpf and other structs (e.g., both systemd and LXC have ptr_to_u64() helpers and so on). Libmount and glibc can hide this away internally as well.
On Mon, Sep 18, 2023 at 3:51 PM Christian Brauner <brauner@kernel.org> wrote: > I really would prefer a properly typed struct and that's what everyone > was happy with in the session as well. So I would not like to change the > main parameters. I completely agree. Just would like to understand this point: struct statmnt *statmnt(u64 mntid, u64 mask, unsigned int flags); What's not properly typed about this interface? I guess the answer is that it's not a syscall interface, which will have an added [void *buf, size_t bufsize], while the buffer sizing is done by a simple libc wrapper. Do you think that's a problem? If so, why? Thanks, Miklos
On Mon, Sep 18, 2023 at 04:14:02PM +0200, Miklos Szeredi wrote: > On Mon, Sep 18, 2023 at 3:51 PM Christian Brauner <brauner@kernel.org> wrote: > > > I really would prefer a properly typed struct and that's what everyone > > was happy with in the session as well. So I would not like to change the > > main parameters. > > I completely agree. Just would like to understand this point: > > struct statmnt *statmnt(u64 mntid, u64 mask, unsigned int flags); > > What's not properly typed about this interface? > > I guess the answer is that it's not a syscall interface, which will > have an added [void *buf, size_t bufsize], while the buffer sizing is > done by a simple libc wrapper. > > Do you think that's a problem? If so, why? Sorry, I think we just talked passed each other. I didn't realize you were talking about a glibc wrapper. I'm not so much concerned with that they can expose this in whathever way they like. But we will have a lot of low-level userspace that will directly use statmount() or not even have glibc like go and other languages. The system call should please have a proper struct like you had in your first proposal. This is what I'm concerned about: int statmount(u64 mnt_id, struct statmnt __user *st, size_t size, unsigned int flags) instead of taking an void pointer.
On Thu, 2023-09-14 at 17:26 +0200, Christian Brauner wrote: > On Thu, Sep 14, 2023 at 12:13:54PM +0200, Miklos Szeredi wrote: > > On Thu, 14 Sept 2023 at 11:28, Christian Brauner <brauner@kernel.org> wrote: > > > > > > On Wed, Sep 13, 2023 at 05:22:35PM +0200, Miklos Szeredi wrote: > > > > Add a way to query attributes of a single mount instead of having to parse > > > > the complete /proc/$PID/mountinfo, which might be huge. > > > > > > > > Lookup the mount by the old (32bit) or new (64bit) mount ID. If a mount > > > > needs to be queried based on path, then statx(2) can be used to first query > > > > the mount ID belonging to the path. > > > > > > > > Design is based on a suggestion by Linus: > > > > > > > > "So I'd suggest something that is very much like "statfsat()", which gets > > > > a buffer and a length, and returns an extended "struct statfs" *AND* > > > > just a string description at the end." > > > > > > So what we agreed to at LSFMM was that we split filesystem option > > > retrieval into a separate system call and just have a very focused > > > statx() for mounts with just binary and non-variable sized information. > > > We even gave David a hard time about this. :) I would really love if we > > > could stick to that. > > > > > > Linus, I realize this was your suggestion a long time ago but I would > > > really like us to avoid structs with variable sized fields at the end of > > > a struct. That's just so painful for userspace and universally disliked. > > > If you care I can even find the LSFMM video where we have users of that > > > api requesting that we please don't do this. So it'd be great if you > > > wouldn't insist on it. > > > > I completely missed that. > > No worries, I think the discussion touching on this starts at: > https://youtu.be/j3fp2MtRr2I?si=f-YBg6uWq80dV3VC&t=1603 > (with David talking quietly without a microphone for some parts > unfortunately...) > > > What I'm thinking is making it even simpler for userspace: > > > > struct statmnt { > > ... > > char *mnt_root; > > char *mountpoint; > > char *fs_type; > > u32 num_opts; > > char *opts; > > }; > > > > I'd still just keep options nul delimited. > > > > Is there a good reason not to return pointers (pointing to within the > > supplied buffer obviously) to userspace? > > It's really unpleasant to program with. Yes, I think you pointed out > before that it often doesn't matter much as long as the system call is > really only relevant to some special purpose userspace. > > But statmount() will be used pretty extensively pretty quickly for the > purpose of finding out mount options on a mount (Querying a whole > sequences of mounts via repeated listmount() + statmount() calls on the > other hand will be rarer.). > > And there's just so many tools that need this: libmount, systemd, all > kinds of container runtimes, path lookup libraries such as libpathrs, > languages like go and rust that expose and wrap these calls and so on. > > Most of these tools don't need to know about filesystem mount options > and if they do they can just query that through an extra system call. No > harm in doing that. > > The agreement we came to to split out listing submounts into a separate > system call was exactly to avoid having to have a variable sized pointer > at the end of the struct statmnt (That's also part of the video above > btw.) and to make it as simple as possible. > > Plus, the format for how to return arbitrary filesystem mount options > warrants a separate discussion imho as that's not really vfs level > information. > > > > This will also allow us to turn statmnt() into an extensible argument > > > system call versioned by size just like we do any new system calls with > > > struct arguments (e.g., mount_setattr(), clone3(), openat2() and so on). > > > Which is how we should do things like that. > > > > The mask mechanism also allow versioning of the struct. > > Yes, but this is done with reserved space which just pushes away the > problem and bloats the struct for the sake of an unknown future. If we > were to use an extensible argument struct we would just version by size. > The only requirement is that you extend by 64 bit (see struct > clone_args) which had been extended. > > Fixed size structs are much nicer to deal with, and most of the fields we're talking about don't change ofetn enough to make trying to strive for perfect atomicity worthwhile. What sort of interface are you thinking for fetching variable-length string info? It sounds a lot like getxattr that uses a mnt_id in place of a pathname. getmntattr() ? > > > > > > Other than that I really think this is on track for what we ultimately > > > want. > > > > > > > +struct stmt_str { > > > > + __u32 off; > > > > + __u32 len; > > > > +}; > > > > + > > > > +struct statmnt { > > > > + __u64 mask; /* What results were written [uncond] */ > > > > + __u32 sb_dev_major; /* Device ID */ > > > > + __u32 sb_dev_minor; > > > > + __u64 sb_magic; /* ..._SUPER_MAGIC */ > > > > + __u32 sb_flags; /* MS_{RDONLY,SYNCHRONOUS,DIRSYNC,LAZYTIME} */ > > > > + __u32 __spare1; > > > > + __u64 mnt_id; /* Unique ID of mount */ > > > > + __u64 mnt_parent_id; /* Unique ID of parent (for root == mnt_id) */ > > > > + __u32 mnt_id_old; /* Reused IDs used in proc/.../mountinfo */ > > > > + __u32 mnt_parent_id_old; > > > > + __u64 mnt_attr; /* MOUNT_ATTR_... */ > > > > + __u64 mnt_propagation; /* MS_{SHARED,SLAVE,PRIVATE,UNBINDABLE} */ > > > > + __u64 mnt_peer_group; /* ID of shared peer group */ > > > > + __u64 mnt_master; /* Mount receives propagation from this ID */ > > > > + __u64 propagate_from; /* Propagation from in current namespace */ > > > > + __u64 __spare[20]; > > > > + struct stmt_str mnt_root; /* Root of mount relative to root of fs */ > > > > + struct stmt_str mountpoint; /* Mountpoint relative to root of process */ > > > > + struct stmt_str fs_type; /* Filesystem type[.subtype] */ > > > A bit tangential to this discussion, but one thing we could consider is adding something like a mnt_change_cookie field that increments on any significant changes on the mount: i.e. remounts with new options, changes to parentage or propagation, etc. That might make it more palatable to do something with separate syscalls for the string-based fields. You could do: statmnt(...); getmntattr(mnt, "mnt.fstype", ...); statmnt(...); ...and then if the mnt_change_cookie hasn't changed, you know that the string option was stable during that window. > > > I think if we want to do this here we should add: > > > > > > __u64 fs_type > > > __u64 fs_subtype > > > > > > fs_type can just be our filesystem magic number and we introduce magic > > > > It's already there: sb_magic. > > > > However it's not a 1:1 mapping (ext* only has one magic). > > That's a very odd choice but probably fixable by giving it a subtype. > > > > > > numbers for sub types as well. So we don't need to use strings here. > > > > Ugh. > > Hm, idk. It's not that bad imho. We'll have to make some ugly tradeoffs.
On Mon, 18 Sept 2023 at 16:25, Christian Brauner <brauner@kernel.org> wrote: > The system call should please have a proper struct like you had in your > first proposal. This is what I'm concerned about: > > int statmount(u64 mnt_id, > struct statmnt __user *st, > size_t size, > unsigned int flags) > > instead of taking an void pointer. So you are not concerned about having ascii strings returned by the syscall? I thought that was the main complaint. Thanks, Miklos
> Fixed size structs are much nicer to deal with, and most of the fields > we're talking about don't change ofetn enough to make trying to strive > for perfect atomicity worthwhile. I think we can live with mnt_root and mnt_mountpoint in struct statmnt if we add a length field for both them and make them __u64 pointers. That's what we did in clone3() for the pid array and bpf is doing that as well for log buffers and pathnames. So if Miklos is fine with that then I'm happy to compromise. And I think that's all the variable length data we want in struct statmount anyway. > ...and then if the mnt_change_cookie hasn't changed, you know that the > string option was stable during that window. Meh, I would really like to sidestep this and keep it as simple as we can. I like the proposal overall I just don't want it to get diluted too much by exploding into another overly broad solution.
On Mon, Sep 18, 2023 at 04:32:30PM +0200, Miklos Szeredi wrote: > On Mon, 18 Sept 2023 at 16:25, Christian Brauner <brauner@kernel.org> wrote: > > > The system call should please have a proper struct like you had in your > > first proposal. This is what I'm concerned about: > > > > int statmount(u64 mnt_id, > > struct statmnt __user *st, > > size_t size, > > unsigned int flags) > > > > instead of taking an void pointer. > > So you are not concerned about having ascii strings returned by the > syscall? I thought that was the main complaint. I'm not following. The original proposals were only returning strings even for basic binary data such as mount flags, propagation options, and so on and we're using the xattr interface for any type of information. What we're talking about here is a nicely typed struct which returns two paths @mnt_root and @mnt_point which can both be represented as u64 pointers with length parameters like we do in other binary structs such as bpf and clone3 and a few others. That is a compromise I can live with. I'm really trying to find as much common ground here as we can.
On Mon, 18 Sept 2023 at 16:40, Christian Brauner <brauner@kernel.org> wrote: > What we're talking about here is a nicely typed struct which returns two > paths @mnt_root and @mnt_point which can both be represented as u64 > pointers with length parameters like we do in other binary structs such > as bpf and clone3 and a few others. That is a compromise I can live > with. I'm really trying to find as much common ground here as we can. So to be clear about your proposal: .mnt_root and .mountpoint are initialized by the caller to buffers that the kernel can copy paths into? If there's an overflow (one of the buffers was too small) the syscall returns -EOVERFLOW? Thanks, Miklos
> So to be clear about your proposal: .mnt_root and .mountpoint are > initialized by the caller to buffers that the kernel can copy paths > into? Yeah, u64 pointer to a buffer and a size (see e.g., @set_tid and @set_tid_size for struct clone_args, @log_buf and @log_size and other args in there). > > If there's an overflow (one of the buffers was too small) the syscall > returns -EOVERFLOW? Yeah, I mean we have to make some things their problem. To me that is an acceptable compromise.
On Mon, 18 Sept 2023 at 17:22, Christian Brauner <brauner@kernel.org> wrote: > > > So to be clear about your proposal: .mnt_root and .mountpoint are > > initialized by the caller to buffers that the kernel can copy paths > > into? > > Yeah, u64 pointer to a buffer and a size (see e.g., @set_tid and > @set_tid_size for struct clone_args, @log_buf and @log_size and other > args in there). > > > > > If there's an overflow (one of the buffers was too small) the syscall > > returns -EOVERFLOW? > > Yeah, I mean we have to make some things their problem. > > To me that is an acceptable compromise. Okay, so there are now (at least) two buffers, and on overflow the caller cannot know which one got overflown. It can resize both, but that doesn't make the caller any simpler to implement. Also the interface is kind of weird in that some struct members are out, some are in (the pointers and the lengths). I'd prefer the single buffer interface, which has none of the above issues. Thanks, Miklos
On Sep 18, 2023, at 7:51 AM, Christian Brauner <brauner@kernel.org> wrote: > > >> The type and subtype are naturally limited to sane sizes, those are >> not an issue. > > What's the limit for fstype actually? I don't think there is one. > There's one by chance but not by design afaict? > > Maybe crazy idea: > That magic number thing that we do in include/uapi/linux/magic.h > is there a good reason for this or why don't we just add a proper, > simple enum: > > enum { > FS_TYPE_ADFS 1 > FS_TYPE_AFFS 2 > FS_TYPE_AFS 3 > FS_TYPE_AUTOFS 4 > FS_TYPE_EXT2 5 > FS_TYPE_EXT3 6 > FS_TYPE_EXT4 7 > . > . > . > FS_TYPE_MAX > } > > that we start returning from statmount(). We can still return both the > old and the new fstype? It always felt a bit odd that fs developers to > just select a magic number. Yes, there is a very good reason that there isn't an enum for filesystem type, which is because this API would be broken if it encounters any filesystem that is not listed there. Often a single filesystem driver in the kernel will have multiple different magic numbers to handle versions, endianness, etc. Having a 32-bit magic number allows decentralized development with low chance of collision, and using new filesystems without having to patch every kernel for this new API to work with that filesystem. Also, filesystems come and go (though more slowly) over time, and keeping the full list of every filesystem ever developed in the kernel enum would be a headache. The field in the statmnt() call would need to be at a fixed-size 32-bit value in any case, so having it return the existing magic will "just work" because userspace tools already know and understand these magic values, while introducing an in-kernel enum would be broken for multiple reasons. Cheers, Andreas
On Mon, Sep 18, 2023 at 11:39 AM Miklos Szeredi <miklos@szeredi.hu> wrote: > Okay, so there are now (at least) two buffers, and on overflow the > caller cannot know which one got overflown. It can resize both, but > that doesn't make the caller any simpler to implement. > > Also the interface is kind of weird in that some struct members are > out, some are in (the pointers and the lengths). > > I'd prefer the single buffer interface, which has none of the above issues. > > Thanks, > Miklos One natural solution is to set either of the two lengths to the expected size if the provided buffer are too small. That way, the caller learns both which of the buffers is too small, and how large they need to be. Replacing a provided size with an expected size in this way already has precedent in existing syscalls: recvmsg(2): The msg argument points to an in/out struct msghdr, and msg->msg_name points to an optional buffer which receives the source address. If msg->msg_namelen is less than the actual size of the source address, the function truncates the address to that length before storing it in msg->msg_name; otherwise, it stores the full address. In either case, it sets msg->msg_namelen to the full size of the source address before returning. (An address buffer size is similarly provided directly as an in/out pointer in accept(2), accept4(2), getpeername(2), getsockname(2), and recvfrom(2).) name_to_handle_at(2): The handle argument points to an in/out struct file_handle, followed by a variable-length char array. If handle->handle_bytes is too small to store the opaque handle, the function returns -EOVERFLOW; otherwise, it succeeds. In either case, it sets handle->handle_bytes to the size of the opaque handle before returning. perf_event_open(2): The attr argument points to an in/out struct perf_event_attr. If attr->size is not a valid size for the struct, the function sets it to the latest size and returns -E2BIG. sched_setattr(2): The attr argument points to an in/out struct sched_attr. If attr->size is not a valid size for the struct, the function sets it to the latest size and returns -E2BIG. The specific pattern of returning the actual size of the strings both on success and on failure, as with recvmsg(2) and name_to_handle_at(2), is beneficial for callers that want to copy the strings elsewhere without having to scan for the null byte. (Also, it would work well if we ever wanted to return variable-size binary data, such as arrays of structs.) Indeed, if we returned the actual size of the string, we could even take a more radical approach of never setting a null byte after the data, leaving the caller to append its own null byte if it really wants one. But perhaps that would be taking it a bit too far; I just don't want this API to end up in an awful situation like strncpy(3) or struct sockaddr_un, where the buffer is always null-terminated except in one particular edge case. Also, if we include a null byte in the returned size, it could invite off-by-one errors in callers that just expect it to be the length of the string. Meanwhile, if this solution of in/out size fields were adopted, then there'd still be the question of what to do when a provided size is too small: should the returned string be truncated (indicating the issue only by the returned size being greater than the provided size), or should the entire call fail with an -EOVERFLOW? IMO, the former is strictly more flexible, since the caller can set a limit on how big a buffer it's willing to dedicate to any particular string, and it can still retrieve the remaining data if that buffer isn't quite big enough. But the latter might be considered a bit more foolproof against callers who don't properly test for truncation. Thank you, Matthew House
On Tue, 19 Sept 2023 at 02:38, Matthew House <mattlloydhouse@gmail.com> wrote: > One natural solution is to set either of the two lengths to the expected > size if the provided buffer are too small. That way, the caller learns both > which of the buffers is too small, and how large they need to be. Replacing > a provided size with an expected size in this way already has precedent in > existing syscalls: This is where the thread started. Knowing the size of the buffer is no good, since the needed buffer could change between calls. We are trying to create a simple interface, no? My proposal would need a helper like this: struct statmnt *statmount(uint64_t mnt_id, uint64_t mask, unsigned int flags) { size_t bufsize = 1 << 15; void *buf; int ret; for (;;) { buf = malloc(bufsize <<= 1); if (!buf) return NULL; ret = syscall(__NR_statmnt, mnt_id, mask, buf, bufsize, flags); if (!ret) return buf; free(buf); if (errno != EOVERFLOW) return NULL; } } Christian's would be (ignoring .fs_type for now): int statmount(uint64_t mnt_id, uint64_t mask, struct statmnt *st, unsigned int flags) { int ret; st->mnt_root_size = 1 << 15; st->mountpoint_size = 1 << 15; for (;;) { st->mnt_root = malloc(st->mnt_root_size <<= 1); st->mountpoint = malloc(st->mountpoint <<= 1); if (!st->mnt_root || !st->mountpoint) { free(st->mnt_root); free(st->mountpoint); return -1; } ret = syscall(__NR_statmnt, mnt_id, mask, st, sizeof(*st), flags); if (!ret || errno != EOVERFLOW) return ret; free(st->mnt_root); free(st->mountpoint); } } It's not hugely more complex, but more complex nonetheless. Also having the helper allocate buffers inside the struct could easily result in leaks since it's not obvious what the caller needs to free, while in the first example it is. Note that I'm not against having the prototype on the kernel interface take a typed pointer. If strings are not needed, both interfaces would work in exactly the same way. Thanks, Miklos
On Tue, Sep 19, 2023 at 10:02:17AM +0200, Miklos Szeredi wrote: > On Tue, 19 Sept 2023 at 02:38, Matthew House <mattlloydhouse@gmail.com> wrote: > > > One natural solution is to set either of the two lengths to the expected > > size if the provided buffer are too small. That way, the caller learns both > > which of the buffers is too small, and how large they need to be. Replacing > > a provided size with an expected size in this way already has precedent in > > existing syscalls: > > This is where the thread started. Knowing the size of the buffer is > no good, since the needed buffer could change between calls. The same problem would exist for the single buffer. Realistically, users will most often simply use a fixed size PATH_MAX buffer that will cover most cases and fallback to allocating a larger buffer in case things go awry. I don't think we need to make this atomic either. Providing a hint for the required buffer size in case this fails is good enough and should be a rather rare occurence and is exactly how other variable-sized buffers are handled. > Also having the helper allocate buffers inside the struct could easily > result in leaks since it's not obvious what the caller needs to free, I don't think we need to be overly concerned with how userspace implements the wrapper here. Leaks can occur in both scenarios and low-level userspace can use automatic cleanup macros (we even support it in the kernel since v6.5) to harden against this. Really, the main things I care about are 64 bit alignment of the whole struct, typed __u64 pointers with __u32 size for mnt_root and mnt_point and that we please spell out "mount" and not use "mnt": so statmount because the new mount api uses "mount" (move_mount(), mount_setattr(), fsmount(), MOUNT_ATTR_*) almost everywhere.
On Tue, 19 Sept 2023 at 11:07, Christian Brauner <brauner@kernel.org> wrote: > > On Tue, Sep 19, 2023 at 10:02:17AM +0200, Miklos Szeredi wrote: > > This is where the thread started. Knowing the size of the buffer is > > no good, since the needed buffer could change between calls. > > The same problem would exist for the single buffer. Realistically, users > will most often simply use a fixed size PATH_MAX buffer that will cover > most cases and fallback to allocating a larger buffer in case things go > awry. Exactly. A large buffer will work in 99.99% of the cases. Good quality implementations will deal with the 0.01% as well, but optimizing that case is nonsense. > Really, the main things I care about are 64 bit alignment of the whole > struct, typed __u64 pointers Okay. > with __u32 size for mnt_root and mnt_point Unnecessary if the strings are nul terminated. > and that we please spell out "mount" and not use "mnt": so statmount > because the new mount api uses "mount" (move_mount(), mount_setattr(), > fsmount(), MOUNT_ATTR_*) almost everywhere. Okay. Incremental below. Also pushed to: git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git#statmount Thanks, Miklos diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl index 0d9a47b0ce9b..a1b3ce7d22cc 100644 --- a/arch/x86/entry/syscalls/syscall_64.tbl +++ b/arch/x86/entry/syscalls/syscall_64.tbl @@ -375,8 +375,8 @@ 451 common cachestat sys_cachestat 452 common fchmodat2 sys_fchmodat2 453 64 map_shadow_stack sys_map_shadow_stack -454 common statmnt sys_statmnt -455 common listmnt sys_listmnt +454 common statmount sys_statmount +455 common listmount sys_listmount # # Due to a historical design error, certain syscalls are numbered differently diff --git a/fs/namespace.c b/fs/namespace.c index 5362b1ffb26f..803003052bfb 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -68,9 +68,8 @@ static u64 event; static DEFINE_IDA(mnt_id_ida); static DEFINE_IDA(mnt_group_ida); -/* Don't allow confusion with mount ID allocated wit IDA */ -#define OLD_MNT_ID_MAX UINT_MAX -static atomic64_t mnt_id_ctr = ATOMIC64_INIT(OLD_MNT_ID_MAX); +/* Don't allow confusion with old 32bit mount ID */ +static atomic64_t mnt_id_ctr = ATOMIC64_INIT(1ULL << 32); static struct hlist_head *mount_hashtable __read_mostly; static struct hlist_head *mountpoint_hashtable __read_mostly; @@ -4679,14 +4678,6 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path, return err; } -static bool mnt_id_match(struct mount *mnt, u64 id) -{ - if (id <= OLD_MNT_ID_MAX) - return id == mnt->mnt_id; - else - return id == mnt->mnt_id_unique; -} - struct vfsmount *lookup_mnt_in_ns(u64 id, struct mnt_namespace *ns) { struct mount *mnt; @@ -4694,7 +4685,7 @@ struct vfsmount *lookup_mnt_in_ns(u64 id, struct mnt_namespace *ns) lock_ns_list(ns); list_for_each_entry(mnt, &ns->list, mnt_list) { - if (!mnt_is_cursor(mnt) && mnt_id_match(mnt, id)) { + if (!mnt_is_cursor(mnt) && id == mnt->mnt_id_unique) { res = &mnt->mnt; break; } @@ -4747,7 +4738,7 @@ static int stmt_string_seq(struct stmt_state *s, stmt_func_t func) } static void stmt_string(struct stmt_state *s, u64 mask, stmt_func_t func, - stmt_str_t *str) + u64 *str) { int ret = s->pos >= s->bufsize ? -EOVERFLOW : 0; struct statmnt *sm = &s->sm; @@ -4767,8 +4758,7 @@ static void stmt_string(struct stmt_state *s, u64 mask, stmt_func_t func, if (copy_to_user(s->buf + s->pos, seq->buf, seq->count)) { s->err = -EFAULT; } else { - str->off = s->pos; - str->len = seq->count - 1; + *str = (unsigned long) (s->buf + s->pos); s->pos += seq->count; } } @@ -4899,39 +4889,10 @@ static int stmt_fs_type(struct stmt_state *s) struct super_block *sb = s->mnt->mnt_sb; seq_puts(seq, sb->s_type->name); - if (sb->s_subtype) { - seq_putc(seq, '.'); - seq_puts(seq, sb->s_subtype); - } - return 0; -} - -static int stmt_sb_opts(struct stmt_state *s) -{ - struct seq_file *seq = &s->seq; - struct super_block *sb = s->mnt->mnt_sb; - char *p, *end, *next, *u = seq->buf; - int err; - - if (!sb->s_op->show_options) - return 0; - - err = sb->s_op->show_options(seq, s->mnt->mnt_root); - if (err || seq_has_overflowed(seq) || !seq->count) - return err; - - end = seq->buf + seq->count; - *end = '\0'; - for (p = seq->buf + 1; p < end; p = next + 1) { - next = strchrnul(p, ','); - *next = '\0'; - u += string_unescape(p, u, 0, UNESCAPE_OCTAL) + 1; - } - seq->count = u - 1 - seq->buf; return 0; } -static int do_statmnt(struct stmt_state *s) +static int do_statmount(struct stmt_state *s) { struct statmnt *sm = &s->sm; struct mount *m = real_mount(s->mnt); @@ -4946,7 +4907,6 @@ static int do_statmnt(struct stmt_state *s) stmt_string(s, STMT_MNT_ROOT, stmt_mnt_root, &sm->mnt_root); stmt_string(s, STMT_MOUNTPOINT, stmt_mountpoint, &sm->mountpoint); stmt_string(s, STMT_FS_TYPE, stmt_fs_type, &sm->fs_type); - stmt_string(s, STMT_SB_OPTS, stmt_sb_opts, &sm->sb_opts); if (s->err) return s->err; @@ -4957,7 +4917,7 @@ static int do_statmnt(struct stmt_state *s) return 0; } -SYSCALL_DEFINE5(statmnt, u64, mnt_id, +SYSCALL_DEFINE5(statmount, u64, mnt_id, u64, mask, struct statmnt __user *, buf, size_t, bufsize, unsigned int, flags) { @@ -4980,7 +4940,7 @@ SYSCALL_DEFINE5(statmnt, u64, mnt_id, }; get_fs_root(current->fs, &s.root); - err = do_statmnt(&s); + err = do_statmount(&s); path_put(&s.root); } up_read(&namespace_sem); @@ -4988,19 +4948,25 @@ SYSCALL_DEFINE5(statmnt, u64, mnt_id, return err; } -static long do_listmnt(struct vfsmount *mnt, u64 __user *buf, size_t bufsize, - const struct path *root) +static long do_listmount(struct vfsmount *mnt, u64 __user *buf, size_t bufsize, + const struct path *root, unsigned int flags) { struct mount *r, *m = real_mount(mnt); struct path rootmnt = { .mnt = root->mnt, .dentry = root->mnt->mnt_root }; long ctr = 0; + bool reachable_only = true; - if (!capable(CAP_SYS_ADMIN) && - !is_path_reachable(m, mnt->mnt_root, &rootmnt)) - return -EPERM; + if (flags & LISTMOUNT_UNREACHABLE) { + if (!capable(CAP_SYS_ADMIN)) + return -EPERM; + reachable_only = false; + } + + if (reachable_only && !is_path_reachable(m, mnt->mnt_root, &rootmnt)) + return capable(CAP_SYS_ADMIN) ? 0 : -EPERM; list_for_each_entry(r, &m->mnt_mounts, mnt_child) { - if (!capable(CAP_SYS_ADMIN) && + if (reachable_only && !is_path_reachable(r, r->mnt.mnt_root, root)) continue; @@ -5015,14 +4981,14 @@ static long do_listmnt(struct vfsmount *mnt, u64 __user *buf, size_t bufsize, return ctr; } -SYSCALL_DEFINE4(listmnt, u64, mnt_id, u64 __user *, buf, size_t, bufsize, +SYSCALL_DEFINE4(listmount, u64, mnt_id, u64 __user *, buf, size_t, bufsize, unsigned int, flags) { struct vfsmount *mnt; struct path root; long err; - if (flags) + if (flags & ~LISTMOUNT_UNREACHABLE) return -EINVAL; down_read(&namespace_sem); @@ -5030,7 +4996,7 @@ SYSCALL_DEFINE4(listmnt, u64, mnt_id, u64 __user *, buf, size_t, bufsize, err = -ENOENT; if (mnt) { get_fs_root(current->fs, &root); - err = do_listmnt(mnt, buf, bufsize, &root); + err = do_listmount(mnt, buf, bufsize, &root, flags); path_put(&root); } up_read(&namespace_sem); diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h index 5d776cdb6f18..a35fb7b2c842 100644 --- a/include/linux/syscalls.h +++ b/include/linux/syscalls.h @@ -74,6 +74,7 @@ struct landlock_ruleset_attr; enum landlock_rule_type; struct cachestat_range; struct cachestat; +struct statmnt; #include <linux/types.h> #include <linux/aio_abi.h> @@ -408,11 +409,11 @@ asmlinkage long sys_statfs64(const char __user *path, size_t sz, asmlinkage long sys_fstatfs(unsigned int fd, struct statfs __user *buf); asmlinkage long sys_fstatfs64(unsigned int fd, size_t sz, struct statfs64 __user *buf); -asmlinkage long sys_statmnt(u64 mnt_id, u64 mask, - struct statmnt __user *buf, size_t bufsize, - unsigned int flags); -asmlinkage long sys_listmnt(u64 mnt_id, u64 __user *buf, size_t bufsize, - unsigned int flags); +asmlinkage long sys_statmount(u64 mnt_id, u64 mask, + struct statmnt __user *buf, size_t bufsize, + unsigned int flags); +asmlinkage long sys_listmount(u64 mnt_id, u64 __user *buf, size_t bufsize, + unsigned int flags); asmlinkage long sys_truncate(const char __user *path, long length); asmlinkage long sys_ftruncate(unsigned int fd, unsigned long length); #if BITS_PER_LONG == 32 diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h index a2b41370f603..8df6a747e21a 100644 --- a/include/uapi/asm-generic/unistd.h +++ b/include/uapi/asm-generic/unistd.h @@ -823,11 +823,11 @@ __SYSCALL(__NR_cachestat, sys_cachestat) #define __NR_fchmodat2 452 __SYSCALL(__NR_fchmodat2, sys_fchmodat2) -#define __NR_statmnt 454 -__SYSCALL(__NR_statmnt, sys_statmnt) +#define __NR_statmount 454 +__SYSCALL(__NR_statmount, sys_statmount) -#define __NR_listmnt 455 -__SYSCALL(__NR_listmnt, sys_listmnt) +#define __NR_listmount 455 +__SYSCALL(__NR_listmount, sys_listmount) #undef __NR_syscalls #define __NR_syscalls 456 diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h index 4ec7308a9259..d98b41024507 100644 --- a/include/uapi/linux/mount.h +++ b/include/uapi/linux/mount.h @@ -138,11 +138,6 @@ struct mount_attr { /* List of all mount_attr versions. */ #define MOUNT_ATTR_SIZE_VER0 32 /* sizeof first published struct */ -struct stmt_str { - __u32 off; - __u32 len; -}; - struct statmnt { __u64 mask; /* What results were written [uncond] */ __u32 sb_dev_major; /* Device ID */ @@ -159,11 +154,10 @@ struct statmnt { __u64 mnt_peer_group; /* ID of shared peer group */ __u64 mnt_master; /* Mount receives propagation from this ID */ __u64 propagate_from; /* Propagation from in current namespace */ - __u64 __spare[20]; - struct stmt_str mnt_root; /* Root of mount relative to root of fs */ - struct stmt_str mountpoint; /* Mountpoint relative to root of process */ - struct stmt_str fs_type; /* Filesystem type[.subtype] */ - struct stmt_str sb_opts; /* Super block string options (nul delimted) */ + __u64 mnt_root; /* [str] Root of mount relative to root of fs */ + __u64 mountpoint; /* [str] Mountpoint relative to root of process */ + __u64 fs_type; /* [srt] Filesystem type */ + __u64 __spare[49]; }; #define STMT_SB_BASIC 0x00000001U /* Want/got sb_... */ @@ -172,6 +166,8 @@ struct statmnt { #define STMT_MNT_ROOT 0x00000008U /* Want/got mnt_root */ #define STMT_MOUNTPOINT 0x00000010U /* Want/got mountpoint */ #define STMT_FS_TYPE 0x00000020U /* Want/got fs_type */ -#define STMT_SB_OPTS 0x00000040U /* Want/got sb_opts */ + +/* listmount(2) flags */ +#define LISTMOUNT_UNREACHABLE 0x01 /* List unreachable mounts too */ #endif /* _UAPI_LINUX_MOUNT_H */
> > with __u32 size for mnt_root and mnt_point > > Unnecessary if the strings are nul terminated. All ok by me so far but how does the kernel know the size of the buffer to copy into? Wouldn't it be better to allow userspace to specify that? I'm probably just missing something but I better ask.
On Mon, Sep 18, 2023 at 02:58:00PM -0600, Andreas Dilger wrote: > On Sep 18, 2023, at 7:51 AM, Christian Brauner <brauner@kernel.org> wrote: > > > > > >> The type and subtype are naturally limited to sane sizes, those are > >> not an issue. > > > > What's the limit for fstype actually? I don't think there is one. > > There's one by chance but not by design afaict? > > > > Maybe crazy idea: > > That magic number thing that we do in include/uapi/linux/magic.h > > is there a good reason for this or why don't we just add a proper, > > simple enum: > > > > enum { > > FS_TYPE_ADFS 1 > > FS_TYPE_AFFS 2 > > FS_TYPE_AFS 3 > > FS_TYPE_AUTOFS 4 > > FS_TYPE_EXT2 5 > > FS_TYPE_EXT3 6 > > FS_TYPE_EXT4 7 > > . > > . > > . > > FS_TYPE_MAX > > } > > > > that we start returning from statmount(). We can still return both the > > old and the new fstype? It always felt a bit odd that fs developers to > > just select a magic number. > > Yes, there is a very good reason that there isn't an enum for filesystem I think this isn't all that relevant to the patchset so I'm not going to spend a lot of time on this discussion but I'm curious. > type, which is because this API would be broken if it encounters any > filesystem that is not listed there. Often a single filesystem driver in > the kernel will have multiple different magic numbers to handle versions, > endianness, etc. Why isn't this a problem for magically chosen numbers? > > Having a 32-bit magic number allows decentralized development with low > chance of collision, and using new filesystems without having to patch > every kernel for this new API to work with that filesystem. Also, We don't care about out of tree filesystems. > filesystems come and go (though more slowly) over time, and keeping the Even if we did ever remove a filesystem we'd obviously leave the enum in place. Same thig we do for deprecated flags, same thing we'd do for magic numbers. > full list of every filesystem ever developed in the kernel enum would be > a headache. I really don't follow this argument. > > The field in the statmnt() call would need to be at a fixed-size 32-bit > value in any case, so having it return the existing magic will "just work" > because userspace tools already know and understand these magic values, > while introducing an in-kernel enum would be broken for multiple reasons. We already do expose the magic number in statmount() but it can't differentiate between ext2, ext3, and ext4 for example which is why I asked. Afaict, none of the points you mention are show stoppers and none of them are unique to an enum.
On Tue, 19 Sept 2023 at 14:41, Christian Brauner <brauner@kernel.org> wrote: > > > > with __u32 size for mnt_root and mnt_point > > > > Unnecessary if the strings are nul terminated. > > All ok by me so far but how does the kernel know the size of the buffer > to copy into? Wouldn't it be better to allow userspace to specify that? > I'm probably just missing something but I better ask. Because size of the buffer is given as the syscall argument. long statmount(u64 mnt_id, u64 mask, struct statmnt __user *buf, size_t bufsize, unsigned int flags); If you are still hung up about this not being properly typed, how about this: struct statmnt { __u64 mask; /* What results were written [uncond] */ __u32 sb_dev_major; /* Device ID */ [...] __u64 fs_type; /* [str] Filesystem type */ __u64 __spare[49]; char __string_buf[]; }; Such variable length structures are used all over the place, this isn't some big invention. The only new thing is that we set pointers to within the tail part of the buffer, to make the interface work for the multiple strings case. Thanks, Miklos
On Tue, Sep 19, 2023 at 02:59:53PM +0200, Miklos Szeredi wrote: > On Tue, 19 Sept 2023 at 14:41, Christian Brauner <brauner@kernel.org> wrote: > > > > > > with __u32 size for mnt_root and mnt_point > > > > > > Unnecessary if the strings are nul terminated. > > > > All ok by me so far but how does the kernel know the size of the buffer > > to copy into? Wouldn't it be better to allow userspace to specify that? > > I'm probably just missing something but I better ask. > > Because size of the buffer is given as the syscall argument. > > long statmount(u64 mnt_id, u64 mask, struct statmnt __user *buf, > size_t bufsize, unsigned int flags); > > If you are still hung up about this not being properly typed, how about this: I really just wasn't clear how exactly you envisioned this. Your proposal as is sounds good to me! I'm on board. I prefer the two offsets as that lets us avoid searching for null bytes. So please leave it as is! Thanks!
On Tue, Sep 19, 2023 at 4:02 AM Miklos Szeredi <miklos@szeredi.hu> wrote: > On Tue, 19 Sept 2023 at 02:38, Matthew House <mattlloydhouse@gmail.com> wrote: > > > One natural solution is to set either of the two lengths to the expected > > size if the provided buffer are too small. That way, the caller learns both > > which of the buffers is too small, and how large they need to be. Replacing > > a provided size with an expected size in this way already has precedent in > > existing syscalls: > > This is where the thread started. Knowing the size of the buffer is > no good, since the needed buffer could change between calls. As Brauner mentioned, this does not change with the single-buffer interface. And since changes are not likely to occur extremely frequently, I feel like it would be better for the caller to only need one retry in the common case rather than N retries for however many doublings it takes to fit the whole buffer. > We are trying to create a simple interface, no? My proposal would > need a helper like this: > > struct statmnt *statmount(uint64_t mnt_id, uint64_t mask, unsigned int flags) > { > size_t bufsize = 1 << 15; > void *buf; > int ret; > > for (;;) { > buf = malloc(bufsize <<= 1); > if (!buf) > return NULL; > ret = syscall(__NR_statmnt, mnt_id, mask, buf, bufsize, flags); > if (!ret) > return buf; > free(buf); > if (errno != EOVERFLOW) > return NULL; > } > } > > Christian's would be (ignoring .fs_type for now): > > int statmount(uint64_t mnt_id, uint64_t mask, struct statmnt *st, > unsigned int flags) > { > int ret; > > st->mnt_root_size = 1 << 15; > st->mountpoint_size = 1 << 15; > for (;;) { > st->mnt_root = malloc(st->mnt_root_size <<= 1); > st->mountpoint = malloc(st->mountpoint <<= 1); > if (!st->mnt_root || !st->mountpoint) { > free(st->mnt_root); > free(st->mountpoint); > return -1; > } > ret = syscall(__NR_statmnt, mnt_id, mask, st, > sizeof(*st), flags); > if (!ret || errno != EOVERFLOW) > return ret; > free(st->mnt_root); > free(st->mountpoint); > } > } > > It's not hugely more complex, but more complex nonetheless. > > Also having the helper allocate buffers inside the struct could easily > result in leaks since it's not obvious what the caller needs to free, > while in the first example it is. There's nothing stopping the userspace helper from exposing a contiguous buffer that can be easily freed, even if the kernel API uses a separate- buffer interface internally. It just takes a bit of addition in the helper to calculate the correct pointers. To wit: struct statmnt *statmount(uint64_t mnt_id, uint64_t mask, unsigned int flags) { uint32_t mnt_root_size = PATH_MAX; uint32_t mountpoint_size = PATH_MAX; struct statmnt *st; int ret; for (;;) { st = malloc(sizeof(*st) + mnt_root_size + mountpoint_size); if (!st) return NULL; st->mnt_root = (char *)st + sizeof(*st); st->mountpoint = (char *)st + sizeof(*st) + mnt_root_size; st->mnt_root_size = mnt_root_size; st->mountpoint_size = mountpoint_size; ret = syscall(__NR_statmnt, mnt_id, mask, st, sizeof(*st), flags); if (ret) { free(st); return NULL; } if (st->mnt_root_size <= mnt_root_size && st->mountpoint_size <= mountpoint_size) return st; mnt_root_size = st->mnt_root_size; mountpoint_size = st->mountpoint_size; free(st); } } (This is also far more helpful for users of the returned struct statmnt *, since they can just dereference the two pointers instead of having to decode the offsets by hand.) More generally speaking, the biggest reason I dislike the current single- buffer interface is that the output is "all or nothing": either the caller has enough space in the buffer to store every single string, or it's unable to get any fields at all, just an -EOVERFLOW. There's no room for the caller to say that it just wants the integer fields and doesn't care about the strings. Thus, to reliably call statmnt() on an arbitrary mount, the ability to dynamically allocate memory is effectively mandatory. The only real solution to this would be additional statx-like flags to select the returned strings. Meanwhile, with a separate-buffer interface, where the caller provides a pointer and capacity for each string, granular output would be trivial: the caller could just specify NULL/0 for any string it doesn't want, and still successfully retrieve all the integer fields. This would also work well if the caller, e.g., wants to set a hard cap of PATH_MAX bytes for each string (since it's using static buffers), but nonetheless wants to retrieve the integer fields if a string is too long. Besides that, if the caller is written in standard C but doesn't want to use malloc(3) to allocate the buffer, then its helper function must be written very carefully (with a wrapper struct around the header and data) to satisfy the aliasing rules, which forbid programs from using a struct statmnt * pointer to read from a declared char[N] array. In practice, callers tend to very rarely exercise this proper care with existing single- buffer interfaces, such as recvmsg(2)'s msg_control buffer, and I would not be very happy if statmnt() further contributed to this widespread issue. Thank you, Matthew House
On Tue, Sep 19, 2023 at 02:50:28PM +0200, Christian Brauner wrote: > On Mon, Sep 18, 2023 at 02:58:00PM -0600, Andreas Dilger wrote: > > On Sep 18, 2023, at 7:51 AM, Christian Brauner <brauner@kernel.org> wrote: > > > > > > > > >> The type and subtype are naturally limited to sane sizes, those are > > >> not an issue. > > > > > > What's the limit for fstype actually? I don't think there is one. > > > There's one by chance but not by design afaict? > > > > > > Maybe crazy idea: > > > That magic number thing that we do in include/uapi/linux/magic.h > > > is there a good reason for this or why don't we just add a proper, > > > simple enum: > > > > > > enum { > > > FS_TYPE_ADFS 1 > > > FS_TYPE_AFFS 2 > > > FS_TYPE_AFS 3 > > > FS_TYPE_AUTOFS 4 > > > FS_TYPE_EXT2 5 > > > FS_TYPE_EXT3 6 > > > FS_TYPE_EXT4 7 > > > . > > > . > > > . > > > FS_TYPE_MAX > > > } > > > > > > that we start returning from statmount(). We can still return both the > > > old and the new fstype? It always felt a bit odd that fs developers to > > > just select a magic number. > > > > Yes, there is a very good reason that there isn't an enum for filesystem > > I think this isn't all that relevant to the patchset so I'm not going to > spend a lot of time on this discussion but I'm curious. > > > type, which is because this API would be broken if it encounters any > > filesystem that is not listed there. Often a single filesystem driver in > > the kernel will have multiple different magic numbers to handle versions, > > endianness, etc. > > Why isn't this a problem for magically chosen numbers? What problem are you asking about? The 32 bit space that contains a few hundred magic numbers remains a vast field of empty space that makes collisions easy to avoid.... > > Having a 32-bit magic number allows decentralized development with low > > chance of collision, and using new filesystems without having to patch > > every kernel for this new API to work with that filesystem. Also, > > We don't care about out of tree filesystems. In this case, we most certainly do care. Downstream distros support all sorts of out of tree filesystems loaded via kernel modules, so a syscall that is used to uniquely identify a filesystem type to userspace *must* have a mechanism for the filesystem to provide that unique identifier to userspace. Fundamentally, the kernel does not and should not dictate what filesystem types it supports; the user decides what filesystem they need to use, and it is the kernel's job to provide infrastructure that works with that user's choice. Remember: it's not just applications that stat the mounted filesystem that know about the filesystem amgic numbers. Apps like grub, libblkid, etc all look at filesystem magic numbers directly on the block device to identify the type of filesystem that is on the device. If we introduce a new identifer specific to mounted kernel filesystems, these sorts of apps now need to use two different identifiers in different contexts instead of the same magic number everywhere. That's not a win for anyone. Magic numbers are also portable - it does not matter what OS you see that FS on, it has the same unique, stable type identifier. You can look at the block device and identify the filesystem by it's magic number, you can stat the mounted filesystem and get the same magic number. It just works the same *everywhere*. Magic numbers have served the purpose of being unique filesystem identifiers for over 40 years. They work just fine for this purpose and nothing has changed in the past couple of decades that has broken them or needs fixing. > > filesystems come and go (though more slowly) over time, and keeping the > > Even if we did ever remove a filesystem we'd obviously leave the enum in > place. Same thig we do for deprecated flags, same thing we'd do for > magic numbers. So why try to replace magic numbers if we must replicate all the same unique, stable behaviour that magic numbers already provide the kernel and userspace with? > > > full list of every filesystem ever developed in the kernel enum would be > > a headache. > > I really don't follow this argument. The kernel currently doesn't need to know about all the potential fuse filesystem types that can be mounted. It doesn't need to know about all the 3rd party filesystems that could be mounted. these all just work and userspace can identify them just fine via their unique magic numbers that are passed through the kernel interfaces from the filesystem. Then enum proposal breaks these existing working use cases unless the enum explicitly includes ever possible filesystem type that the kernel might expose to userspace. The kernel *should not care* what filesystems it exposes to userspace and that's the whole point of using a filesystem supplied magic number as the unique identifier for the filesystem... > > The field in the statmnt() call would need to be at a fixed-size 32-bit > > value in any case, so having it return the existing magic will "just work" > > because userspace tools already know and understand these magic values, > > while introducing an in-kernel enum would be broken for multiple reasons. > > We already do expose the magic number in statmount() but it can't > differentiate between ext2, ext3, and ext4 for example which is why I > asked. That's just an extN quirk, and it's trivial to fix for the new interface. Define new magic numbers for ext3 and ext4 and only use them in the new interface, leave the old interfaces using the ext2 magic number for all of them. -Dave.
On Tue, 19 Sept 2023 at 23:28, Matthew House <mattlloydhouse@gmail.com> wrote: > More generally speaking, the biggest reason I dislike the current single- > buffer interface is that the output is "all or nothing": either the caller > has enough space in the buffer to store every single string, or it's unable > to get any fields at all, just an -EOVERFLOW. There's no room for the > caller to say that it just wants the integer fields and doesn't care about > the strings. Thus, to reliably call statmnt() on an arbitrary mount, the > ability to dynamically allocate memory is effectively mandatory. The only > real solution to this would be additional statx-like flags to select the > returned strings. It's already there: #define STMT_MNT_ROOT 0x00000008U /* Want/got mnt_root */ #define STMT_MNT_POINT 0x00000010U /* Want/got mnt_point */ #define STMT_FS_TYPE 0x00000020U /* Want/got fs_type */ For example, it's perfectly fine to do the following, and it's guaranteed not to return EOVERFLOW: struct statmnt st; unsigned int mask = STMT_SB_BASIC | STMT_MNT_BASIC; ret = statmount(mnt_id, mask, &st, sizeof(st), flags); > Besides that, if the caller is written in standard C but doesn't want to > use malloc(3) to allocate the buffer, then its helper function must be > written very carefully (with a wrapper struct around the header and data) > to satisfy the aliasing rules, which forbid programs from using a struct > statmnt * pointer to read from a declared char[N] array. I think you interpret aliasing rules incorrectly. The issue with aliasing is if you access the same piece of memory though different types. Which is not the case here. In fact with the latest incarnation of the interface[1] there's no need to access the underlying buffer at all: printf("mnt_root: <%s>\n", st->str + st->mnt_root); So the following is perfectly safe to do (as long as you don't care about buffer overflow): char buf[10000]; struct statmnt *st = (void *) buf; ret = statmount(mnt_id, mask, st, sizeof(buf), flags); If you do care about handling buffer overflows, then dynamic allocation is the only sane way. And before you dive into how this is going to be horrible because the buffer size needs to be doubled an unknown number of times, think a bit: have you *ever* seen a line in /proc/self/mountinfo longer than say 1000 characters? So if the buffer starts out at 64k, how often will this doubling happen? Right: practically never. Adding complexity to handle this case is nonsense, as I've said many times. And there is definitely nonzero complexity involved (just see the special casing in getxattr and listxattr implementations all over the place). Thanks, Miklos [1] git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git#statmount-v2
From: Jeff Layton > Sent: 18 September 2023 15:30 .... > A bit tangential to this discussion, but one thing we could consider is > adding something like a mnt_change_cookie field that increments on any > significant changes on the mount: i.e. remounts with new options, > changes to parentage or propagation, etc. > > That might make it more palatable to do something with separate syscalls > for the string-based fields. You could do: > > statmnt(...); > getmntattr(mnt, "mnt.fstype", ...); > statmnt(...); > > ...and then if the mnt_change_cookie hasn't changed, you know that the > string option was stable during that window. That would also help with the problem of the mount options being changed while processing a page fault on the user buffer. Of, with a call to just get the cookie, could find that nothing has changed so there is no point looking again. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
On Wed, Sep 20, 2023 at 5:42 AM Miklos Szeredi <miklos@szeredi.hu> wrote: > On Tue, 19 Sept 2023 at 23:28, Matthew House <mattlloydhouse@gmail.com> wrote: > > > More generally speaking, the biggest reason I dislike the current single- > > buffer interface is that the output is "all or nothing": either the caller > > has enough space in the buffer to store every single string, or it's unable > > to get any fields at all, just an -EOVERFLOW. There's no room for the > > caller to say that it just wants the integer fields and doesn't care about > > the strings. Thus, to reliably call statmnt() on an arbitrary mount, the > > ability to dynamically allocate memory is effectively mandatory. The only > > real solution to this would be additional statx-like flags to select the > > returned strings. > > It's already there: > > #define STMT_MNT_ROOT 0x00000008U /* Want/got mnt_root */ > #define STMT_MNT_POINT 0x00000010U /* Want/got mnt_point */ > #define STMT_FS_TYPE 0x00000020U /* Want/got fs_type */ > > For example, it's perfectly fine to do the following, and it's > guaranteed not to return EOVERFLOW: > > struct statmnt st; > unsigned int mask = STMT_SB_BASIC | STMT_MNT_BASIC; > > ret = statmount(mnt_id, mask, &st, sizeof(st), flags); Whoops, my apologies; perhaps I should try to learn to read for once. (I just saw the undecorated sequence of stmt_numeric() and stmt_string() calls and didn't notice the early exits within the functions.) I withdraw that particular objection. > > Besides that, if the caller is written in standard C but doesn't want to > > use malloc(3) to allocate the buffer, then its helper function must be > > written very carefully (with a wrapper struct around the header and data) > > to satisfy the aliasing rules, which forbid programs from using a struct > > statmnt * pointer to read from a declared char[N] array. > > I think you interpret aliasing rules incorrectly. The issue with > aliasing is if you access the same piece of memory though different > types. Which is not the case here. In fact with the latest > incarnation of the interface[1] there's no need to access the > underlying buffer at all: > > printf("mnt_root: <%s>\n", st->str + st->mnt_root); > > So the following is perfectly safe to do (as long as you don't care > about buffer overflow): > > char buf[10000]; > struct statmnt *st = (void *) buf; > > ret = statmount(mnt_id, mask, st, sizeof(buf), flags); The declared type of a variable *is* one of the different types, as far as the aliasing rules are concerned. In C17, section 6.5 ("Expressions"): > The *effective type* of an object for an access to its stored value is > the declared type of the object, if any. [More rules about objects with > no declared type, i.e., those created with malloc(3) or realloc(3)...] > > An object shall have its stored value accessed only by an lvalue > expression that has one of the following types: > > -- a type compatible with the effective type of the object, > > -- a qualified version of a type compatible with the effective type of > the object, > > -- a type that is the signed or unsigned type corresponding to the > effective type of the object, > > -- a type that is the signed or unsigned type corresponding to a > qualified version of the effective type of the object, > > -- an aggregate or union type that includes one of the aforementioned > types among its members (including, recursively, a member of a > subaggregate or contained union), or > > -- a character type. In this case, buf is declared in the program as a char[10000] array, so the declared type of each element is char, and the effective type of each element is also char. If we want to access, say, st->mnt_id, the lvalue expression has type __u64, and it tries to access 8 of the char objects. However, the integer type that __u64 expands to doesn't meet any of those criteria, so the aliasing rules are violated and the behavior is undefined. (The statmount() helper could in theory avoid UB by saying the struct statmnt object is stored in the buffer as if by memcpy(3), but it would still be UB for the caller to access the fields of that pointer directly instead of memcpy'ing them back out of the buffer. And practically no one does that in the real world.) It's a common misconception that the aliasing rules as written are about accessing the same object through two different pointer types. That corollary is indeed what compilers mainly care about, but the C/C++ standards further say that objects in memory "remember" the types they were created with, and they demand that programs respect objects' original types when trying to access them (except when accessing their raw representations via a pointer of character type). > If you do care about handling buffer overflows, then dynamic > allocation is the only sane way. > > And before you dive into how this is going to be horrible because the > buffer size needs to be doubled an unknown number of times, think a > bit: have you *ever* seen a line in /proc/self/mountinfo longer than > say 1000 characters? So if the buffer starts out at 64k, how often > will this doubling happen? Right: practically never. Adding > complexity to handle this case is nonsense, as I've said many times. > And there is definitely nonzero complexity involved (just see the > special casing in getxattr and listxattr implementations all over the > place). > > Thanks, > Miklos I've always felt that capacity doubling is a bit wasteful, but it's definitely something I can live with, especially if providing size feedback is as complex as you suggest. Still, I'm not a big fan of single-buffer interfaces in general, with how poorly they tend to interact with C's aliasing rules. (Also, those kinds of interfaces also invite alignment errors: for instance, your snippet above is missing the necessary union to prevent the buffer from being misaligned, which would cause UB when you cast it to a struct statmnt *.) Thank you, Matthew House
On Wed, 20 Sept 2023 at 15:26, Matthew House <mattlloydhouse@gmail.com> wrote: > The declared type of a variable *is* one of the different types, as far as > the aliasing rules are concerned. In C17, section 6.5 ("Expressions"): > > > The *effective type* of an object for an access to its stored value is > > the declared type of the object, if any. [More rules about objects with > > no declared type, i.e., those created with malloc(3) or realloc(3)...] > > > > An object shall have its stored value accessed only by an lvalue > > expression that has one of the following types: > > > > -- a type compatible with the effective type of the object, > > > > -- a qualified version of a type compatible with the effective type of > > the object, > > > > -- a type that is the signed or unsigned type corresponding to the > > effective type of the object, > > > > -- a type that is the signed or unsigned type corresponding to a > > qualified version of the effective type of the object, > > > > -- an aggregate or union type that includes one of the aforementioned > > types among its members (including, recursively, a member of a > > subaggregate or contained union), or > > > > -- a character type. > > In this case, buf is declared in the program as a char[10000] array, so the > declared type of each element is char, and the effective type of each > element is also char. If we want to access, say, st->mnt_id, the lvalue > expression has type __u64, and it tries to access 8 of the char objects. > However, the integer type that __u64 expands to doesn't meet any of those > criteria, so the aliasing rules are violated and the behavior is undefined. Some of the above is new information for me. However for all practical purposes the code doesn't violate aliasing rules. Even the most aggressive "-Wstrict-aliasing=1" doesn't trigger a warning. I guess this is because gcc takes the definition to be symmetric, i.e. anything may safely be aliased to a char pointer and a char pointer may safely be aliased to anything. I'm not saying that that is what the language definition says, just that gcc interprets the language definition that way. Also plain "-Wstrict-aliasing" doesn't trigger even if the type of the array is not char, because gcc tries hard not to warn about cases where there's no dereference of the aliased pointer. This is consistent with what I said and what the gcc manpage says: only accesses count, declarations don't. > > I've always felt that capacity doubling is a bit wasteful, but it's > definitely something I can live with, especially if providing size feedback > is as complex as you suggest. Still, I'm not a big fan of single-buffer > interfaces in general, with how poorly they tend to interact with C's > aliasing rules. (Also, those kinds of interfaces also invite alignment > errors: for instance, your snippet above is missing the necessary union to > prevent the buffer from being misaligned, which would cause UB when you > cast it to a struct statmnt *.) Okay, alignment is a different story. I'll note this in the man page. Thanks, Miklos
On Wed, Sep 13, 2023, at 17:22, Miklos Szeredi wrote: > asmlinkage long sys_fstatfs64(unsigned int fd, size_t sz, > struct statfs64 __user *buf); > +asmlinkage long sys_statmnt(u64 mnt_id, u64 mask, > + struct statmnt __user *buf, size_t bufsize, > + unsigned int flags); This definition is problematic on 32-bit architectures for two reasons: - 64-bit register arguments are passed in pairs of registers on two architectures, so anything passing those needs to have a separate entry point for compat syscalls on 64-bit architectures. I would suggest also using the same one on 32-bit ones, so you don't rely on the compiler splitting up the long arguments into pairs. - There is a limit of six argument registers for system call entry points, but with two pairs and three single registers you end up with seven of them. The listmnt syscall in patch 3 also has the first problem, but not the second. Arnd
On Mon, Sep 25, 2023 at 02:57:31PM +0200, Arnd Bergmann wrote: > On Wed, Sep 13, 2023, at 17:22, Miklos Szeredi wrote: > > > asmlinkage long sys_fstatfs64(unsigned int fd, size_t sz, > > struct statfs64 __user *buf); > > +asmlinkage long sys_statmnt(u64 mnt_id, u64 mask, > > + struct statmnt __user *buf, size_t bufsize, > > + unsigned int flags); > > This definition is problematic on 32-bit architectures for two > reasons: > > - 64-bit register arguments are passed in pairs of registers > on two architectures, so anything passing those needs to > have a separate entry point for compat syscalls on 64-bit > architectures. I would suggest also using the same one on > 32-bit ones, so you don't rely on the compiler splitting > up the long arguments into pairs. > > - There is a limit of six argument registers for system call > entry points, but with two pairs and three single registers > you end up with seven of them. > > The listmnt syscall in patch 3 also has the first problem, > but not the second. Both fields could also just be moved into the struct itself just like we did for clone3() and others.
On Mon, 25 Sept 2023 at 15:04, Christian Brauner <brauner@kernel.org> wrote: > > On Mon, Sep 25, 2023 at 02:57:31PM +0200, Arnd Bergmann wrote: > > On Wed, Sep 13, 2023, at 17:22, Miklos Szeredi wrote: > > > > > asmlinkage long sys_fstatfs64(unsigned int fd, size_t sz, > > > struct statfs64 __user *buf); > > > +asmlinkage long sys_statmnt(u64 mnt_id, u64 mask, > > > + struct statmnt __user *buf, size_t bufsize, > > > + unsigned int flags); > > > > This definition is problematic on 32-bit architectures for two > > reasons: > > > > - 64-bit register arguments are passed in pairs of registers > > on two architectures, so anything passing those needs to > > have a separate entry point for compat syscalls on 64-bit > > architectures. I would suggest also using the same one on > > 32-bit ones, so you don't rely on the compiler splitting > > up the long arguments into pairs. > > > > - There is a limit of six argument registers for system call > > entry points, but with two pairs and three single registers > > you end up with seven of them. > > > > The listmnt syscall in patch 3 also has the first problem, > > but not the second. > > Both fields could also just be moved into the struct itself just like we > did for clone3() and others. Let's not mix in and out args, please. How about passing u64 *? Thanks, Miklos
> How about passing u64 *?
struct statmnt_req {
__u64 mnt_id;
__u64 mask;
};
?
On Mon, 25 Sept 2023 at 15:19, Christian Brauner <brauner@kernel.org> wrote: > > > How about passing u64 *? > > struct statmnt_req { > __u64 mnt_id; > __u64 mask; > }; > > ? I'm fine with that as well. Thanks, Miklos
On Mon, Sep 25, 2023, at 15:20, Miklos Szeredi wrote: > On Mon, 25 Sept 2023 at 15:19, Christian Brauner <brauner@kernel.org> wrote: >> >> > How about passing u64 *? >> >> struct statmnt_req { >> __u64 mnt_id; >> __u64 mask; >> }; >> >> ? > > I'm fine with that as well. Yes, this looks fine for the compat syscall purpose. Not sure if losing visibility of the mnt_id and mask in ptrace or seccomp/bpf is a problem though. Arnd
On Mon, Sep 25, 2023 at 05:46:59PM +0200, Arnd Bergmann wrote: > On Mon, Sep 25, 2023, at 15:20, Miklos Szeredi wrote: > > On Mon, 25 Sept 2023 at 15:19, Christian Brauner <brauner@kernel.org> wrote: > >> > >> > How about passing u64 *? > >> > >> struct statmnt_req { > >> __u64 mnt_id; > >> __u64 mask; > >> }; > >> > >> ? > > > > I'm fine with that as well. > > Yes, this looks fine for the compat syscall purpose. > > Not sure if losing visibility of the mnt_id and mask in ptrace > or seccomp/bpf is a problem though. It's an information retrieval syscall so there shouldn't be any need to block it and I think that this ship has sailed in general. Container workloads should migrate from seccomp to landlock if they need to filter system calls like this.
* Miklos Szeredi: > On Mon, Sep 18, 2023 at 3:51 PM Christian Brauner <brauner@kernel.org> wrote: > >> I really would prefer a properly typed struct and that's what everyone >> was happy with in the session as well. So I would not like to change the >> main parameters. > > I completely agree. Just would like to understand this point: > > struct statmnt *statmnt(u64 mntid, u64 mask, unsigned int flags); > > What's not properly typed about this interface? > > I guess the answer is that it's not a syscall interface, which will > have an added [void *buf, size_t bufsize], while the buffer sizing is > done by a simple libc wrapper. > > Do you think that's a problem? If so, why? Try-and-resize interfaces can be quite bad for data obtained from the network. If the first call provides the minimum buffer size (like getgroups, but unlike readlink or the glibc *_r interfaces for NSS), this could at least allow us to avoid allocating too much. In userspace, we cannot reduce the size of the heap allocation without knowing where the pointers are and what they mean. I also don't quite understand the dislike of variable-sized records. Don't getdents, inotify, Netlink all use them? And I think at least for Netlink, more stuff is added all the time? Thanks, Florian
On Tue, 26 Sept 2023 at 15:49, Florian Weimer <fweimer@redhat.com> wrote: > > * Miklos Szeredi: > > > On Mon, Sep 18, 2023 at 3:51 PM Christian Brauner <brauner@kernel.org> wrote: > > > >> I really would prefer a properly typed struct and that's what everyone > >> was happy with in the session as well. So I would not like to change the > >> main parameters. > > > > I completely agree. Just would like to understand this point: > > > > struct statmnt *statmnt(u64 mntid, u64 mask, unsigned int flags); > > > > What's not properly typed about this interface? > > > > I guess the answer is that it's not a syscall interface, which will > > have an added [void *buf, size_t bufsize], while the buffer sizing is > > done by a simple libc wrapper. > > > > Do you think that's a problem? If so, why? > > Try-and-resize interfaces can be quite bad for data obtained from the > network. In this particular case it's all local information. > If the first call provides the minimum buffer size (like > getgroups, but unlike readlink or the glibc *_r interfaces for NSS), > this could at least allow us to avoid allocating too much. In > userspace, we cannot reduce the size of the heap allocation without > knowing where the pointers are and what they mean. Does it matter if the heap allocation is say 32k instead of 589bytes? The returned strings are not limited in size, but are quite unlikely to be over PATH_MAX. E.g. getdents apparently uses 32k buffers, which is really a tiny amount of heap these days, but more than enough for the purpose. Not sure if this is hard coded into libc or if it's the result of some heuristic based on available memory, but I don't see why similar treatment couldn't be applied to the statmount(2) syscall. > I also don't quite understand the dislike of variable-sized records. > Don't getdents, inotify, Netlink all use them? And I think at least for > Netlink, more stuff is added all the time? What do you mean by variable sized records? Thanks, Miklos
> I also don't quite understand the dislike of variable-sized records. > Don't getdents, inotify, Netlink all use them? And I think at least for > Netlink, more stuff is added all the time? Netlink is absolutely atrocious to work with because everything is variable sized and figuring out the correct allocation size is a complete nightmare even with the "helpful" macros that are provided. The bigger problem however is the complete untypedness even of the most basic things. For example, retrieving the mtu of a network interface through netlink is a complete nightmare. getdents, inotify, fanotify, open_by_handle_at()'s struct fiel_handle are all fine. But let's absolutely not take netlink as a model for anything related to mounts. And no one is against again variable sized records per se. I think we're coming to a good compromise here.
* Miklos Szeredi: >> Try-and-resize interfaces can be quite bad for data obtained from the >> network. > > In this particular case it's all local information. That's good. >> If the first call provides the minimum buffer size (like >> getgroups, but unlike readlink or the glibc *_r interfaces for NSS), >> this could at least allow us to avoid allocating too much. In >> userspace, we cannot reduce the size of the heap allocation without >> knowing where the pointers are and what they mean. > > Does it matter if the heap allocation is say 32k instead of 589bytes? > The returned strings are not limited in size, but are quite unlikely > to be over PATH_MAX. It matters if the application needs to keep a copy. > E.g. getdents apparently uses 32k buffers, which is really a tiny > amount of heap these days, but more than enough for the purpose. Not > sure if this is hard coded into libc or if it's the result of some > heuristic based on available memory, but I don't see why similar > treatment couldn't be applied to the statmount(2) syscall. getdents gets away with this buffer size because applications can copy out all the data from struct dirent if they need long-term storage. They have to do that because the usual readdir interface overwrites the buffer, potentially at the next readdir call. This means the buffer size does not introduce an amount of memory fragmention that is dependent on the directory size. With an opaque, pointer-carrying struct, copying out the data is not possible in a generic fashion. Only the parts that the application knows about can be copied out. So I think it's desirable to have a fairly exact allocation. >> I also don't quite understand the dislike of variable-sized records. >> Don't getdents, inotify, Netlink all use them? And I think at least for >> Netlink, more stuff is added all the time? > > What do you mean by variable sized records? Iterating through d_reclen-sized subojects (for getdents). Thanks, Florian
On Tue, 26 Sept 2023 at 16:19, Florian Weimer <fweimer@redhat.com> wrote: > getdents gets away with this buffer size because applications can copy > out all the data from struct dirent if they need long-term storage. > They have to do that because the usual readdir interface overwrites the > buffer, potentially at the next readdir call. This means the buffer > size does not introduce an amount of memory fragmention that is > dependent on the directory size. > > With an opaque, pointer-carrying struct, copying out the data is not > possible in a generic fashion. Only the parts that the application > knows about can be copied out. So I think it's desirable to have a > fairly exact allocation. Okay, so let's add a 'size' field to the struct, which is set to the size used (as opposed to the size of the buffer). That should solve copying without wasting a single byte of memory. Otherwise the format is fully copyable, since the strings are denoted with an offset, which doesn't change after the buffer is copied. Thanks, Miklos
> With an opaque, pointer-carrying struct, copying out the data is not > possible in a generic fashion. Only the parts that the application > knows about can be copied out. So I think it's desirable to have a > fairly exact allocation. This could easily be added if we added size parameters like I originally suggested for the variable sized mnt_root and mnt_point records into struct statmount. If the user specified that they want to retrieve the mnt_root and mnt_mountpoint in @mask and the size for the relevant field is zero then we fill in the required size for the relevant field. If they aren't zero we just try to copy in the data in the relevant pointer field. I prefer this interface as it allows for both strategies: * users that don't care about exact allocation size can just pass a guesstimated buffer usually PATH_MAX/2 or sm * users that care about exact allocation size can query the kernel
* Miklos Szeredi: > On Tue, 26 Sept 2023 at 16:19, Florian Weimer <fweimer@redhat.com> wrote: > >> getdents gets away with this buffer size because applications can copy >> out all the data from struct dirent if they need long-term storage. >> They have to do that because the usual readdir interface overwrites the >> buffer, potentially at the next readdir call. This means the buffer >> size does not introduce an amount of memory fragmention that is >> dependent on the directory size. >> >> With an opaque, pointer-carrying struct, copying out the data is not >> possible in a generic fashion. Only the parts that the application >> knows about can be copied out. So I think it's desirable to have a >> fairly exact allocation. > > Okay, so let's add a 'size' field to the struct, which is set to the > size used (as opposed to the size of the buffer). That should solve > copying without wasting a single byte of memory. That would be helpful. > Otherwise the format is fully copyable, since the strings are denoted > with an offset, which doesn't change after the buffer is copied. I missed the development in that direction. Yes, offsets would work nicely in this context. They help with compat syscalls, too. If the buffer is relocatable like that, we can even try first with a reasonably sized on-stack buffer and create an exactly-sized heap allocation from that. Thanks, Florian
On Mon, 25 Sept 2023 at 15:20, Miklos Szeredi <miklos@szeredi.hu> wrote: > > On Mon, 25 Sept 2023 at 15:19, Christian Brauner <brauner@kernel.org> wrote: > > > > > How about passing u64 *? > > > > struct statmnt_req { > > __u64 mnt_id; > > __u64 mask; > > }; > > > > ? > > I'm fine with that as well. So after a bit more thinking: this is okay to make life easier for 32bit archs, but only on the kernel ABI. On the library API the args should *not* be multiplexed, as it's just a pointless complication. This is just an internal implementation detail for the sake of legacy architectures, instead of being good API design. And because it's an internal thingy, my feeling is that this struct could be reused for passing mnt_id to listmount(2) as well, despite the fact that the mask would be unused. But I'm ready to be convinced otherwise... Thanks, Miklos
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl index 1d6eee30eceb..6d807c30cd16 100644 --- a/arch/x86/entry/syscalls/syscall_64.tbl +++ b/arch/x86/entry/syscalls/syscall_64.tbl @@ -375,6 +375,7 @@ 451 common cachestat sys_cachestat 452 common fchmodat2 sys_fchmodat2 453 64 map_shadow_stack sys_map_shadow_stack +454 common statmnt sys_statmnt # # Due to a historical design error, certain syscalls are numbered differently diff --git a/fs/internal.h b/fs/internal.h index d64ae03998cc..8f75271428aa 100644 --- a/fs/internal.h +++ b/fs/internal.h @@ -83,6 +83,11 @@ int path_mount(const char *dev_name, struct path *path, const char *type_page, unsigned long flags, void *data_page); int path_umount(struct path *path, int flags); +/* + * proc_namespace.c + */ +int show_path(struct seq_file *m, struct dentry *root); + /* * fs_struct.c */ diff --git a/fs/namespace.c b/fs/namespace.c index de47c5f66e17..088a52043bba 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -69,7 +69,8 @@ static DEFINE_IDA(mnt_id_ida); static DEFINE_IDA(mnt_group_ida); /* Don't allow confusion with mount ID allocated wit IDA */ -static atomic64_t mnt_id_ctr = ATOMIC64_INIT(1ULL << 32); +#define OLD_MNT_ID_MAX UINT_MAX +static atomic64_t mnt_id_ctr = ATOMIC64_INIT(OLD_MNT_ID_MAX); static struct hlist_head *mount_hashtable __read_mostly; static struct hlist_head *mountpoint_hashtable __read_mostly; @@ -4678,6 +4679,315 @@ SYSCALL_DEFINE5(mount_setattr, int, dfd, const char __user *, path, return err; } +static bool mnt_id_match(struct mount *mnt, u64 id) +{ + if (id <= OLD_MNT_ID_MAX) + return id == mnt->mnt_id; + else + return id == mnt->mnt_id_unique; +} + +struct vfsmount *lookup_mnt_in_ns(u64 id, struct mnt_namespace *ns) +{ + struct mount *mnt; + struct vfsmount *res = NULL; + + lock_ns_list(ns); + list_for_each_entry(mnt, &ns->list, mnt_list) { + if (!mnt_is_cursor(mnt) && mnt_id_match(mnt, id)) { + res = &mnt->mnt; + break; + } + } + unlock_ns_list(ns); + return res; +} + +struct stmt_state { + void __user *const buf; + size_t const bufsize; + struct vfsmount *const mnt; + u64 const mask; + struct seq_file seq; + struct path root; + struct statmnt sm; + size_t pos; + int err; +}; + +typedef int (*stmt_func_t)(struct stmt_state *); + +static int stmt_string_seq(struct stmt_state *s, stmt_func_t func) +{ + struct seq_file *seq = &s->seq; + int ret; + + seq->count = 0; + seq->size = min_t(size_t, seq->size, s->bufsize - s->pos); + seq->buf = kvmalloc(seq->size, GFP_KERNEL_ACCOUNT); + if (!seq->buf) + return -ENOMEM; + + ret = func(s); + if (ret) + return ret; + + if (seq_has_overflowed(seq)) { + if (seq->size == s->bufsize - s->pos) + return -EOVERFLOW; + seq->size *= 2; + if (seq->size > MAX_RW_COUNT) + return -ENOMEM; + kvfree(seq->buf); + return 0; + } + + /* Done */ + return 1; +} + +static void stmt_string(struct stmt_state *s, u64 mask, stmt_func_t func, + stmt_str_t *str) +{ + int ret = s->pos >= s->bufsize ? -EOVERFLOW : 0; + struct statmnt *sm = &s->sm; + struct seq_file *seq = &s->seq; + + if (s->err || !(s->mask & mask)) + return; + + seq->size = PAGE_SIZE; + while (!ret) + ret = stmt_string_seq(s, func); + + if (ret < 0) { + s->err = ret; + } else { + seq->buf[seq->count++] = '\0'; + if (copy_to_user(s->buf + s->pos, seq->buf, seq->count)) { + s->err = -EFAULT; + } else { + str->off = s->pos; + str->len = seq->count - 1; + s->pos += seq->count; + } + } + kvfree(seq->buf); + sm->mask |= mask; +} + +static void stmt_numeric(struct stmt_state *s, u64 mask, stmt_func_t func) +{ + if (s->err || !(s->mask & mask)) + return; + + s->err = func(s); + s->sm.mask |= mask; +} + +static u64 mnt_to_attr_flags(struct vfsmount *mnt) +{ + unsigned int mnt_flags = READ_ONCE(mnt->mnt_flags); + u64 attr_flags = 0; + + if (mnt_flags & MNT_READONLY) + attr_flags |= MOUNT_ATTR_RDONLY; + if (mnt_flags & MNT_NOSUID) + attr_flags |= MOUNT_ATTR_NOSUID; + if (mnt_flags & MNT_NODEV) + attr_flags |= MOUNT_ATTR_NODEV; + if (mnt_flags & MNT_NOEXEC) + attr_flags |= MOUNT_ATTR_NOEXEC; + if (mnt_flags & MNT_NODIRATIME) + attr_flags |= MOUNT_ATTR_NODIRATIME; + if (mnt_flags & MNT_NOSYMFOLLOW) + attr_flags |= MOUNT_ATTR_NOSYMFOLLOW; + + if (mnt_flags & MNT_NOATIME) + attr_flags |= MOUNT_ATTR_NOATIME; + else if (mnt_flags & MNT_RELATIME) + attr_flags |= MOUNT_ATTR_RELATIME; + else + attr_flags |= MOUNT_ATTR_STRICTATIME; + + if (is_idmapped_mnt(mnt)) + attr_flags |= MOUNT_ATTR_IDMAP; + + return attr_flags; +} + +static u64 mnt_to_propagation_flags(struct mount *m) +{ + u64 propagation = 0; + + if (IS_MNT_SHARED(m)) + propagation |= MS_SHARED; + if (IS_MNT_SLAVE(m)) + propagation |= MS_SLAVE; + if (IS_MNT_UNBINDABLE(m)) + propagation |= MS_UNBINDABLE; + if (!propagation) + propagation |= MS_PRIVATE; + + return propagation; +} + +static int stmt_sb_basic(struct stmt_state *s) +{ + struct super_block *sb = s->mnt->mnt_sb; + + s->sm.sb_dev_major = MAJOR(sb->s_dev); + s->sm.sb_dev_minor = MINOR(sb->s_dev); + s->sm.sb_magic = sb->s_magic; + s->sm.sb_flags = sb->s_flags & (SB_RDONLY|SB_SYNCHRONOUS|SB_DIRSYNC|SB_LAZYTIME); + + return 0; +} + +static int stmt_mnt_basic(struct stmt_state *s) +{ + struct mount *m = real_mount(s->mnt); + + s->sm.mnt_id = m->mnt_id_unique; + s->sm.mnt_parent_id = m->mnt_parent->mnt_id_unique; + s->sm.mnt_id_old = m->mnt_id; + s->sm.mnt_parent_id_old = m->mnt_parent->mnt_id; + s->sm.mnt_attr = mnt_to_attr_flags(&m->mnt); + s->sm.mnt_propagation = mnt_to_propagation_flags(m); + s->sm.mnt_peer_group = IS_MNT_SHARED(m) ? m->mnt_group_id : 0; + s->sm.mnt_master = IS_MNT_SLAVE(m) ? m->mnt_master->mnt_group_id : 0; + + return 0; +} + +static int stmt_propagate_from(struct stmt_state *s) +{ + struct mount *m = real_mount(s->mnt); + + if (!IS_MNT_SLAVE(m)) + return 0; + + s->sm.propagate_from = get_dominating_id(m, ¤t->fs->root); + + return 0; +} + +static int stmt_mnt_root(struct stmt_state *s) +{ + struct seq_file *seq = &s->seq; + int err = show_path(seq, s->mnt->mnt_root); + + if (!err && !seq_has_overflowed(seq)) { + seq->buf[seq->count] = '\0'; + seq->count = string_unescape_inplace(seq->buf, UNESCAPE_OCTAL); + } + return err; +} + +static int stmt_mountpoint(struct stmt_state *s) +{ + struct vfsmount *mnt = s->mnt; + struct path mnt_path = { .dentry = mnt->mnt_root, .mnt = mnt }; + int err = seq_path_root(&s->seq, &mnt_path, &s->root, ""); + + return err == SEQ_SKIP ? 0 : err; +} + +static int stmt_fs_type(struct stmt_state *s) +{ + struct seq_file *seq = &s->seq; + struct super_block *sb = s->mnt->mnt_sb; + + seq_puts(seq, sb->s_type->name); + if (sb->s_subtype) { + seq_putc(seq, '.'); + seq_puts(seq, sb->s_subtype); + } + return 0; +} + +static int stmt_sb_opts(struct stmt_state *s) +{ + struct seq_file *seq = &s->seq; + struct super_block *sb = s->mnt->mnt_sb; + char *p, *end, *next, *u = seq->buf; + int err; + + if (!sb->s_op->show_options) + return 0; + + err = sb->s_op->show_options(seq, s->mnt->mnt_root); + if (err || seq_has_overflowed(seq) || !seq->count) + return err; + + end = seq->buf + seq->count; + *end = '\0'; + for (p = seq->buf + 1; p < end; p = next + 1) { + next = strchrnul(p, ','); + *next = '\0'; + u += string_unescape(p, u, 0, UNESCAPE_OCTAL) + 1; + } + seq->count = u - 1 - seq->buf; + return 0; +} + +static int do_statmnt(struct stmt_state *s) +{ + struct statmnt *sm = &s->sm; + struct mount *m = real_mount(s->mnt); + + if (!capable(CAP_SYS_ADMIN) && + !is_path_reachable(m, m->mnt.mnt_root, &s->root)) + return -EPERM; + + stmt_numeric(s, STMT_SB_BASIC, stmt_sb_basic); + stmt_numeric(s, STMT_MNT_BASIC, stmt_mnt_basic); + stmt_numeric(s, STMT_PROPAGATE_FROM, stmt_propagate_from); + stmt_string(s, STMT_MNT_ROOT, stmt_mnt_root, &sm->mnt_root); + stmt_string(s, STMT_MOUNTPOINT, stmt_mountpoint, &sm->mountpoint); + stmt_string(s, STMT_FS_TYPE, stmt_fs_type, &sm->fs_type); + stmt_string(s, STMT_SB_OPTS, stmt_sb_opts, &sm->sb_opts); + + if (s->err) + return s->err; + + if (copy_to_user(s->buf, sm, min_t(size_t, s->bufsize, sizeof(*sm)))) + return -EFAULT; + + return 0; +} + +SYSCALL_DEFINE5(statmnt, u64, mnt_id, + u64, mask, struct statmnt __user *, buf, + size_t, bufsize, unsigned int, flags) +{ + struct vfsmount *mnt; + int err; + + if (flags) + return -EINVAL; + + down_read(&namespace_sem); + mnt = lookup_mnt_in_ns(mnt_id, current->nsproxy->mnt_ns); + err = -ENOENT; + if (mnt) { + struct stmt_state s = { + .mask = mask, + .buf = buf, + .bufsize = bufsize, + .mnt = mnt, + .pos = sizeof(*buf), + }; + + get_fs_root(current->fs, &s.root); + err = do_statmnt(&s); + path_put(&s.root); + } + up_read(&namespace_sem); + + return err; +} + static void __init init_mount_tree(void) { struct vfsmount *mnt; diff --git a/fs/proc_namespace.c b/fs/proc_namespace.c index 250eb5bf7b52..20681d1f6798 100644 --- a/fs/proc_namespace.c +++ b/fs/proc_namespace.c @@ -132,6 +132,15 @@ static int show_vfsmnt(struct seq_file *m, struct vfsmount *mnt) return err; } +int show_path(struct seq_file *m, struct dentry *root) +{ + if (root->d_sb->s_op->show_path) + return root->d_sb->s_op->show_path(m, root); + + seq_dentry(m, root, " \t\n\\"); + return 0; +} + static int show_mountinfo(struct seq_file *m, struct vfsmount *mnt) { struct proc_mounts *p = m->private; @@ -142,13 +151,9 @@ static int show_mountinfo(struct seq_file *m, struct vfsmount *mnt) seq_printf(m, "%i %i %u:%u ", r->mnt_id, r->mnt_parent->mnt_id, MAJOR(sb->s_dev), MINOR(sb->s_dev)); - if (sb->s_op->show_path) { - err = sb->s_op->show_path(m, mnt->mnt_root); - if (err) - goto out; - } else { - seq_dentry(m, mnt->mnt_root, " \t\n\\"); - } + err = show_path(m, mnt->mnt_root); + if (err) + goto out; seq_putc(m, ' '); /* mountpoints outside of chroot jail will give SEQ_SKIP on this */ diff --git a/fs/statfs.c b/fs/statfs.c index 96d1c3edf289..cc774c2e2c9a 100644 --- a/fs/statfs.c +++ b/fs/statfs.c @@ -9,6 +9,7 @@ #include <linux/security.h> #include <linux/uaccess.h> #include <linux/compat.h> +#include <uapi/linux/mount.h> #include "internal.h" static int flags_by_mnt(int mnt_flags) diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h index 22bc6bc147f8..1099bd307fa7 100644 --- a/include/linux/syscalls.h +++ b/include/linux/syscalls.h @@ -408,6 +408,9 @@ asmlinkage long sys_statfs64(const char __user *path, size_t sz, asmlinkage long sys_fstatfs(unsigned int fd, struct statfs __user *buf); asmlinkage long sys_fstatfs64(unsigned int fd, size_t sz, struct statfs64 __user *buf); +asmlinkage long sys_statmnt(u64 mnt_id, u64 mask, + struct statmnt __user *buf, size_t bufsize, + unsigned int flags); asmlinkage long sys_truncate(const char __user *path, long length); asmlinkage long sys_ftruncate(unsigned int fd, unsigned long length); #if BITS_PER_LONG == 32 diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h index abe087c53b4b..640997231ff6 100644 --- a/include/uapi/asm-generic/unistd.h +++ b/include/uapi/asm-generic/unistd.h @@ -823,8 +823,11 @@ __SYSCALL(__NR_cachestat, sys_cachestat) #define __NR_fchmodat2 452 __SYSCALL(__NR_fchmodat2, sys_fchmodat2) +#define __NR_statmnt 454 +__SYSCALL(__NR_statmnt, sys_statmnt) + #undef __NR_syscalls -#define __NR_syscalls 453 +#define __NR_syscalls 455 /* * 32 bit systems traditionally used different diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h index bb242fdcfe6b..4ec7308a9259 100644 --- a/include/uapi/linux/mount.h +++ b/include/uapi/linux/mount.h @@ -138,4 +138,40 @@ struct mount_attr { /* List of all mount_attr versions. */ #define MOUNT_ATTR_SIZE_VER0 32 /* sizeof first published struct */ +struct stmt_str { + __u32 off; + __u32 len; +}; + +struct statmnt { + __u64 mask; /* What results were written [uncond] */ + __u32 sb_dev_major; /* Device ID */ + __u32 sb_dev_minor; + __u64 sb_magic; /* ..._SUPER_MAGIC */ + __u32 sb_flags; /* MS_{RDONLY,SYNCHRONOUS,DIRSYNC,LAZYTIME} */ + __u32 __spare1; + __u64 mnt_id; /* Unique ID of mount */ + __u64 mnt_parent_id; /* Unique ID of parent (for root == mnt_id) */ + __u32 mnt_id_old; /* Reused IDs used in proc/.../mountinfo */ + __u32 mnt_parent_id_old; + __u64 mnt_attr; /* MOUNT_ATTR_... */ + __u64 mnt_propagation; /* MS_{SHARED,SLAVE,PRIVATE,UNBINDABLE} */ + __u64 mnt_peer_group; /* ID of shared peer group */ + __u64 mnt_master; /* Mount receives propagation from this ID */ + __u64 propagate_from; /* Propagation from in current namespace */ + __u64 __spare[20]; + struct stmt_str mnt_root; /* Root of mount relative to root of fs */ + struct stmt_str mountpoint; /* Mountpoint relative to root of process */ + struct stmt_str fs_type; /* Filesystem type[.subtype] */ + struct stmt_str sb_opts; /* Super block string options (nul delimted) */ +}; + +#define STMT_SB_BASIC 0x00000001U /* Want/got sb_... */ +#define STMT_MNT_BASIC 0x00000002U /* Want/got mnt_... */ +#define STMT_PROPAGATE_FROM 0x00000004U /* Want/got propagate_from */ +#define STMT_MNT_ROOT 0x00000008U /* Want/got mnt_root */ +#define STMT_MOUNTPOINT 0x00000010U /* Want/got mountpoint */ +#define STMT_FS_TYPE 0x00000020U /* Want/got fs_type */ +#define STMT_SB_OPTS 0x00000040U /* Want/got sb_opts */ + #endif /* _UAPI_LINUX_MOUNT_H */
Add a way to query attributes of a single mount instead of having to parse the complete /proc/$PID/mountinfo, which might be huge. Lookup the mount by the old (32bit) or new (64bit) mount ID. If a mount needs to be queried based on path, then statx(2) can be used to first query the mount ID belonging to the path. Design is based on a suggestion by Linus: "So I'd suggest something that is very much like "statfsat()", which gets a buffer and a length, and returns an extended "struct statfs" *AND* just a string description at the end." The interface closely mimics that of statx. Handle ASCII attributes by appending after the end of the structure (as per above suggestion). Allow querying multiple string attributes with individual offset/length for each. String are nul terminated (termination isn't counted in length). Mount options are also delimited with nul characters. Unlike proc, special characters are not quoted. Link: https://lore.kernel.org/all/CAHk-=wh5YifP7hzKSbwJj94+DZ2czjrZsczy6GBimiogZws=rg@mail.gmail.com/ Signed-off-by: Miklos Szeredi <mszeredi@redhat.com> --- arch/x86/entry/syscalls/syscall_64.tbl | 1 + fs/internal.h | 5 + fs/namespace.c | 312 ++++++++++++++++++++++++- fs/proc_namespace.c | 19 +- fs/statfs.c | 1 + include/linux/syscalls.h | 3 + include/uapi/asm-generic/unistd.h | 5 +- include/uapi/linux/mount.h | 36 +++ 8 files changed, 373 insertions(+), 9 deletions(-)