| Message ID | 20211116054237.100814-5-memxor@gmail.com (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | BPF |
| Headers | show |
| Series | Introduce BPF iterators for io_uring and epoll | expand |
On 11/15/21 9:42 PM, Kumar Kartikeya Dwivedi wrote: > This patch adds eBPF iterator for epoll items (epitems) registered in an > epoll instance. It gives access to the eventpoll ctx, and the registered > epoll item (struct epitem). This allows the iterator to inspect the > registered file and be able to use others iterators to associate it with > a task's fdtable. > > The primary usecase this is enabling is expediting existing eventpoll > checkpoint/restore support in the CRIU project. This iterator allows us > to switch from a worst case O(n^2) algorithm to a single O(n) pass over > task and epoll registered descriptors. > > We also make sure we're iterating over a live file, one that is not > going away. The case we're concerned about is a file that has its > f_count as zero, but is waiting for iterator bpf_seq_read to release > ep->mtx, so that it can remove its epitem. Since such a file will > disappear once iteration is done, and it is being destructed, we use > get_file_rcu to ensure it is alive when invoking the BPF program. > > Getting access to a file that is going to disappear after iteration > is not useful anyway. This does have a performance overhead however > (since file reference will be raised and dropped for each file). > > The rcu_read_lock around get_file_rcu isn't strictly required for > lifetime management since fput path is serialized on ep->mtx to call > ep_remove, hence the epi->ffd.file pointer remains stable during our > seq_start/seq_stop bracketing. > > To be able to continue back from the position we were iterating, we > store the epi->ffi.fd and use ep_find_tfd to find the target file again. > It would be more appropriate to use both struct file pointer and fd > number to find the last file, but see below for why that cannot be done. > > Taking reference to struct file and walking RB-Tree to find it again > will lead to reference cycle issue if the iterator after partial read > takes reference to socket which later is used in creating a descriptor > cycle using SCM_RIGHTS. An example that was encountered when working on > this is mentioned below. > > Let there be Unix sockets SK1, SK2, epoll fd EP, and epoll iterator > ITER. > Let SK1 be registered in EP, then on a partial read it is possible > that ITER returns from read and takes reference to SK1 to be able to > find it later in RB-Tree and continue the iteration. If SK1 sends > ITER over to SK2 using SCM_RIGHTS, and SK2 sends SK2 over to SK1 using > SCM_RIGHTS, and both fds are not consumed on the corresponding receive > ends, a cycle is created. When all of SK1, SK2, EP, and ITER are > closed, SK1's receive queue holds reference to SK2, and SK2's receive > queue holds reference to ITER, which holds a reference to SK1. > All file descriptors except EP leak. > > To resolve it, we would need to hook into the Unix Socket GC mechanism, > but the alternative of using ep_find_tfd is much more simpler. The > finding of the last position in face of concurrent modification of the > epoll set is at best an approximation anyway. For the case of CRIU, the > epoll set remains stable. > > Cc: Alexander Viro <viro@zeniv.linux.org.uk> > Cc: linux-fsdevel@vger.kernel.org > Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com> > --- > fs/eventpoll.c | 196 ++++++++++++++++++++++++++++++++- > include/linux/bpf.h | 5 +- > include/uapi/linux/bpf.h | 3 + > tools/include/uapi/linux/bpf.h | 3 + > 4 files changed, 205 insertions(+), 2 deletions(-) > [...] > + > +static const struct bpf_iter_seq_info bpf_epoll_seq_info = { > + .seq_ops = &bpf_epoll_seq_ops, > + .init_seq_private = bpf_epoll_init_seq, > + .seq_priv_size = sizeof(struct bpf_epoll_iter_seq_info), > +}; > + > +static struct bpf_iter_reg epoll_reg_info = { > + .target = "epoll", > + .feature = BPF_ITER_RESCHED, > + .attach_target = bpf_epoll_iter_attach, > + .detach_target = bpf_epoll_iter_detach, implement show_fdinfo and fill_link_info? There are some bpftool work needed as well to show the information in user space. An example is e60495eafdba ("bpftool: Implement link_query for bpf iterators"). > + .ctx_arg_info_size = 2, > + .ctx_arg_info = { > + { offsetof(struct bpf_iter__epoll, ep), > + PTR_TO_BTF_ID }, > + { offsetof(struct bpf_iter__epoll, epi), > + PTR_TO_BTF_ID_OR_NULL }, > + }, > + .seq_info = &bpf_epoll_seq_info, > +}; > + > +static int __init epoll_iter_init(void) > +{ > + epoll_reg_info.ctx_arg_info[0].btf_id = btf_epoll_ids[0]; > + epoll_reg_info.ctx_arg_info[1].btf_id = btf_epoll_ids[1]; > + return bpf_iter_reg_target(&epoll_reg_info); > +} > +late_initcall(epoll_iter_init); > + > +#endif > diff --git a/include/linux/bpf.h b/include/linux/bpf.h > index fe7b499da781..eb1c9acdc40b 100644 > --- a/include/linux/bpf.h > +++ b/include/linux/bpf.h > @@ -1512,7 +1512,10 @@ int bpf_obj_get_user(const char __user *pathname, int flags); > struct io_ring_ctx; > struct bpf_iter_aux_info { > struct bpf_map *map; > - struct io_ring_ctx *ctx; > + union { > + struct io_ring_ctx *ctx; > + struct file *ep; > + }; You changed to union here. I think we can change "struct bpf_iter_aux_info" to "union bpf_iter_aux_info". This should make code simpler and easy to understand. > }; > > typedef int (*bpf_iter_attach_target_t)(struct bpf_prog *prog, > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h > index b70e9da3d722..64e18c1dcfca 100644 > --- a/include/uapi/linux/bpf.h > +++ b/include/uapi/linux/bpf.h > @@ -94,6 +94,9 @@ union bpf_iter_link_info { > struct { > __u32 io_uring_fd; > } io_uring; > + struct { > + __u32 epoll_fd; > + } epoll; > }; > > /* BPF syscall commands, see bpf(2) man-page for more details. */ > diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h > index b70e9da3d722..64e18c1dcfca 100644 > --- a/tools/include/uapi/linux/bpf.h > +++ b/tools/include/uapi/linux/bpf.h > @@ -94,6 +94,9 @@ union bpf_iter_link_info { > struct { > __u32 io_uring_fd; > } io_uring; > + struct { > + __u32 epoll_fd; > + } epoll; > }; > > /* BPF syscall commands, see bpf(2) man-page for more details. */ >
diff --git a/fs/eventpoll.c b/fs/eventpoll.c index 06f4c5ae1451..aa21628b6307 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -37,6 +37,7 @@ #include <linux/seq_file.h> #include <linux/compat.h> #include <linux/rculist.h> +#include <linux/btf_ids.h> #include <net/busy_poll.h> /* @@ -985,7 +986,6 @@ static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd) return epir; } -#ifdef CONFIG_KCMP static struct epitem *ep_find_tfd(struct eventpoll *ep, int tfd, unsigned long toff) { struct rb_node *rbp; @@ -1005,6 +1005,7 @@ static struct epitem *ep_find_tfd(struct eventpoll *ep, int tfd, unsigned long t return NULL; } +#ifdef CONFIG_KCMP struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd, unsigned long toff) { @@ -2385,3 +2386,196 @@ static int __init eventpoll_init(void) return 0; } fs_initcall(eventpoll_init); + +#ifdef CONFIG_BPF_SYSCALL + +BTF_ID_LIST(btf_epoll_ids) +BTF_ID(struct, eventpoll) +BTF_ID(struct, epitem) + +struct bpf_epoll_iter_seq_info { + struct eventpoll *ep; + struct rb_node *rbp; + int tfd; +}; + +static int bpf_epoll_init_seq(void *priv_data, struct bpf_iter_aux_info *aux) +{ + struct bpf_epoll_iter_seq_info *info = priv_data; + + info->ep = aux->ep->private_data; + info->tfd = -1; + return 0; +} + +static int bpf_epoll_iter_attach(struct bpf_prog *prog, + union bpf_iter_link_info *linfo, + struct bpf_iter_aux_info *aux) +{ + struct file *file; + int ret; + + file = fget(linfo->epoll.epoll_fd); + if (!file) + return -EBADF; + + ret = -EOPNOTSUPP; + if (unlikely(!is_file_epoll(file))) + goto out_fput; + + aux->ep = file; + return 0; +out_fput: + fput(file); + return ret; +} + +static void bpf_epoll_iter_detach(struct bpf_iter_aux_info *aux) +{ + fput(aux->ep); +} + +struct bpf_iter__epoll { + __bpf_md_ptr(struct bpf_iter_meta *, meta); + __bpf_md_ptr(struct eventpoll *, ep); + __bpf_md_ptr(struct epitem *, epi); +}; + +static void *bpf_epoll_seq_start(struct seq_file *seq, loff_t *pos) +{ + struct bpf_epoll_iter_seq_info *info = seq->private; + struct epitem *epi; + + mutex_lock(&info->ep->mtx); + /* already iterated? */ + if (info->tfd == -2) + return NULL; + /* partially iterated */ + if (info->tfd >= 0) { + epi = ep_find_tfd(info->ep, info->tfd, 0); + if (!epi) + return NULL; + info->rbp = &epi->rbn; + return epi; + } + WARN_ON(info->tfd != -1); + /* first iteration */ + info->rbp = rb_first_cached(&info->ep->rbr); + if (!info->rbp) + return NULL; + if (*pos == 0) + ++*pos; + return rb_entry(info->rbp, struct epitem, rbn); +} + +static void *bpf_epoll_seq_next(struct seq_file *seq, void *v, loff_t *pos) +{ + struct bpf_epoll_iter_seq_info *info = seq->private; + + ++*pos; + info->rbp = rb_next(info->rbp); + return info->rbp ? rb_entry(info->rbp, struct epitem, rbn) : NULL; +} + +DEFINE_BPF_ITER_FUNC(epoll, struct bpf_iter_meta *meta, struct eventpoll *ep, + struct epitem *epi) + +static int __bpf_epoll_seq_show(struct seq_file *seq, void *v, bool in_stop) +{ + struct bpf_epoll_iter_seq_info *info = seq->private; + struct bpf_iter__epoll ctx; + struct bpf_iter_meta meta; + struct bpf_prog *prog; + int ret; + + meta.seq = seq; + prog = bpf_iter_get_info(&meta, in_stop); + if (!prog) + return 0; + + ctx.meta = &meta; + ctx.ep = info->ep; + ctx.epi = v; + if (ctx.epi) { + /* The file we are going to pass to prog may already have its f_count as + * 0, hence before invoking the prog, we always try to get the reference + * if it isn't zero, failing which we skip the file. This is usually the + * case for files that are closed before calling EPOLL_CTL_DEL for them, + * which would wait for us to release ep->mtx before doing ep_remove. + */ + rcu_read_lock(); + ret = get_file_rcu(ctx.epi->ffd.file); + rcu_read_unlock(); + if (!ret) + return 0; + } + ret = bpf_iter_run_prog(prog, &ctx); + /* fput queues work asynchronously, so in our case, either task_work + * for non-exiting task, and otherwise delayed_fput, so holding + * ep->mtx and calling fput (which will take the same lock) in + * this context will not deadlock us, in case f_count is 1 at this + * point. + */ + if (ctx.epi) + fput(ctx.epi->ffd.file); + return ret; +} + +static int bpf_epoll_seq_show(struct seq_file *seq, void *v) +{ + return __bpf_epoll_seq_show(seq, v, false); +} + +static void bpf_epoll_seq_stop(struct seq_file *seq, void *v) +{ + struct bpf_epoll_iter_seq_info *info = seq->private; + struct epitem *epi; + + if (!v) { + __bpf_epoll_seq_show(seq, v, true); + /* done iterating */ + info->tfd = -2; + } else { + epi = rb_entry(info->rbp, struct epitem, rbn); + info->tfd = epi->ffd.fd; + } + mutex_unlock(&info->ep->mtx); +} + +static const struct seq_operations bpf_epoll_seq_ops = { + .start = bpf_epoll_seq_start, + .next = bpf_epoll_seq_next, + .stop = bpf_epoll_seq_stop, + .show = bpf_epoll_seq_show, +}; + +static const struct bpf_iter_seq_info bpf_epoll_seq_info = { + .seq_ops = &bpf_epoll_seq_ops, + .init_seq_private = bpf_epoll_init_seq, + .seq_priv_size = sizeof(struct bpf_epoll_iter_seq_info), +}; + +static struct bpf_iter_reg epoll_reg_info = { + .target = "epoll", + .feature = BPF_ITER_RESCHED, + .attach_target = bpf_epoll_iter_attach, + .detach_target = bpf_epoll_iter_detach, + .ctx_arg_info_size = 2, + .ctx_arg_info = { + { offsetof(struct bpf_iter__epoll, ep), + PTR_TO_BTF_ID }, + { offsetof(struct bpf_iter__epoll, epi), + PTR_TO_BTF_ID_OR_NULL }, + }, + .seq_info = &bpf_epoll_seq_info, +}; + +static int __init epoll_iter_init(void) +{ + epoll_reg_info.ctx_arg_info[0].btf_id = btf_epoll_ids[0]; + epoll_reg_info.ctx_arg_info[1].btf_id = btf_epoll_ids[1]; + return bpf_iter_reg_target(&epoll_reg_info); +} +late_initcall(epoll_iter_init); + +#endif diff --git a/include/linux/bpf.h b/include/linux/bpf.h index fe7b499da781..eb1c9acdc40b 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -1512,7 +1512,10 @@ int bpf_obj_get_user(const char __user *pathname, int flags); struct io_ring_ctx; struct bpf_iter_aux_info { struct bpf_map *map; - struct io_ring_ctx *ctx; + union { + struct io_ring_ctx *ctx; + struct file *ep; + }; }; typedef int (*bpf_iter_attach_target_t)(struct bpf_prog *prog, diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h index b70e9da3d722..64e18c1dcfca 100644 --- a/include/uapi/linux/bpf.h +++ b/include/uapi/linux/bpf.h @@ -94,6 +94,9 @@ union bpf_iter_link_info { struct { __u32 io_uring_fd; } io_uring; + struct { + __u32 epoll_fd; + } epoll; }; /* BPF syscall commands, see bpf(2) man-page for more details. */ diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h index b70e9da3d722..64e18c1dcfca 100644 --- a/tools/include/uapi/linux/bpf.h +++ b/tools/include/uapi/linux/bpf.h @@ -94,6 +94,9 @@ union bpf_iter_link_info { struct { __u32 io_uring_fd; } io_uring; + struct { + __u32 epoll_fd; + } epoll; }; /* BPF syscall commands, see bpf(2) man-page for more details. */
This patch adds eBPF iterator for epoll items (epitems) registered in an epoll instance. It gives access to the eventpoll ctx, and the registered epoll item (struct epitem). This allows the iterator to inspect the registered file and be able to use others iterators to associate it with a task's fdtable. The primary usecase this is enabling is expediting existing eventpoll checkpoint/restore support in the CRIU project. This iterator allows us to switch from a worst case O(n^2) algorithm to a single O(n) pass over task and epoll registered descriptors. We also make sure we're iterating over a live file, one that is not going away. The case we're concerned about is a file that has its f_count as zero, but is waiting for iterator bpf_seq_read to release ep->mtx, so that it can remove its epitem. Since such a file will disappear once iteration is done, and it is being destructed, we use get_file_rcu to ensure it is alive when invoking the BPF program. Getting access to a file that is going to disappear after iteration is not useful anyway. This does have a performance overhead however (since file reference will be raised and dropped for each file). The rcu_read_lock around get_file_rcu isn't strictly required for lifetime management since fput path is serialized on ep->mtx to call ep_remove, hence the epi->ffd.file pointer remains stable during our seq_start/seq_stop bracketing. To be able to continue back from the position we were iterating, we store the epi->ffi.fd and use ep_find_tfd to find the target file again. It would be more appropriate to use both struct file pointer and fd number to find the last file, but see below for why that cannot be done. Taking reference to struct file and walking RB-Tree to find it again will lead to reference cycle issue if the iterator after partial read takes reference to socket which later is used in creating a descriptor cycle using SCM_RIGHTS. An example that was encountered when working on this is mentioned below. Let there be Unix sockets SK1, SK2, epoll fd EP, and epoll iterator ITER. Let SK1 be registered in EP, then on a partial read it is possible that ITER returns from read and takes reference to SK1 to be able to find it later in RB-Tree and continue the iteration. If SK1 sends ITER over to SK2 using SCM_RIGHTS, and SK2 sends SK2 over to SK1 using SCM_RIGHTS, and both fds are not consumed on the corresponding receive ends, a cycle is created. When all of SK1, SK2, EP, and ITER are closed, SK1's receive queue holds reference to SK2, and SK2's receive queue holds reference to ITER, which holds a reference to SK1. All file descriptors except EP leak. To resolve it, we would need to hook into the Unix Socket GC mechanism, but the alternative of using ep_find_tfd is much more simpler. The finding of the last position in face of concurrent modification of the epoll set is at best an approximation anyway. For the case of CRIU, the epoll set remains stable. Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: linux-fsdevel@vger.kernel.org Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com> --- fs/eventpoll.c | 196 ++++++++++++++++++++++++++++++++- include/linux/bpf.h | 5 +- include/uapi/linux/bpf.h | 3 + tools/include/uapi/linux/bpf.h | 3 + 4 files changed, 205 insertions(+), 2 deletions(-)