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[76.182.20.124]) by smtp.gmail.com with ESMTPSA id af79cd13be357-79bce936027sm432916485a.115.2024.06.25.11.56.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 25 Jun 2024 11:56:54 -0700 (PDT) From: Josef Bacik To: alx@kernel.org, linux-man@vger.kernel.org, brauner@kernel.org, linux-fsdevel@vger.kernel.org, mszeredi@redhat.com, kernel-team@fb.com Subject: [PATCH 2/3] statmount.2: New page describing the statmount syscall Date: Tue, 25 Jun 2024 14:56:05 -0400 Message-ID: X-Mailer: git-send-email 2.43.0 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Add some documentation on the new statmount syscall. Signed-off-by: Josef Bacik --- man/man2/statmount.2 | 274 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 274 insertions(+) create mode 100644 man/man2/statmount.2 diff --git a/man/man2/statmount.2 b/man/man2/statmount.2 new file mode 100644 index 000000000..6d9f505f9 --- /dev/null +++ b/man/man2/statmount.2 @@ -0,0 +1,274 @@ +'\" t +.\" Copyright (c) 2024 Josef Bacik +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.TH statmount 2 (date) "Linux man-pages (unreleased)" +.SH NAME +statmount \- get a mount status +.SH LIBRARY +Standard C library +.RI ( libc ", " \-lc ) +.SH SYNOPSIS +.nf +.BR "#include " " /* Definition of STATMOUNT_* constants */" +.B #include +.P +.BI "int syscall(SYS_statmount, struct mnt_id_req * " req , +.BI " struct statmount * " statmountbuf ", size_t " bufsize , +.BI " unsigned long " flags " ); +.P +.EX +.B struct mnt_id_req { +.BR " __u32 size;" " /* sizeof(struct mnt_id_req) */" +.BR " __u64 mnt_id;" " /* The mnt_id being queried */" +.BR " __u64 param;" " /* An ORed combination of the STATMOUNT_ constants */" +.B }; +.EE +.fi +.P +.IR Note : +glibc provides no wrapper for +.BR statmount (), +necessitating the use of +.BR syscall (2). +.SH DESCRIPTION +To access a mount's status, you must have CAP_SYS_ADMIN in the user namespace. +.P +This function returns information about a mount, storing it in the buffer +pointed to by +.IR buf . +The returned buffer is a structure of the following type: +.P +.in +4n +.EX +struct statmount { + __u32 size; + __u64 mask; + __u32 sb_dev_major; + __u32 sb_dev_minor; + __u64 sb_magic; + __u32 sb_flags; + __u32 fs_type; + __u64 mnt_id; + __u64 mnt_parent_id; + __u32 mnt_id_old; + __u32 mnt_parent_id_old; + __u64 mnt_attr; + __u64 mnt_propagation; + __u64 mnt_peer_group; + __u64 mnt_master; + __u64 propagate_from; + __u32 mnt_root; + __u32 mnt_point; + char str[]; +}; +.EE +.in +.P +(Note that reserved space and padding is omitted.) +.SS The mnt_id_req structure +.I req.size +is used by the kernel to determine which struct +.I mnt_id_req +is being passed in, it should always be set to sizeof(struct mnt_id req). +.P +.I req.mnt_id +can be obtained from either +.BR statx (2) +using +.B STATX_MNT_ID_UNIQUE +or from +.BR listmount (2) +and is used as the identifier to query the status of the desired mount point. +.P +.I req.param +is used to tell the kernel which fields the caller is interested in. It is an +ORed combination of the following constants +.P +.in +4n +.TS +lBl. +STATMOUNT_SB_BASIC /* Want/got sb_... */ +STATMOUNT_MNT_BASIC /* Want/got mnt_... */ +STATMOUNT_PROPAGATE_FROM /* Want/got propagate_from */ +STATMOUNT_MNT_ROOT /* Want/got mnt_root */ +STATMOUNT_MNT_POINT /* Want/got mnt_point */ +STATMOUNT_FS_TYPE /* Want/got fs_type */ +.TE +.in +.P +Note that, in general, the kernel does +.I not +reject values in +.I req.param +other than the above. +(For an exception, see +.B EINVAL +in errors.) +Instead, it simply informs the caller which values are supported +by this kernel and filesystem via the +.I statmount.mask +field. +Therefore, +.I "do not" +simply set +.I req.param +to +.B UINT_MAX +(all bits set), +as one or more bits may, in the future, be used to specify an +extension to the buffer. +.SS +The returned information +The status information for the target mount is returned in the +.I statmount +structure pointed to by +.IR statmountbuf . +Included in this is +.I size +which indicates the size of the +.I statmountbuf +that was filled in, including any strings. +.I mask +which indicates what information in the structure has been filled in. +.P +It should be noted that the kernel may return fileds that weren't requested and +may fail to return fields that were requested, depending on what the backing +file system and kernel supports. +In either case, +.I req.param +will not be equal to +.IR mask . +.P +Apart from +.I mask +(which is described above), the fields in the +.I statmount +structure are: +.TP +.I size +The size of the returned +.I statmountbuf +structure. +.TP +.IR sb_dev_major " and " sb_dev_minor +The device that is mounted at this mount point. +.TP +.I sb_magic +The file system specific super block magic. +.TP +.I sb_flags +The flags that are set on the super block, an ORed combination of +.BR SB_RDONLY , +.BR SB_SYNCHRONOUS , +.BR SB_DIRSYNC , +.BR SB_LAZYTIME . +.TP +.I fs_type +The offset to the location in the +.I statmount.str +buffer that contains the string representation of the mounted file system. It is +a NULL terminated string. +.TP +.I mnt_id +The unique mount ID of the mount. +.TP +.I mnt_parent_id +The unique mount ID of the parent mount point of this mount. If this is the +root mount point then +.B mnt_id +== +.BR parent_mount_id . +.TP +.I mnt_id_old +This corresponds to the mount ID that is exported by /proc/$PID/mountinfo. +.TP +.I mnt_parent_id_old +This corresponds to the parent mount ID that is exported by +/proc/$PID/mountinfo. +.TP +.I mnt_attr +The +.B MOUNT_ATTR_ +flags set on this mount point. +.TP +.I mnt_propagation +The mount propagation flags, which can be one of +.BR MS_SHARED , +.BR MS_SLAVE , +.BR MS_PRIVATE , +.BR MS_UNBINDABLE . +.TP +.I mnt_peer_group +The ID of the shared peer group. +.TP +.I mnt_master +The mount point receives its propagation from this mount ID. +.TP +.I propagate_from +The ID from the namespace we propagated from. +.TP +.I mnt_root +The offset to the location in the +.I statmount.str +buffer that contains the string representation of the mount relative to the root +of the file system. It is a NULL terminated string. +.TP +.I mnt_point +The offset to the location in the +.I statmount.str +buffer that contains the string representation of the mount relative to the +current root (ie if you are in a +.BR chroot ). +It is a NULL terminated string. +.SH RETURN VALUE +On success, zero is returned. +On error, \-1 is returned, and +.I errno +is set to indicate the error. +.SH ERRORS +.TP +.B EPERM +Permission is denied for accessing this mount. +.TP +.B EFAULT +.I req +or +.I statmountbuf +is NULL or points to a location outside the process's +accessible address space. +.TP +.B EINVAL +Invalid flag specified in +.IR flags . +.TP +.B EINVAL +.I req +is of insufficient size to be utilized. +.B E2BIG +.I req +is too large, the limit is the architectures page size. +.TP +.B EOVERFLOW +The size of +.I statmountbuf +is too small to contain either the +.IR statmountbuf.fs_type , +.IR statmountbuf.mnt_root , +or +.IR statmountbuf.mnt_point . +Allocate a larger buffer and retry the call. +.TP +.B ENOENT +The specified +.I req.mnt_id +doesn't exist. +.TP +.B ENOMEM +Out of memory (i.e., kernel memory). +.SH STANDARDS +Linux. +.SH SEE ALSO +.BR listmount (2), +.BR statx (2)