[PATCH v2 1/2] statmount.2: New page describing the statmount syscall

Josef Bacik
2024-06-26T15:56:51Z

Add some documentation on the new statmount syscall.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
man/man2/statmount.2 | 289 +++++++++++++++++++++++++++++++++++++++++++
1 file changed, 289 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..9eefebde7
--- /dev/null
+++ b/man/man2/statmount.2
@@ -0,0 +1,289 @@
+'\" t
+.\" Copyright (c) 2024 Josef Bacik <josef@toxicpanda.com>
+.\"
+.\" 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 <linux/mount.h>" " /* Definition of STATMOUNT_* constants */"
+.B #include <unistd.h>
+.P
+.BI "int syscall(SYS_statmount, struct mnt_id_req * " req ,
+.BI " struct statmount * " statmountbuf ", size_t " bufsize ,
+.BI " unsigned long " flags " );
+.P
+.B #include <linux/mount.h>
+.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
+.P
+.EX
+.B struct statmount {
+.B " __u32 size;"
+.B " __u64 mask;"
+.B " __u32 sb_dev_major;"
+.B " __u32 sb_dev_minor;"
+.B " __u64 sb_magic;"
+.B " __u32 sb_flags;"
+.B " __u32 fs_type;"
+.B " __u64 mnt_id;"
+.B " __u64 mnt_parent_id;"
+.B " __u32 mnt_id_old;"
+.B " __u32 mnt_parent_id_old;"
+.B " __u64 mnt_attr;"
+.B " __u64 mnt_propagation;"
+.B " __u64 mnt_peer_group;"
+.B " __u64 mnt_master;"
+.B " __u64 propagate_from;"
+.B " __u32 mnt_root;"
+.B " __u32 mnt_point;"
+.B " char str[];"
+.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 statmountbuf .
+The returned buffer is a
+.I struct statmount
+with the fields filled in as described below.
+.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 fields 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
+.I sb_dev_major
+.TQ
+.I 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
+.IR mnt_id\~==\~parent_mount_id .
+.TP
+.I mnt_id_old
+This corresponds to the mount ID that is exported by
+.IR /proc/ pid /mountinfo .
+.TP
+.I mnt_parent_id_old
+This corresponds to the parent mount ID that is exported by
+.IR /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)
--
2.43.0