man-pages: add documentation for statmount/listmount

Add some documentation on the new statmount syscall.

Signed-off-by Josef Bacik <[email protected]>

---
 man/man2/statmount.2 | 281 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 281 insertions(+)
 create mode 100644 man/man2/statmount.2

man/man2/statmount.2 new file +281 -0

new file mode 100644
index 000000000..2ae1f1bc3
@@ -0,0 +1,281 @@
+'\" t
+.\" Copyright (c) 2024 Josef Bacik <[email protected]>
+.\"
+.\" 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 * " smbuf ", size_t " bufsize ,
+.BI "            unsigned long " flags );
+.P
+.B #include <linux/mount.h>
+.P
+.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 };
+.P
+.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 };
+.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,
+the caller 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 smbuf .
+The returned buffer is a
+.I struct statmount
+which is of size
+.I bufsize
+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
+.I struct\~mnt_id_req
+is being passed in;
+it should always be set to
+.IR 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
+lB l.
+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
+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 smbuf .
+.P
+The fields in the
+.I statmount
+structure are:
+.TP
+.I smbuf.size
+The size of the returned
+.I smbuf
+structure,
+including any of the strings fields that were filled.
+.TP
+.I smbuf.mask
+The ORed combination of
+.BI STATMOUNT_ *
+flags indicating which fields were filled in and thus valid.
+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 .
+.TP
+.I smbuf.sb_dev_major
+.TQ
+.I smbuf.sb_dev_minor
+The device that is mounted at this mount point.
+.TP
+.I smbuf.sb_magic
+The file system specific super block magic.
+.TP
+.I smbuf.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 smbuf.fs_type
+The offset to the location in the
+.I smbuf.str
+buffer that contains the string representation of the mounted file system.
+It is a null-terminated string.
+.TP
+.I smbuf.mnt_id
+The unique mount ID of the mount.
+.TP
+.I smbuf.mnt_parent_id
+The unique mount ID of the parent mount point of this mount.
+If this is the root mount point then
+.IR smbuf.mnt_id\~==\~smbuf.parent_mount_id .
+.TP
+.I smbuf.mnt_id_old
+This corresponds to the mount ID that is exported by
+.IR /proc/ pid /mountinfo .
+.TP
+.I smbuf.mnt_parent_id_old
+This corresponds to the parent mount ID that is exported by
+.IR /proc/ pid /mountinfo .
+.TP
+.I smbuf.mnt_attr
+The
+.BI MOUNT_ATTR_ *
+flags set on this mount point.
+.TP
+.I smbuf.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 smbuf.mnt_peer_group
+The ID of the shared peer group.
+.TP
+.I smbuf.mnt_master
+The mount point receives its propagation from this mount ID.
+.TP
+.I smbuf.propagate_from
+The ID from the namespace we propagated from.
+.TP
+.I smbuf.mnt_root
+The offset to the location in the
+.I smbuf.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 smbuf.mnt_point
+The offset to the location in the
+.I smbuf.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 smbuf
+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.
+.TP
+.B E2BIG
+.I req
+is too large.
+.TP
+.B EOVERFLOW
+The size of
+.I smbuf
+is too small to contain either the
+.IR smbuf.fs_type ,
+.IR smbuf.mnt_root ,
+or
+.IR smbuf.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)

Signature · 1 line

2.43.0

Add some documentation for the new listmount syscall.

Signed-off-by Josef Bacik <[email protected]>

---
 man/man2/listmount.2 | 112 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)
 create mode 100644 man/man2/listmount.2

man/man2/listmount.2 new file +112 -0

new file mode 100644
index 000000000..212929fb6
@@ -0,0 +1,112 @@
+.\" Copyright (c) 2024 Josef Bacik <[email protected]>
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH listmount 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+listmount \- get a list of mount ID's
+.SH LIBRARY
+Standard C library
+.RI ( libc ", " \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <linux/mount.h>" "  /* Definition of struct mnt_id_req constants */"
+.B #include <unistd.h>
+.P
+.BI "int syscall(SYS_listmount, struct mnt_id_req * " req ,
+.BI "            u64 * " mnt_ids ", size_t " nr_mnt_ids ,
+.BI "            unsigned long " flags );
+.P
+.B #include <linux/mount.h>
+.P
+.B struct mnt_id_req {
+.BR "    __u32 size;" "    /* sizeof(struct mnt_id_req) */"
+.BR "    __u64 mnt_id;" "  /* The parent mnt_id being searched */"
+.BR "    __u64 param;" "   /* The next mnt_id we want to find */"
+.B };
+.fi
+.P
+.IR Note :
+glibc provides no wrapper for
+.BR listmount (),
+necessitating the use of
+.BR syscall (2).
+.SH DESCRIPTION
+To access the mounts in your namespace,
+you must have CAP_SYS_ADMIN in the user namespace.
+.P
+This function returns a list of mount IDs under the
+.BR req.mnt_id .
+This is meant to be used in conjuction with
+.BR statmount (2)
+in order to provide a way to iterate and discover mounted file systems.
+.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
+is the parent mnt_id that we will list from,
+which can either be
+.B LSMT_ROOT
+which means the root mount of the current mount namespace,
+or a mount ID obtained from either
+.BR statx (2)
+using
+.B STATX_MNT_ID_UNIQUE
+or from
+.BR listmount (2) .
+.P
+.I req.param
+is used to tell the kernel what mount ID to start the list from.
+This is useful if multiple calls to
+.BR listmount (2)
+are required.
+This can be set to the last mount ID returned + 1 in order to
+resume from a previous spot in the list.
+.SH RETURN VALUE
+On success, the number of entries filled into
+.I mnt_ids
+is returned, 0 if there are no more mounts left.
+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 mnt_ids
+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.
+.TP
+.B E2BIG
+.I req
+is too large,
+the limit is the architectures page size.
+.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 statmount (2),
+.BR statx (2)

Signature · 1 line

2.43.0

Hi Josef,

Quoted 3 lines · Josef Bacik

Add some documentation for the new listmount syscall.

Signed-off-by Josef Bacik <[email protected]>

Thanks! I've applied the patch with some minor tweaks:

	diff --git i/man/man2/listmount.2 w/man/man2/listmount.2
	index 212929fb6..8f7c7afaa 100644
	--- i/man/man2/listmount.2
	+++ w/man/man2/listmount.2
	@@ -4,7 +4,9 @@
	 .\"
	 .TH listmount 2 (date) "Linux man-pages (unreleased)"
	 .SH NAME
	-listmount \- get a list of mount ID's
	+listmount
	+\-
	+get a list of mount ID's
	 .SH LIBRARY
	 Standard C library
	 .RI ( libc ", " \-lc )
	@@ -14,15 +16,15 @@ .SH SYNOPSIS
	 .B #include <unistd.h>
	 .P
	 .BI "int syscall(SYS_listmount, struct mnt_id_req * " req ,
	-.BI "            u64 * " mnt_ids ", size_t " nr_mnt_ids ,
	+.BI "            uint64_t * " mnt_ids ", size_t " nr_mnt_ids ,
	 .BI "            unsigned long " flags );
	 .P
	 .B #include <linux/mount.h>
	 .P
	 .B struct mnt_id_req {
	-.BR "    __u32 size;" "    /* sizeof(struct mnt_id_req) */"
	-.BR "    __u64 mnt_id;" "  /* The parent mnt_id being searched */"
	-.BR "    __u64 param;" "   /* The next mnt_id we want to find */"
	+.BR "    __u32  size;" "    /* sizeof(struct mnt_id_req) */"
	+.BR "    __u64  mnt_id;" "  /* The parent mnt_id being searched */"
	+.BR "    __u64  param;" "   /* The next mnt_id we want to find */"
	 .B };
	 .fi
	 .P
	@@ -45,7 +47,8 @@ .SS The mnt_id_req structure
	 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).
	+it should always be set to
	+.IR \%sizeof(struct\~mnt_id_req) .
	 .P
	 .I req.mnt_id
	 is the parent mnt_id that we will list from,
	@@ -69,7 +72,8 @@ .SS The mnt_id_req structure
	 .SH RETURN VALUE
	 On success, the number of entries filled into
	 .I mnt_ids
	-is returned, 0 if there are no more mounts left.
	+is returned;
	+0 if there are no more mounts left.
	 On error, \-1 is returned, and
	 .I errno
	 is set to indicate the error.

Would you mind adding an example program in a new patch?

Quoted 1 line

Would you mind adding an example program in a new patch?

Yup I can do that, I was going to follow-up with a patch for the new extensions that have landed in this merge window, after the final release has been cut. Is it cool if I wait until then, or would you like something sooner? Thanks,

Josef

Add some documentation for the new listmount syscall.

Signed-off-by Josef Bacik <[email protected]>

---
 man/man2/listmount.2 | 112 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)
 create mode 100644 man/man2/listmount.2

man/man2/listmount.2 new file +26 -0

new file mode 100644
index 000000000..212929fb6
@@ -0,0 +1,112 @@
+.\" Copyright (c) 2024 Josef Bacik <[email protected]>
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH listmount 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+listmount \- get a list of mount ID's
+.SH LIBRARY
+Standard C library
+.RI ( libc ", " \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <linux/mount.h>" "  /* Definition of struct mnt_id_req constants */"
+.B #include <unistd.h>
+.P
+.BI "int syscall(SYS_listmount, struct mnt_id_req * " req ,
+.BI "            u64 * " mnt_ids ", size_t " nr_mnt_ids ,
+.BI "            unsigned long " flags );
+.P
+.B #include <linux/mount.h>
+.P
+.B struct mnt_id_req {
+.BR "    __u32 size;" "    /* sizeof(struct mnt_id_req) */"
+.BR "    __u64 mnt_id;" "  /* The parent mnt_id being searched */"
+.BR "    __u64 param;" "   /* The next mnt_id we want to find */"
+.B };

man/man2/listmount.2 new file +14 -0

+.P
+This function returns a list of mount IDs under the
+.BR req.mnt_id .
+This is meant to be used in conjuction with
+.BR statmount (2)
+in order to provide a way to iterate and discover mounted file systems.
+.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

man/man2/listmount.2 new file +52 -0

+.P
+.I req.param
+is used to tell the kernel what mount ID to start the list from.
+This is useful if multiple calls to
+.BR listmount (2)
+are required.
+This can be set to the last mount ID returned + 1 in order to
+resume from a previous spot in the list.
+.SH RETURN VALUE
+On success, the number of entries filled into
+.I mnt_ids
+is returned, 0 if there are no more mounts left.
+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 mnt_ids
+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.
+.TP
+.B E2BIG
+.I req
+is too large,
+the limit is the architectures page size.
+.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 statmount (2),
+.BR statx (2)