man/man2/statmount.2: Document New Features

---
 man/man2/statmount.2 | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

man/man2/statmount.2 +20 -3

index cdc96da92..4ffddc43a 100644
@@ -23,9 +23,10 @@ .SH SYNOPSIS
 .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 */"
+.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 */"
+.BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"
 .B };
 .P
 .B struct statmount {

@@ -47,6 +48,7 @@ .SH SYNOPSIS
 .B "    __u64  propagate_from;"
 .B "    __u32  mnt_root;"
 .B "    __u32  mnt_point;"
+.B "    __u64  mnt_ns_id;"
 .B "    char   str[];"
 .B };
 .EE

@@ -59,6 +61,9 @@ .SH SYNOPSIS
 .SH DESCRIPTION
 To access a mount's status,
 the caller must have CAP_SYS_ADMIN in the user namespace.
+In case of accessing a mount in a foreign mount namespace (specified via
+.IR req.mnt_ns_id ),
+the foreign mount namespace should be child of the current namespace.
 .P
 This function returns information about a mount,
 storing it in the buffer pointed to by

@@ -102,6 +107,7 @@ .SS The mnt_id_req structure
 STATMOUNT_MNT_ROOT	/* Want/got mnt_root  */
 STATMOUNT_MNT_POINT	/* Want/got mnt_point */
 STATMOUNT_FS_TYPE	/* Want/got fs_type */
+STATMOUNT_MNT_NS_ID	/* Want/got mnt_ns_id */
 .TE
 .in
 .P

@@ -130,6 +136,14 @@ .SS The mnt_id_req structure
 as one or more bits may,
 in the future,
 be used to specify an extension to the buffer.
+.P
+.I req.mnt_ns_id
+can be obtained from
+.B NS_GET_MNTNS_ID
+.BR ioctl (2)
+operation and is used to specify a foreign mount namespace in which to query
+.I req.mnt_id
+(since Linux 6.11).
 .SS The returned information
 The status information for the target mount is returned in the
 .I statmount

@@ -232,6 +246,9 @@ .SS The returned information
 relative to the current root (ie if you are in a
 .BR chroot ).
 It is a null-terminated string.
+.TP
+.IR smbuf.mnt_ns_id " (since Linux 6.11)"
+The unique ID of the mount namespace the mount belongs to.
 .SH RETURN VALUE
 On success, zero is returned.
 On error, \-1 is returned, and

Signature · 1 line

2.53.0

Bhavik Sachdev

author

2026-02-22 14:51 UTC #2 lore raw

PATCH v2 2/10 man/man2/statmount.2: Document STATMOUNT_MNT_OPTS

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f9af549d1fd31487bbbc666b5b158cfc940ccc17>

Document STATMOUNT_MNT_OPTS flag and the corresponding field introduced by it.

STATMOUNT_MNT_OPTS is used to get all mount options on the mount separated by commas. Similiar to how they are displayed in /proc/[pid]/mountinfo.

This text is based on this commit [1].

---
 man/man2/statmount.2 | 10 ++++++++++
 1 file changed, 10 insertions(+)

man/man2/statmount.2 +10 -0

index 4ffddc43a..c2682646c 100644
@@ -31,6 +31,7 @@ .SH SYNOPSIS
 .P
 .B struct statmount {
 .B "    __u32  size;"
+.B "    __u32  mnt_opts;"
 .B "    __u64  mask;"
 .B "    __u32  sb_dev_major;"
 .B "    __u32  sb_dev_minor;"

@@ -108,6 +109,7 @@ .SS The mnt_id_req structure
 STATMOUNT_MNT_POINT	/* Want/got mnt_point */
 STATMOUNT_FS_TYPE	/* Want/got fs_type */
 STATMOUNT_MNT_NS_ID	/* Want/got mnt_ns_id */
+STATMOUNT_MNT_OPTS	/* Want/got mnt_opts */
 .TE
 .in
 .P

@@ -160,6 +162,14 @@ .SS The returned information
 structure,
 including any of the strings fields that were filled.
 .TP
+.IR smbuf.mnt_opts " (since Linux 6.11)"
+The offset to the location in the
+.I smbuf.str
+buffer that contains a comma separated list of mount options,
+similiar to those in
+.BR proc_pid_mountinfo (5) .
+It is a null-terminated string.
+.TP
 .I smbuf.mask
 The ORed combination of
 .BI STATMOUNT_ *

Signature · 1 line

2.53.0

Bhavik Sachdev

author

2026-02-22 14:51 UTC #3 lore raw

PATCH v2 3/10 man/man2/statmount.2: Document STATMOUNT_FS_SUBTYPE

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=ed9d95f691c29748f21bc019de9566b698fdfab7>

Document STATMOUNT_FS_SUBTYPE flag and the corresponding field introduced by it.

STATMOUNT_FS_SUBTYPE retrieves the subtype of a file system, useful for FUSE mounts (example: fuse.sshfs, where "sshfs" is the subtype).

This work is based on this commit message [1].

---
 man/man2/statmount.2 | 9 +++++++++
 1 file changed, 9 insertions(+)

man/man2/statmount.2 +9 -0

index c2682646c..29080246b 100644
@@ -50,6 +50,7 @@ .SH SYNOPSIS
 .B "    __u32  mnt_root;"
 .B "    __u32  mnt_point;"
 .B "    __u64  mnt_ns_id;"
+.B "    __u32  fs_subtype;"
 .B "    char   str[];"
 .B };
 .EE

@@ -110,6 +111,7 @@ .SS The mnt_id_req structure
 STATMOUNT_FS_TYPE	/* Want/got fs_type */
 STATMOUNT_MNT_NS_ID	/* Want/got mnt_ns_id */
 STATMOUNT_MNT_OPTS	/* Want/got mnt_opts */
+STATMOUNT_FS_SUBTYPE	/* Want/got fs_subtype */
 .TE
 .in
 .P

@@ -259,6 +261,13 @@ .SS The returned information
 .TP
 .IR smbuf.mnt_ns_id " (since Linux 6.11)"
 The unique ID of the mount namespace the mount belongs to.
+.TP
+.IR smbuf.fs_subtype " (since Linux 6.13)"
+The offset to the location in the
+.I smbuf.str
+buffer that contains the string representation of the file system subtype.
+This is particularly useful for disambiguating FUSE mounts.
+It is a null-terminated string.
 .SH RETURN VALUE
 On success, zero is returned.
 On error, \-1 is returned, and

Signature · 1 line

2.53.0

Bhavik Sachdev

author

2026-02-22 14:51 UTC #4 lore raw

PATCH v2 4/10 man/man2/statmount.2: Document STATMOUNT_SB_SOURCE

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=44010543fc8bedad172aa5b6c43480e5d2124497>

Document STATMOUNT_SB_SOURCE flag and the corresponding field introduced by it. This text is based on this commit message [1].

---
 man/man2/statmount.2 | 8 ++++++++
 1 file changed, 8 insertions(+)

man/man2/statmount.2 +8 -0

index 29080246b..f7a2feff9 100644
@@ -51,6 +51,7 @@ .SH SYNOPSIS
 .B "    __u32  mnt_point;"
 .B "    __u64  mnt_ns_id;"
 .B "    __u32  fs_subtype;"
+.B "    __u32  sb_source;"
 .B "    char   str[];"
 .B };
 .EE

@@ -112,6 +113,7 @@ .SS The mnt_id_req structure
 STATMOUNT_MNT_NS_ID	/* Want/got mnt_ns_id */
 STATMOUNT_MNT_OPTS	/* Want/got mnt_opts */
 STATMOUNT_FS_SUBTYPE	/* Want/got fs_subtype */
+STATMOUNT_SB_SOURCE	/* Want/got sb_source */
 .TE
 .in
 .P

@@ -268,6 +270,12 @@ .SS The returned information
 buffer that contains the string representation of the file system subtype.
 This is particularly useful for disambiguating FUSE mounts.
 It is a null-terminated string.
+.TP
+.IR smbuf.sb_source " (since Linux 6.13)"
+The offset to the location in the
+.I smbuf.str
+that contains the source of the mount.
+It is a null-terminated string.
 .SH RETURN VALUE
 On success, zero is returned.
 On error, \-1 is returned, and

Signature · 1 line

2.53.0

Bhavik Sachdev

author

2026-02-22 14:52 UTC #5 lore raw

PATCH v2 5/10 man/man2/statmount.2: Document STATMOUNT_OPT_ARRAY

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2f4d4503e9e5ab765a7948f98bc5deef7850f607>

Document STATMOUNT_OPT_ARRAY flag and the corresponding fields introduced by it. This text is based on this commit message [1].

Retrieve only file system options, separated by null bytes. For security opts STATMOUNT_OPT_SEC_ARRAY is used.

---
 man/man2/statmount.2 | 13 +++++++++++++
 1 file changed, 13 insertions(+)

man/man2/statmount.2 +13 -0

index f7a2feff9..c238a3d9f 100644
@@ -52,6 +52,8 @@ .SH SYNOPSIS
 .B "    __u64  mnt_ns_id;"
 .B "    __u32  fs_subtype;"
 .B "    __u32  sb_source;"
+.B "    __u32  opt_num;"
+.B "    __u32  opt_array;"
 .B "    char   str[];"
 .B };
 .EE

@@ -114,6 +116,7 @@ .SS The mnt_id_req structure
 STATMOUNT_MNT_OPTS	/* Want/got mnt_opts */
 STATMOUNT_FS_SUBTYPE	/* Want/got fs_subtype */
 STATMOUNT_SB_SOURCE	/* Want/got sb_source */
+STATMOUNT_OPT_ARRAY	/* Want/got opt_... */
 .TE
 .in
 .P

@@ -276,6 +279,16 @@ .SS The returned information
 .I smbuf.str
 that contains the source of the mount.
 It is a null-terminated string.
+.TP
+.IR smbuf.opt_num " (since Linux 6.13)"
+The number of filesystem options set on the mount.
+.TP
+.IR smbuf.opt_array " (since Linux 6.13)"
+The offset to the location in the
+.I smbuf.str
+buffer that contains file system options separated by null bytes.
+They can be iterated over with the help of
+.IR smbuf.opt_num .
 .SH RETURN VALUE
 On success, zero is returned.
 On error, \-1 is returned, and

Signature · 1 line

2.53.0

Bhavik Sachdev

author

2026-02-22 14:52 UTC #6 lore raw

PATCH v2 6/10 man/man2/statmount.2: Document STATMOUNT_OPT_SEC_ARRAY

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=aefff51e1c2986e16f2780ca8e4c97b784800ab5>

Document STATMOUNT_OPT_SEC_ARRAY flag and the corresponding fields introduced by it. This text is based this commit message [1].

Retrieves only security options separated by null bytes.

---
 man/man2/statmount.2 | 13 +++++++++++++
 1 file changed, 13 insertions(+)

man/man2/statmount.2 +13 -0

index c238a3d9f..c4200d76e 100644
@@ -54,6 +54,8 @@ .SH SYNOPSIS
 .B "    __u32  sb_source;"
 .B "    __u32  opt_num;"
 .B "    __u32  opt_array;"
+.B "    __u32  opt_sec_num;"
+.B "    __u32  opt_sec_array;"
 .B "    char   str[];"
 .B };
 .EE

@@ -117,6 +119,7 @@ .SS The mnt_id_req structure
 STATMOUNT_FS_SUBTYPE	/* Want/got fs_subtype */
 STATMOUNT_SB_SOURCE	/* Want/got sb_source */
 STATMOUNT_OPT_ARRAY	/* Want/got opt_... */
+STATMOUNT_OPT_SEC_ARRAY	/* Want/got opt_sec... */
 .TE
 .in
 .P

@@ -289,6 +292,16 @@ .SS The returned information
 buffer that contains file system options separated by null bytes.
 They can be iterated over with the help of
 .IR smbuf.opt_num .
+.TP
+.IR smbuf.opt_sec_num " (since Linux 6.13)"
+The number of security options set on the mount.
+.TP
+.IR smbuf.opt_sec_array " (since Linux 6.13)"
+The offset to the location in the
+.I smbuf.str
+buffer that contains security options separated by null bytes.
+They can be iterated over with the help of
+.IR smbuf.opt_sec_num .
 .SH RETURN VALUE
 On success, zero is returned.
 On error, \-1 is returned, and

Signature · 1 line

2.53.0

Bhavik Sachdev

author

2026-02-22 14:52 UTC #7 lore raw

PATCH v2 7/10 man/man2/statmount.2: Document STATMOUNT_{UIDMAP,GIDMAP}

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=37c4a9590e1efcae7749682239fc22a330d2d325>

Document the STATMOUNT_{UIDMAP,GIDMAP} flags and the corresponding fields introduced by them. Most of this is based on the commit message provided here [1].

---
 man/man2/statmount.2 | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

man/man2/statmount.2 +38 -0

index c4200d76e..e24827ad4 100644
@@ -56,6 +56,10 @@ .SH SYNOPSIS
 .B "    __u32  opt_array;"
 .B "    __u32  opt_sec_num;"
 .B "    __u32  opt_sec_array;"
+.B "    __u32  mnt_uidmap_num;"
+.B "    __u32  mnt_uidmap;"
+.B "    __u32  mnt_gidmap_num;"
+.B "    __u32  mnt_gidmap;"
 .B "    char   str[];"
 .B };
 .EE

@@ -120,6 +124,8 @@ .SS The mnt_id_req structure
 STATMOUNT_SB_SOURCE	/* Want/got sb_source */
 STATMOUNT_OPT_ARRAY	/* Want/got opt_... */
 STATMOUNT_OPT_SEC_ARRAY	/* Want/got opt_sec... */
+STATMOUNT_MNT_UIDMAP	/* Want/got uidmap... */
+STATMOUNT_MNT_GIDMAP	/* Want/got gidmap... */
 .TE
 .in
 .P

@@ -302,6 +308,38 @@ .SS The returned information
 buffer that contains security options separated by null bytes.
 They can be iterated over with the help of
 .IR smbuf.opt_sec_num .
+.TP
+.IR smbuf.mnt_uidmap_num " (since Linux 6.15)"
+The number of uid mappings applied on the mount.
+If
+.I smbuf.mask
+has STATMOUNT_UIDMAP set and this field is 0,
+the mount is not an idmapped mount.
+.TP
+.IR smbuf.mnt_uidmap " (since Linux 6.15)"
+The offset to the location in the
+.I smbuf.str
+buffer that contains uid mappings as consecutive null-terminated strings.
+They can be iterated over with the help of
+.IR smbuf.mnt_uidmap_num .
+The whole range of uid mappings must be resolvable in the user namespace
+of the caller.
+.TP
+.IR smbuf.mnt_gidmap_num " (since Linux 6.15)"
+The number of gid mappings applied on the mount.
+If
+.I smbuf.mask
+has STATMOUNT_GIDMAP set and this field is 0,
+the mount is not an idmapped mount.
+.TP
+.IR smbuf.mnt_gidmap " (since Linux 6.15)"
+The offset to the location in the
+.I smbuf.str
+buffer that contains gid mappings as consecutive null-terminated strings.
+They can be iterated over with the help of
+.IR smbuf.mnt_gidmap_num .
+The whole range of gid mappings must be resolvable in the user namespace
+of the caller.
 .SH RETURN VALUE
 On success, zero is returned.
 On error, \-1 is returned, and

Signature · 1 line

2.53.0

Bhavik Sachdev

author

2026-02-22 14:52 UTC #8 lore raw

PATCH v2 8/10 man/man2/statmount.2: Document STATMOUNT_SUPPORTED_MASK

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8f6116b5b77b0536d2ad7482ee42bfe58b8fac01>

Document STATMOUNT_SUPPORTED_MASK flag and the corresponding field introduced by it. It is used to determine the flags supported by the current kernel. The text is based on this commit message [1].

---
 man/man2/statmount.2 | 7 +++++++
 1 file changed, 7 insertions(+)

man/man2/statmount.2 +7 -0

index e24827ad4..7951c8391 100644
@@ -60,6 +60,7 @@ .SH SYNOPSIS
 .B "    __u32  mnt_uidmap;"
 .B "    __u32  mnt_gidmap_num;"
 .B "    __u32  mnt_gidmap;"
+.B "    __u64  supported_mask;"
 .B "    char   str[];"
 .B };
 .EE

@@ -126,6 +127,7 @@ .SS The mnt_id_req structure
 STATMOUNT_OPT_SEC_ARRAY	/* Want/got opt_sec... */
 STATMOUNT_MNT_UIDMAP	/* Want/got uidmap... */
 STATMOUNT_MNT_GIDMAP	/* Want/got gidmap... */
+STATMOUNT_SUPPORTED_MASK	/* Want/got supported mask flags */
 .TE
 .in
 .P

@@ -340,6 +342,11 @@ .SS The returned information
 .IR smbuf.mnt_gidmap_num .
 The whole range of gid mappings must be resolvable in the user namespace
 of the caller.
+.TP
+.IR smbuf.supported_mask " (since Linux 6.15)"
+The ORed combination of
+.BI STATMOUNT_ *
+flags supported by the current kernel.
 .SH RETURN VALUE
 On success, zero is returned.
 On error, \-1 is returned, and

Signature · 1 line

2.53.0

Bhavik Sachdev

author

2026-02-22 14:52 UTC #9 lore raw

PATCH v2 9/10 man/man2/statmount.2: Document req.mnt_ns_fd

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9020d0d844ad58a051f90b1e5b82ba34123925b9>

Document the new mnt_ns_fd parameter to struct mnt_id_req.

req.mnt_ns_fd can be used to query for a mount in a foreign mount namespace.

The mnt_ns_fd parameter description is based on this commit message [1].

---
 man/man2/statmount.2 | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

man/man2/statmount.2 +21 -1

index 7951c8391..0b3a65bb3 100644
@@ -24,6 +24,7 @@ .SH SYNOPSIS
 .EX
 .B struct mnt_id_req {
 .BR "    __u32  size;" "        /* sizeof(struct mnt_id_req) */"
+.BR "    __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
 .BR "    __u64  mnt_id;" "      /* The mnt_id being queried */"
 .BR "    __u64  param;" "       /* An ORed combination of the STATMOUNT_ constants */"
 .BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"

@@ -74,7 +75,9 @@ .SH DESCRIPTION
 To access a mount's status,
 the caller must have CAP_SYS_ADMIN in the user namespace.
 In case of accessing a mount in a foreign mount namespace (specified via
-.IR req.mnt_ns_id ),
+.I req.mnt_ns_id
+or
+.IR req.mnt_ns_fd ),
 the foreign mount namespace should be child of the current namespace.
 .P
 This function returns information about a mount,

@@ -97,6 +100,16 @@ .SS The mnt_id_req structure
 it should always be set to
 .IR sizeof(struct\~mnt_id_req) .
 .P
+.I req.mnt_ns_fd
+can be obtained from
+.B PIDFD_GET_MNT_NAMESPACE
+.BR ioctl (2)
+operation or by opening a file descriptor to
+.IR /proc/ pid /ns/mnt
+and is used to specify a foreign mount namespace in which to query
+.I req.mnt_id
+(Since Linux 6.18).
+.P
 .I req.mnt_id
 can be obtained from either
 .BR statx (2)

@@ -372,6 +385,13 @@ .SH ERRORS
 .I req
 is of insufficient size to be utilized.
 .TP
+.B EINVAL
+Both
+.I req.mnt_ns_id
+and
+.I req.mnt_ns_fd
+were set.
+.TP
 .B E2BIG
 .I req
 is too large.

Signature · 1 line

2.53.0

Bhavik Sachdev

author

2026-02-22 14:52 UTC #10 lore raw

PATCH v2 10/10 man/man2/statmount.2: Document STATMOUNT_BY_FD

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?h=next-20260126&id=d5bc4e31f2a3f301b4214858bec834c67bb2be5c>

STATMOUNT_BY_FD introduces the ability to get information about a mount using a fd on the mount. This functionality is currently in linux-next [1].

---
 man/man2/statmount.2 | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

man/man2/statmount.2 +35 -2

index 0b3a65bb3..761df60dc 100644
@@ -24,7 +24,10 @@ .SH SYNOPSIS
 .EX
 .B struct mnt_id_req {
 .BR "    __u32  size;" "        /* sizeof(struct mnt_id_req) */"
-.BR "    __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "    union {"
+.BR "           __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "           __u32  mnt_fd;" "      /* fd on the mount being queried */"
+.BR "    };"
 .BR "    __u64  mnt_id;" "      /* The mnt_id being queried */"
 .BR "    __u64  param;" "       /* An ORed combination of the STATMOUNT_ constants */"
 .BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"

@@ -89,7 +92,7 @@ .SH DESCRIPTION
 .I bufsize
 with the fields filled in as described below.
 .I flags
-must be 0.
+must either be 0 or STATMOUNT_BY_FD.
 .P
 (Note that reserved space and padding is omitted.)
 .SS The mnt_id_req structure

@@ -110,6 +113,25 @@ .SS The mnt_id_req structure
 .I req.mnt_id
 (Since Linux 6.18).
 .P
+.I req.mnt_fd
+is a file descriptor on a mount.
+If STATMOUNT_BY_FD flag is specified,
+.I req.mnt_id
+and
+.I req.mnt_ns_id
+are zeroed, the function will return information about the mount the fd is on
+(Since Linux 7.0).
+.P
+The fd can also be on a mount that has been lazily unmounted (see
+.BR umount2 (2)
+with
+.BR MNT_DETACH ).
+In this case,
+.BR STATMOUNT_MNT_POINT
+and
+.BR STATMOUNT_MNT_NS_ID
+will be unset, since an unmounted mount is no longer a part of the filesystem.
+.P
 .I req.mnt_id
 can be obtained from either
 .BR statx (2)

@@ -392,6 +414,17 @@ .SH ERRORS
 .I req.mnt_ns_fd
 were set.
 .TP
+.B EINVAL
+.I req.mnt_id
+or
+.I req.mnt_ns_id
+was specified alongside
+.IR req.mnt_fd .
+.TP
+.B EBADF
+.I req.mnt_fd
+is an invalid file descriptor.
+.TP
 .B E2BIG
 .I req
 is too large.

Signature · 1 line

2.53.0

Alejandro Colomar

maintainer

2026-02-22 21:52 UTC #11 lore raw

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=37c4a9590e1efcae7749682239fc22a330d2d325>

Hi Bhavik,

Quoted 29 lines · Bhavik Sachdev

Hello Everyone!
This patchset documents all the new features added to statmount() since the syscall was first introduced.
I did work on the STATMOUNT_BY_FD changes and I have tried my best to understand rest of the features and document them.
I have cc'ed the authors of all these new features, please let me know your thoughts.
This patchset is also available on github [1].
Changes from v1 to v2:
- Appended Linux versions to document in which version new fields were introduced.
- Fixed formatting issues and wording problems pointed out by Alex during review.
- Reworded STATMOUNT_{UIDMAP,GIDMAP} to better explain the feature.
Changes from RFC to v1:
- Document req.mnt_ns_fd as well.
- Fix all lint errors generated by running
make -R lint-man man/man2/statmount.2.
[1]: https://github.com/bsach64/man-pages/tree/man.2.statmount.v2

Quoted 7 lines

Bhavik Sachdev (10):
man/man2/statmount.2: Document req.mnt_ns_id and STATMOUNT_MNT_NS_ID man/man2/statmount.2: Document STATMOUNT_MNT_OPTS man/man2/statmount.2: Document STATMOUNT_FS_SUBTYPE man/man2/statmount.2: Document STATMOUNT_SB_SOURCE man/man2/statmount.2: Document STATMOUNT_OPT_ARRAY man/man2/statmount.2: Document STATMOUNT_OPT_SEC_ARRAY

I've applied patches 1..6. They're in a branch:
<https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/?h=statmount>

I've done some amendments to patches 5 and 6, and the following ones don't apply anymore. Would you mind rebasing and sending the rest?

Have a lovely night!
Alex

Quoted 12 lines

man/man2/statmount.2: Document STATMOUNT_{UIDMAP,GIDMAP} man/man2/statmount.2: Document STATMOUNT_SUPPORTED_MASK man/man2/statmount.2: Document req.mnt_ns_fd man/man2/statmount.2: Document STATMOUNT_BY_FD
 man/man2/statmount.2 | 176 ++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 172 insertions(+), 4 deletions(-)
Signature · 1 line
2.53.0

Signature · 1 line

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Bhavik Sachdev

author

2026-02-23 14:13 UTC #12 lore raw

PATCH v2 1/4 man/man2/statmount.2: Document STATMOUNT_{UIDMAP,GIDMAP}

Document the STATMOUNT_{UIDMAP,GIDMAP} flags and the corresponding fields introduced by them. Most of this is based on the commit message provided here [1].

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?h=next-20260126&id=d5bc4e31f2a3f301b4214858bec834c67bb2be5c>

Message-ID: <6146fa1d59006387ad7996f28f5651787125d956.1771855135.git.b.sachdev1904@gmail.com>
---
Hey Alex!

Hopefully this is what you wanted!

Thanks,
Bhavik

 man/man2/statmount.2 | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

man/man2/statmount.2 +38 -0

index f8d2302ef..f5a3721eb 100644
@@ -56,6 +56,10 @@ .SH SYNOPSIS
 .B "    __u32  opt_array;"
 .B "    __u32  opt_sec_num;"
 .B "    __u32  opt_sec_array;"
+.B "    __u32  mnt_uidmap_num;"
+.B "    __u32  mnt_uidmap;"
+.B "    __u32  mnt_gidmap_num;"
+.B "    __u32  mnt_gidmap;"
 .B "    char   str[];"
 .B };
 .EE

@@ -120,6 +124,8 @@ .SS The mnt_id_req structure
 STATMOUNT_SB_SOURCE	/* Want/got sb_source */
 STATMOUNT_OPT_ARRAY	/* Want/got opt_{num,array} */
 STATMOUNT_OPT_SEC_ARRAY	/* Want/got opt_sec_{num,array} */
+STATMOUNT_MNT_UIDMAP	/* Want/got uidmap... */
+STATMOUNT_MNT_GIDMAP	/* Want/got gidmap... */
 .TE
 .in
 .P

@@ -302,6 +308,38 @@ .SS The returned information
 buffer that contains security options separated by null bytes.
 They can be iterated over with the help of
 .IR smbuf.opt_sec_num .
+.TP
+.IR smbuf.mnt_uidmap_num " (since Linux 6.15)"
+The number of uid mappings applied on the mount.
+If
+.I smbuf.mask
+has STATMOUNT_UIDMAP set and this field is 0,
+the mount is not an idmapped mount.
+.TP
+.IR smbuf.mnt_uidmap " (since Linux 6.15)"
+The offset to the location in the
+.I smbuf.str
+buffer that contains uid mappings as consecutive null-terminated strings.
+They can be iterated over with the help of
+.IR smbuf.mnt_uidmap_num .
+The whole range of uid mappings must be resolvable in the user namespace
+of the caller.
+.TP
+.IR smbuf.mnt_gidmap_num " (since Linux 6.15)"
+The number of gid mappings applied on the mount.
+If
+.I smbuf.mask
+has STATMOUNT_GIDMAP set and this field is 0,
+the mount is not an idmapped mount.
+.TP
+.IR smbuf.mnt_gidmap " (since Linux 6.15)"
+The offset to the location in the
+.I smbuf.str
+buffer that contains gid mappings as consecutive null-terminated strings.
+They can be iterated over with the help of
+.IR smbuf.mnt_gidmap_num .
+The whole range of gid mappings must be resolvable in the user namespace
+of the caller.
 .SH RETURN VALUE
 On success, zero is returned.
 On error, \-1 is returned, and

Signature · 1 line

2.53.0

Alejandro Colomar

maintainer

2026-02-23 15:41 UTC #13 lore raw

Re: [PATCH v2 1/4] man/man2/statmount.2: Document STATMOUNT_{UIDMAP,GIDMAP}

Hi Bhavik,

Quoted 7 lines · Bhavik Sachdev

Document the STATMOUNT_{UIDMAP,GIDMAP} flags and the corresponding fields introduced by them. Most of this is based on the commit message provided here [1].
Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=37c4a9590e1efcae7749682239fc22a330d2d325>

Quoted 6 lines

Signed-off-by Bhavik Sachdev <[email protected]>
Message-ID: <6146fa1d59006387ad7996f28f5651787125d956.1771855135.git.b.sachdev1904@gmail.com>
---
Hey Alex!
Hopefully this is what you wanted!

Yup, that was it. :)

I've applied patches v2 1..3. I've done some minor amends, and patch 4 doesn't apply. Would you mind rebasing that one?

Have a lovely day!
Alex

Quoted 73 lines

Thanks,
Bhavik

 man/man2/statmount.2 | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

man/man2/statmount.2 +38 -0

index f8d2302ef..f5a3721eb 100644
@@ -56,6 +56,10 @@ .SH SYNOPSIS
 .B "    __u32  opt_array;"
 .B "    __u32  opt_sec_num;"
 .B "    __u32  opt_sec_array;"
+.B "    __u32  mnt_uidmap_num;"
+.B "    __u32  mnt_uidmap;"
+.B "    __u32  mnt_gidmap_num;"
+.B "    __u32  mnt_gidmap;"
 .B "    char   str[];"
 .B };
 .EE

@@ -120,6 +124,8 @@ .SS The mnt_id_req structure
 STATMOUNT_SB_SOURCE	/* Want/got sb_source */
 STATMOUNT_OPT_ARRAY	/* Want/got opt_{num,array} */
 STATMOUNT_OPT_SEC_ARRAY	/* Want/got opt_sec_{num,array} */
+STATMOUNT_MNT_UIDMAP	/* Want/got uidmap... */
+STATMOUNT_MNT_GIDMAP	/* Want/got gidmap... */
 .TE
 .in
 .P

@@ -302,6 +308,38 @@ .SS The returned information
 buffer that contains security options separated by null bytes.
 They can be iterated over with the help of
 .IR smbuf.opt_sec_num .
+.TP
+.IR smbuf.mnt_uidmap_num " (since Linux 6.15)"
+The number of uid mappings applied on the mount.
+If
+.I smbuf.mask
+has STATMOUNT_UIDMAP set and this field is 0,
+the mount is not an idmapped mount.
+.TP
+.IR smbuf.mnt_uidmap " (since Linux 6.15)"
+The offset to the location in the
+.I smbuf.str
+buffer that contains uid mappings as consecutive null-terminated strings.
+They can be iterated over with the help of
+.IR smbuf.mnt_uidmap_num .
+The whole range of uid mappings must be resolvable in the user namespace
+of the caller.
+.TP
+.IR smbuf.mnt_gidmap_num " (since Linux 6.15)"
+The number of gid mappings applied on the mount.
+If
+.I smbuf.mask
+has STATMOUNT_GIDMAP set and this field is 0,
+the mount is not an idmapped mount.
+.TP
+.IR smbuf.mnt_gidmap " (since Linux 6.15)"
+The offset to the location in the
+.I smbuf.str
+buffer that contains gid mappings as consecutive null-terminated strings.
+They can be iterated over with the help of
+.IR smbuf.mnt_gidmap_num .
+The whole range of gid mappings must be resolvable in the user namespace
+of the caller.
 .SH RETURN VALUE
 On success, zero is returned.
 On error, \-1 is returned, and

Signature · 1 line

2.53.0

Signature · 1 line

<https://www.alejandro-colomar.es>

Bhavik Sachdev

author

2026-02-23 18:17 UTC #14 lore raw

PATCH v2 1/1 man/man2/statmount.2: Document STATMOUNT_BY_FD

STATMOUNT_BY_FD introduces the ability to get information about a mount using a fd on the mount. This functionality is currently in linux-next [1].

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?h=next-20260126&id=d5bc4e31f2a3f301b4214858bec834c67bb2be5c>

Message-ID: <57c96336ccfbdc05f60b7875c315a8c1dd0d14b8.1771870334.git.b.sachdev1904@gmail.com>

---
 man/man2/statmount.2 | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

man/man2/statmount.2 +35 -2

index 5ac96796c..a5b135bee 100644
@@ -24,7 +24,10 @@ .SH SYNOPSIS
 .EX
 .B struct mnt_id_req {
 .BR "    __u32  size;" "        /* sizeof(struct mnt_id_req) */"
-.BR "    __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "    union {"
+.BR "           __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "           __u32  mnt_fd;" "      /* fd on the mount being queried */"
+.BR "    };"
 .BR "    __u64  mnt_id;" "      /* The mnt_id being queried */"
 .BR "    __u64  param;" "       /* ORed combination of the STATMOUNT_ constants */"
 .BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"

@@ -89,7 +92,7 @@ .SH DESCRIPTION
 .I bufsize
 with the fields filled in as described below.
 .I flags
-must be 0.
+must either be 0 or STATMOUNT_BY_FD.
 .P
 (Note that reserved space and padding is omitted.)
 .SS The mnt_id_req structure

@@ -110,6 +113,25 @@ .SS The mnt_id_req structure
 .I req.mnt_id
 (since Linux 6.18).
 .P
+.I req.mnt_fd
+is a file descriptor on a mount.
+If STATMOUNT_BY_FD flag is specified,
+.I req.mnt_id
+and
+.I req.mnt_ns_id
+are zeroed, the function will return information about the mount the fd is on
+(Since Linux 7.0).
+.P
+The fd can also be on a mount that has been lazily unmounted (see
+.BR umount2 (2)
+with
+.BR MNT_DETACH ).
+In this case,
+.BR STATMOUNT_MNT_POINT
+and
+.BR STATMOUNT_MNT_NS_ID
+will be unset, since an unmounted mount is no longer a part of the filesystem.
+.P
 .I req.mnt_id
 can be obtained from either
 .BR statx (2)

@@ -392,6 +414,17 @@ .SH ERRORS
 .I req.mnt_ns_fd
 were set.
 .TP
+.B EINVAL
+.I req.mnt_id
+or
+.I req.mnt_ns_id
+was specified alongside
+.IR req.mnt_fd .
+.TP
+.B EBADF
+.I req.mnt_fd
+is an invalid file descriptor.
+.TP
 .B E2BIG
 .I req
 is too large.

Signature · 1 line

2.53.0

Alejandro Colomar

maintainer

2026-02-24 00:21 UTC #15 lore raw

Re: [PATCH v2] man/man2/statmount.2: Document STATMOUNT_BY_FD

Hi Bhavik,

On 2026-02-23T23:47:03+0530, Bhavik Sachdev wrote:
[...]

Quoted 17 lines

man/man2/statmount.2 +4 -1

index 5ac96796c..a5b135bee 100644
@@ -24,7 +24,10 @@ .SH SYNOPSIS
 .EX
 .B struct mnt_id_req {
 .BR "    __u32  size;" "        /* sizeof(struct mnt_id_req) */"
-.BR "    __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "    union {"
+.BR "           __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "           __u32  mnt_fd;" "      /* fd on the mount being queried */"
+.BR "    };"
 .BR "    __u64  mnt_id;" "      /* The mnt_id being queried */"
 .BR "    __u64  param;" "       /* ORed combination of the STATMOUNT_ constants */"
 .BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"

man/man2/statmount.2 .SH DESCRIPTION · line 95

Quoted 5 lines

man/man2/statmount.2 +1 -1

@@ -89,7 +92,7 @@ .SH DESCRIPTION
 .I bufsize
 with the fields filled in as described below.
 .I flags
-must be 0.
+must either be 0 or STATMOUNT_BY_FD.

The macro should go in bold (.B, .BR) here and elsewhere.

Also, should we say the same as elsewhere?:

"It is an ORed combination of the following constants"

and then a list which contains only STATMOUNT_BY_FD?

Have a lovely night!
Alex

Quoted 50 lines

man/man2/statmount.2 +30 -0

 .P
 (Note that reserved space and padding is omitted.)
 .SS The mnt_id_req structure

@@ -110,6 +113,25 @@ .SS The mnt_id_req structure
 .I req.mnt_id
 (since Linux 6.18).
 .P
+.I req.mnt_fd
+is a file descriptor on a mount.
+If STATMOUNT_BY_FD flag is specified,
+.I req.mnt_id
+and
+.I req.mnt_ns_id
+are zeroed, the function will return information about the mount the fd is on
+(Since Linux 7.0).
+.P
+The fd can also be on a mount that has been lazily unmounted (see
+.BR umount2 (2)
+with
+.BR MNT_DETACH ).
+In this case,
+.BR STATMOUNT_MNT_POINT
+and
+.BR STATMOUNT_MNT_NS_ID
+will be unset, since an unmounted mount is no longer a part of the filesystem.
+.P
 .I req.mnt_id
 can be obtained from either
 .BR statx (2)

@@ -392,6 +414,17 @@ .SH ERRORS
 .I req.mnt_ns_fd
 were set.
 .TP
+.B EINVAL
+.I req.mnt_id
+or
+.I req.mnt_ns_id
+was specified alongside
+.IR req.mnt_fd .
+.TP
+.B EBADF
+.I req.mnt_fd
+is an invalid file descriptor.
+.TP
 .B E2BIG
 .I req
 is too large.

Signature · 1 line

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Signature · 1 line

<https://www.alejandro-colomar.es>

Bhavik Sachdev

author

2026-02-26 03:15 UTC #16 lore raw

PATCH v3 1/1 man/man2/statmount.2: Document STATMOUNT_BY_FD

STATMOUNT_BY_FD introduces the ability to get information about a mount using a fd on the mount. This functionality is currently in linux-next [1].

Message-ID: <57c96336ccfbdc05f60b7875c315a8c1dd0d14b8.1771870334.git.b.sachdev1904@gmail.com>
Message-ID: <7d4b22c595feeadb3be6df8a8781344597120f7e.1771870502.git.b.sachdev1904@gmail.com>
---
Hey Alex!

Quoted 3 lines

Also, should we say the same as elsewhere?:
"It is an ORed combination of the following constants"
and then a list which contains only STATMOUNT_BY_FD?

I am not really sure that statmount flags will be a ORed combination in the future i.e (STATMOUNT_BY_FD | STATMOUNT_NEW_FLAG) would be something that is valid.

I think for now, it is better we don't do that.

Thanks,
Bhavik

 man/man2/statmount.2 | 40 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)

man/man2/statmount.2 +38 -2

index 5ac96796c..1556342de 100644
@@ -24,7 +24,10 @@ .SH SYNOPSIS
 .EX
 .B struct mnt_id_req {
 .BR "    __u32  size;" "        /* sizeof(struct mnt_id_req) */"
-.BR "    __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "    union {"
+.BR "           __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "           __u32  mnt_fd;" "      /* fd on the mount being queried */"
+.BR "    };"
 .BR "    __u64  mnt_id;" "      /* The mnt_id being queried */"
 .BR "    __u64  param;" "       /* ORed combination of the STATMOUNT_ constants */"
 .BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"

@@ -89,7 +92,8 @@ .SH DESCRIPTION
 .I bufsize
 with the fields filled in as described below.
 .I flags
-must be 0.
+must either be 0 or
+.BR STATMOUNT_BY_FD .
 .P
 (Note that reserved space and padding is omitted.)
 .SS The mnt_id_req structure

@@ -110,6 +114,27 @@ .SS The mnt_id_req structure
 .I req.mnt_id
 (since Linux 6.18).
 .P
+.I req.mnt_fd
+is a file descriptor on a mount.
+If
+.B STATMOUNT_BY_FD
+flag is specified,
+.I req.mnt_id
+and
+.I req.mnt_ns_id
+are zeroed, the function will return information about the mount the fd is on
+(Since Linux 7.0).
+.P
+The fd can also be on a mount that has been lazily unmounted (see
+.BR umount2 (2)
+with
+.BR MNT_DETACH ).
+In this case,
+.BR STATMOUNT_MNT_POINT
+and
+.BR STATMOUNT_MNT_NS_ID
+will be unset, since an unmounted mount is no longer a part of the filesystem.
+.P
 .I req.mnt_id
 can be obtained from either
 .BR statx (2)

@@ -392,6 +417,17 @@ .SH ERRORS
 .I req.mnt_ns_fd
 were set.
 .TP
+.B EINVAL
+.I req.mnt_id
+or
+.I req.mnt_ns_id
+was specified alongside
+.IR req.mnt_fd .
+.TP
+.B EBADF
+.I req.mnt_fd
+is an invalid file descriptor.
+.TP
 .B E2BIG
 .I req
 is too large.

Signature · 1 line

2.53.0

Alejandro Colomar

maintainer

2026-03-04 14:58 UTC #17 lore raw

Re: [PATCH v3] man/man2/statmount.2: Document STATMOUNT_BY_FD

Hi Bhavik,

Sorry for the delay; I had an issue with my mail provider. It's now resolved.

Quoted 56 lines · Bhavik Sachdev

STATMOUNT_BY_FD introduces the ability to get information about a mount using a fd on the mount. This functionality is currently in linux-next [1].
Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?h=next-20260126&id=d5bc4e31f2a3f301b4214858bec834c67bb2be5c>
Signed-off-by Bhavik Sachdev <[email protected]>
Message-ID: <57c96336ccfbdc05f60b7875c315a8c1dd0d14b8.1771870334.git.b.sachdev1904@gmail.com>
Message-ID: <7d4b22c595feeadb3be6df8a8781344597120f7e.1771870502.git.b.sachdev1904@gmail.com>
---
Hey Alex!
Quoted 3 lines
Also, should we say the same as elsewhere?:
"It is an ORed combination of the following constants"
and then a list which contains only STATMOUNT_BY_FD?
I am not really sure that statmount flags will be a ORed combination in the future i.e (STATMOUNT_BY_FD | STATMOUNT_NEW_FLAG) would be something that is valid.
I think for now, it is better we don't do that.
Thanks,
Bhavik
 man/man2/statmount.2 | 40 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)
man/man2/statmount.2 +6 -2
index 5ac96796c..1556342de 100644
@@ -24,7 +24,10 @@ .SH SYNOPSIS
 .EX
 .B struct mnt_id_req {
 .BR "    __u32  size;" "        /* sizeof(struct mnt_id_req) */"
-.BR "    __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "    union {"
+.BR "           __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "           __u32  mnt_fd;" "      /* fd on the mount being queried */"
+.BR "    };"
 .BR "    __u64  mnt_id;" "      /* The mnt_id being queried */"
 .BR "    __u64  param;" "       /* ORed combination of the STATMOUNT_ constants */"
 .BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"
@@ -89,7 +92,8 @@ .SH DESCRIPTION
 .I bufsize
 with the fields filled in as described below.
 .I flags
-must be 0.
+must either be 0 or
+.BR STATMOUNT_BY_FD .
 .P
 (Note that reserved space and padding is omitted.)
 .SS The mnt_id_req structure

man/man2/statmount.2 .SS The mnt_id_req structure · line 118

Quoted 5 lines

man/man2/statmount.2 +2 -0

@@ -110,6 +114,27 @@ .SS The mnt_id_req structure
 .I req.mnt_id
 (since Linux 6.18).
 .P
+.I req.mnt_fd
+is a file descriptor on a mount.

Is this the same as a "mount object file descriptor" as describer in fsopen(2)? If so, we should use the same language, I think.

CC += Aleksa, Askar

man/man2/statmount.2 .SS The mnt_id_req structure · line 125

Quoted 7 lines

man/man2/statmount.2 +7 -0

+If
+.B STATMOUNT_BY_FD
+flag is specified,
+.I req.mnt_id
+and
+.I req.mnt_ns_id
+are zeroed, the function will return information about the mount the fd is on

We always spell "file descriptor", not fd.

Aleksa, Askar, would you mind reviewing this patch? You may have comments on some specific terms used here, as they might relate to fsopen(2).

man/man2/statmount.2 .SS The mnt_id_req structure · line 126

Quoted 1 line

man/man2/statmount.2 +1 -0
+(Since Linux 7.0).

s/Since/since/

man/man2/statmount.2 .SS The mnt_id_req structure · line 133

Quoted 7 lines

man/man2/statmount.2 +7 -0

+.P
+The fd can also be on a mount that has been lazily unmounted (see
+.BR umount2 (2)
+with
+.BR MNT_DETACH ).
+In this case,
+.BR STATMOUNT_MNT_POINT

s/BR/B/

BR is for alternating bold and roman.

Other than the questios/doubts about mounts and file descriptors, and these minor formatting/source issues (which I would have amended otherwise), the patch looks good to me.

Have a lovely day!
Alex

Quoted 28 lines

man/man2/statmount.2 +15 -0

+and
+.BR STATMOUNT_MNT_NS_ID
+will be unset, since an unmounted mount is no longer a part of the filesystem.
+.P
 .I req.mnt_id
 can be obtained from either
 .BR statx (2)

@@ -392,6 +417,17 @@ .SH ERRORS
 .I req.mnt_ns_fd
 were set.
 .TP
+.B EINVAL
+.I req.mnt_id
+or
+.I req.mnt_ns_id
+was specified alongside
+.IR req.mnt_fd .
+.TP
+.B EBADF
+.I req.mnt_fd
+is an invalid file descriptor.
+.TP
 .B E2BIG
 .I req
 is too large.

Signature · 1 line

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Signature · 1 line

<https://www.alejandro-colomar.es>

Askar Safin

2026-03-05 00:15 UTC #18 lore raw

Re: [PATCH v3] man/man2/statmount.2: Document STATMOUNT_BY_FD

Alejandro Colomar <[email protected]>:

Quoted 2 lines

Is this the same as a "mount object file descriptor" as describer in fsopen(2)? If so, we should use the same language, I think.

The term "mount object file descriptor" is probably confusing. In fact fsmount simply creates detached mount and returns O_PATH fd, referring to the root of the new detached mount. (Note: I'm talking about fsmount here, not fsopen.) This fd is similar to what open(O_PATH)
creates. So probably we should just replace this "mount object file descriptor" in fsopen(2) with "O_PATH fd".

The only difference between fd, returned by fsmount, and normal O_PATH descriptor is that fd, returned by fsmount, has FMODE_NEED_UNMOUNT flag on file description:
https://elixir.bootlin.com/linux/v7.0-rc1/source/fs/namespace.c#L4495 .

This FMODE_NEED_UNMOUNT means that when we close fd, the mount is unmounted.

This statmount patch refers to any fd inside mount in question. I. e. any fd referring to any file inside the mount. This may include O_PATH descriptors and, yes, descriptors returned by fsmount. So, current wording in this statmount patch is correct.

Signature · 1 line

Askar Safin

Alejandro Colomar

maintainer

2026-03-05 00:43 UTC #19 lore raw

Re: [PATCH v3] man/man2/statmount.2: Document STATMOUNT_BY_FD

Quoted 10 lines · Askar Safin

Alejandro Colomar <[email protected]>:
Quoted 2 lines
Is this the same as a "mount object file descriptor" as describer in fsopen(2)? If so, we should use the same language, I think.
The term "mount object file descriptor" is probably confusing. In fact fsmount simply creates detached mount and returns O_PATH fd, referring to the root of the new detached mount. (Note: I'm talking about fsmount here, not fsopen.) This fd is similar to what open(O_PATH)
creates. So probably we should just replace this "mount object file descriptor" in fsopen(2) with "O_PATH fd".

Except for expanding fd by file descriptor, what you say sounds reasonable to me.

Quoted 6 lines

The only difference between fd, returned by fsmount, and normal O_PATH descriptor is that fd, returned by fsmount, has FMODE_NEED_UNMOUNT flag on file description:
https://elixir.bootlin.com/linux/v7.0-rc1/source/fs/namespace.c#L4495 .

Quoted 7 lines

This FMODE_NEED_UNMOUNT means that when we close fd, the mount is unmounted.
This statmount patch refers to any fd inside mount in question. I. e. any fd referring to any file inside the mount. This may include O_PATH descriptors and, yes, descriptors returned by fsmount. So, current wording in this statmount patch is correct.

Thanks for having a look! And thanks for confirming it's correct!

Have a lovely night!
Alex

Quoted 4 lines

Signature · 2 lines
-- 
Askar Safin

Signature · 1 line

<https://www.alejandro-colomar.es>

Aleksa Sarai

2026-03-05 02:18 UTC #20 lore raw

Re: [PATCH v3] man/man2/statmount.2: Document STATMOUNT_BY_FD

In reply to Askar Safin

Quoted 10 lines · Askar Safin

Alejandro Colomar <[email protected]>:
Quoted 2 lines
Is this the same as a "mount object file descriptor" as describer in fsopen(2)? If so, we should use the same language, I think.
The term "mount object file descriptor" is probably confusing. In fact fsmount simply creates detached mount and returns O_PATH fd, referring to the root of the new detached mount. (Note: I'm talking about fsmount here, not fsopen.) This fd is similar to what open(O_PATH)
creates. So probably we should just replace this "mount object file descriptor" in fsopen(2) with "O_PATH fd".

No. From the perspective of userspace these are different types of objects, calling them both O_PATH file descriptors is just more confusing.

The fact that they are "just" an O_PATH to a root mount in an anonymous namespace with FMODE_NEED_UNMOUNT is an implementation detail, users reading the docs will be confused if we start referring to two objects with different semantics as "O_PATH file descriptors" (especially once you consider that O_PATH file descriptor semantics are also woefully under-specified).

Quoted 4 lines

This statmount patch refers to any fd inside mount in question. I. e. any fd referring to any file inside the mount. This may include O_PATH descriptors and, yes, descriptors returned by fsmount. So, current wording in this statmount patch is correct.

I agree.

Signature · 2 lines

Aleksa Sarai
https://www.cyphar.com/

Alejandro Colomar

maintainer

2026-03-05 17:19 UTC #21 lore raw

Re: [PATCH v3] man/man2/statmount.2: Document STATMOUNT_BY_FD

Hi,

Quoted 16 lines · Alejandro Colomar

Hi Bhavik,
Sorry for the delay; I had an issue with my mail provider. It's now resolved.
Quoted 10 lines · Bhavik Sachdev
STATMOUNT_BY_FD introduces the ability to get information about a mount using a fd on the mount. This functionality is currently in linux-next [1].
Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?h=next-20260126&id=d5bc4e31f2a3f301b4214858bec834c67bb2be5c>
Signed-off-by Bhavik Sachdev <[email protected]>
Message-ID: <57c96336ccfbdc05f60b7875c315a8c1dd0d14b8.1771870334.git.b.sachdev1904@gmail.com>
Message-ID: <7d4b22c595feeadb3be6df8a8781344597120f7e.1771870502.git.b.sachdev1904@gmail.com>

I've applied the patch. I've amended the few formatting issues I reported below. Since the only important issue was the wording one, and Askar confirmed it's good, I took the patch.

BTW, Branden, I'm getting a diagnostic from groff:

	TROFF		.tmp/man/man2/statmount.2.pdf.set
	! (troff -mandoc -Tpdf -wbreak -rS12 -rU0  <.tmp/man/man2/statmount.2.pdf.troff 2>&1 >.tmp/man/man2/statmount.2.pdf.set) \
	| grep ^ >&2
	.tmp/man/man2/statmount.2:155: warning: table row does not fit on page 2

But the PDF looks good to me. Would you mind looking at that page and telling me what you'd do with that diagnostic, and whether I should care about it?

Have a lovely day!
Alex

Quoted 127 lines

Quoted 46 lines
---
Hey Alex!
Quoted 3 lines
Also, should we say the same as elsewhere?:
"It is an ORed combination of the following constants"
and then a list which contains only STATMOUNT_BY_FD?
I am not really sure that statmount flags will be a ORed combination in the future i.e (STATMOUNT_BY_FD | STATMOUNT_NEW_FLAG) would be something that is valid.
I think for now, it is better we don't do that.
Thanks,
Bhavik
 man/man2/statmount.2 | 40 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)
man/man2/statmount.2 +6 -2
index 5ac96796c..1556342de 100644
@@ -24,7 +24,10 @@ .SH SYNOPSIS
 .EX
 .B struct mnt_id_req {
 .BR "    __u32  size;" "        /* sizeof(struct mnt_id_req) */"
-.BR "    __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "    union {"
+.BR "           __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "           __u32  mnt_fd;" "      /* fd on the mount being queried */"
+.BR "    };"
 .BR "    __u64  mnt_id;" "      /* The mnt_id being queried */"
 .BR "    __u64  param;" "       /* ORed combination of the STATMOUNT_ constants */"
 .BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"
@@ -89,7 +92,8 @@ .SH DESCRIPTION
 .I bufsize
 with the fields filled in as described below.
 .I flags
-must be 0.
+must either be 0 or
+.BR STATMOUNT_BY_FD .
 .P
 (Note that reserved space and padding is omitted.)
 .SS The mnt_id_req structure
man/man2/statmount.2 .SS The mnt_id_req structure · line 118
Quoted 5 lines
man/man2/statmount.2 +2 -0
@@ -110,6 +114,27 @@ .SS The mnt_id_req structure
 .I req.mnt_id
 (since Linux 6.18).
 .P
+.I req.mnt_fd
+is a file descriptor on a mount.
Is this the same as a "mount object file descriptor" as describer in fsopen(2)? If so, we should use the same language, I think.
CC += Aleksa, Askar
man/man2/statmount.2 .SS The mnt_id_req structure · line 125
Quoted 7 lines
man/man2/statmount.2 +7 -0
+If
+.B STATMOUNT_BY_FD
+flag is specified,
+.I req.mnt_id
+and
+.I req.mnt_ns_id
+are zeroed, the function will return information about the mount the fd is on
We always spell "file descriptor", not fd.
Aleksa, Askar, would you mind reviewing this patch? You may have comments on some specific terms used here, as they might relate to fsopen(2).
man/man2/statmount.2 .SS The mnt_id_req structure · line 126
Quoted 1 line
man/man2/statmount.2 +1 -0
+(Since Linux 7.0).
s/Since/since/
man/man2/statmount.2 .SS The mnt_id_req structure · line 133
Quoted 7 lines
man/man2/statmount.2 +7 -0
+.P
+The fd can also be on a mount that has been lazily unmounted (see
+.BR umount2 (2)
+with
+.BR MNT_DETACH ).
+In this case,
+.BR STATMOUNT_MNT_POINT
s/BR/B/
BR is for alternating bold and roman.
Other than the questios/doubts about mounts and file descriptors, and these minor formatting/source issues (which I would have amended otherwise), the patch looks good to me.
Have a lovely day!
Alex
Quoted 28 lines
man/man2/statmount.2 +15 -0
+and
+.BR STATMOUNT_MNT_NS_ID
+will be unset, since an unmounted mount is no longer a part of the filesystem.
+.P
 .I req.mnt_id
 can be obtained from either
 .BR statx (2)
@@ -392,6 +417,17 @@ .SH ERRORS
 .I req.mnt_ns_fd
 were set.
 .TP
+.B EINVAL
+.I req.mnt_id
+or
+.I req.mnt_ns_id
+was specified alongside
+.IR req.mnt_fd .
+.TP
+.B EBADF
+.I req.mnt_fd
+is an invalid file descriptor.
+.TP
 .B E2BIG
 .I req
 is too large.
Signature · 1 line
2.53.0
Signature · 1 line
<https://www.alejandro-colomar.es>

Signature · 1 line

<https://www.alejandro-colomar.es>

G. Branden Robinson

2026-03-05 22:24 UTC #22 lore raw

Re: [PATCH v3] man/man2/statmount.2: Document STATMOUNT_BY_FD

[CC list trimmed way down since this is a *roff/formatting issue]

Hi Alex,

Quoted 10 lines · Alejandro Colomar

BTW, Branden, I'm getting a diagnostic from groff:
	TROFF		.tmp/man/man2/statmount.2.pdf.set
	! (troff -mandoc -Tpdf -wbreak -rS12 -rU0  <.tmp/man/man2/statmount.2.pdf.troff 2>&1 >.tmp/man/man2/statmount.2.pdf.set) \
	| grep ^ >&2
	.tmp/man/man2/statmount.2:155: warning: table row does not fit on page 2
But the PDF looks good to me. Would you mind looking at that page and telling me what you'd do with that diagnostic, and whether I should care about it?

I can't reproduce this diagnostic with groff 1.23.0 or 1.24.0.

Quoted 1 line

.tmp/man/man2/statmount.2:155: warning: table row does not fit on page 2

This warning means that a row of the table might be vertically encroaching into the footer margin; in extreme cases, might overprint the page footer itself; and in really loony cases, format text beyond the bottom of the page.

My copy of the document is this one:

	commit 7178f0e15db66a3616a900fc8a4007466633159b
	Author: Bhavik Sachdev <[email protected]>
	Date:   Thu Feb 26 08:40:16 2026 +0530

$ ~/groff-1.24.0/bin/groff -t -ww -mandoc -Tpdf -rS12 -rU0 -z \
  man/man2/statmount.2 2>&1 | grep . || echo NO DIAGNOSTICS
NO DIAGNOSTICS
$ ~/groff-1.23.0/bin/groff -t -ww -mandoc -Tpdf -rS12 -rU0 -z \
  man/man2/statmount.2 2>&1 | grep . || echo NO DIAGNOSTICS
NO DIAGNOSTICS

As you can see, I've turned on _all_ warnings (-ww).

Looking at the table source:

$ sed -n '154,171p' man/man2/statmount.2
.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 */
STATMOUNT_MNT_NS_ID     /* Want/got mnt_ns_id */
STATMOUNT_MNT_OPTS      /* Want/got mnt_opts */
STATMOUNT_FS_SUBTYPE    /* Want/got fs_subtype */
STATMOUNT_SB_SOURCE     /* Want/got sb_source */
STATMOUNT_OPT_ARRAY     /* Want/got opt_{num,array} */
STATMOUNT_OPT_SEC_ARRAY /* Want/got opt_sec_{num,array} */
STATMOUNT_MNT_UIDMAP    /* Want/got uidmap{_num,} */
STATMOUNT_MNT_GIDMAP    /* Want/got gidmap{_num,} */
STATMOUNT_SUPPORTED_MASK        /* Want/got supported mask flags */
.TE

...I don't see any reason why you should get that diagnostic.

1. You're not using any text blocks. Text blocks are formatted in a diversion and contain (line) breaks, so they can get "tall".

2. You're not using any other features to make a table row "tall", for example by performing "local motion" with the \v escape sequence. Formatting equations in a table cell is the more traditional means of encountering this problem.

Can you reproduce this problem from the command line, without using your Makefile?

Regards,
Branden

Alejandro Colomar

maintainer

2026-03-05 22:45 UTC #23 lore raw

Re: [PATCH v3] man/man2/statmount.2: Document STATMOUNT_BY_FD

Hi Branden,

Quoted 12 lines · G. Branden Robinson

[CC list trimmed way down since this is a *roff/formatting issue]
Hi Alex,
Quoted 7 lines · Alejandro Colomar
BTW, Branden, I'm getting a diagnostic from groff:
	TROFF		.tmp/man/man2/statmount.2.pdf.set
	! (troff -mandoc -Tpdf -wbreak -rS12 -rU0  <.tmp/man/man2/statmount.2.pdf.troff 2>&1 >.tmp/man/man2/statmount.2.pdf.set) \
	| grep ^ >&2
	.tmp/man/man2/statmount.2:155: warning: table row does not fit on page 2

Quoted 5 lines

Quoted 3 lines
But the PDF looks good to me. Would you mind looking at that page and telling me what you'd do with that diagnostic, and whether I should care about it?
I can't reproduce this diagnostic with groff 1.23.0 or 1.24.0.

Huh... I'm reproducing it on my server, but not on my desktop. On my server, I have:

	GNU troff (groff) version 1.23.0.1254-19a18

On my desktop, I have:

	GNU grops (groff) version 1.23.0.2695-49927

I'll assume this is a fixed issue, since the newer groff(1) from my desktop doesn't reproduce it.

Quoted 7 lines

Quoted 2 lines
.tmp/man/man2/statmount.2:155: warning: table row does not fit on page 2
This warning means that a row of the table might be vertically encroaching into the footer margin; in extreme cases, might overprint the page footer itself; and in really loony cases, format text beyond the bottom of the page.

Yeah, that's what I was expecting, but the page renders fine, and I don't see anything near the margins, which made it weird.

Quoted 5 lines

My copy of the document is this one:

	commit 7178f0e15db66a3616a900fc8a4007466633159b
	Author: Bhavik Sachdev <[email protected]>
	Date:   Thu Feb 26 08:40:16 2026 +0530

Yup, that's correct.

Quoted 37 lines

$ ~/groff-1.24.0/bin/groff -t -ww -mandoc -Tpdf -rS12 -rU0 -z \
  man/man2/statmount.2 2>&1 | grep . || echo NO DIAGNOSTICS
NO DIAGNOSTICS
$ ~/groff-1.23.0/bin/groff -t -ww -mandoc -Tpdf -rS12 -rU0 -z \
  man/man2/statmount.2 2>&1 | grep . || echo NO DIAGNOSTICS
NO DIAGNOSTICS

As you can see, I've turned on _all_ warnings (-ww).

Looking at the table source:

$ sed -n '154,171p' man/man2/statmount.2
.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 */
STATMOUNT_MNT_NS_ID     /* Want/got mnt_ns_id */
STATMOUNT_MNT_OPTS      /* Want/got mnt_opts */
STATMOUNT_FS_SUBTYPE    /* Want/got fs_subtype */
STATMOUNT_SB_SOURCE     /* Want/got sb_source */
STATMOUNT_OPT_ARRAY     /* Want/got opt_{num,array} */
STATMOUNT_OPT_SEC_ARRAY /* Want/got opt_sec_{num,array} */
STATMOUNT_MNT_UIDMAP    /* Want/got uidmap{_num,} */
STATMOUNT_MNT_GIDMAP    /* Want/got gidmap{_num,} */
STATMOUNT_SUPPORTED_MASK        /* Want/got supported mask flags */
.TE

...I don't see any reason why you should get that diagnostic.

1. You're not using any text blocks. Text blocks are formatted in a diversion and contain (line) breaks, so they can get "tall".

Quoted 7 lines

2. You're not using any other features to make a table row "tall", for example by performing "local motion" with the \v escape sequence. Formatting equations in a table cell is the more traditional means of encountering this problem.
Can you reproduce this problem from the command line, without using your Makefile?

Yup. On my server:

	$ groff -t -ww -mandoc -Tpdf -rS12 -rU0 -z man/man2/statmount.2 2>&1 \
	| grep . \
	|| echo NO DIAGNOSTICS;
	man/man2/statmount.2:155: warning: table row does not fit on page 2

Using

	$ groff --version | tail -n1
	GNU troff (groff) version 1.23.0.1254-19a18

Have a lovely night!
Alex

Signature · 1 line

<https://www.alejandro-colomar.es>

G. Branden Robinson

2026-03-05 22:55 UTC #24 lore raw

Re: [PATCH v3] man/man2/statmount.2: Document STATMOUNT_BY_FD

Hi Alex,

Quoted 19 lines · Alejandro Colomar

Quoted 12 lines · G. Branden Robinson
[CC list trimmed way down since this is a *roff/formatting issue]
Hi Alex,
Quoted 7 lines · Alejandro Colomar
BTW, Branden, I'm getting a diagnostic from groff:
	TROFF		.tmp/man/man2/statmount.2.pdf.set
	! (troff -mandoc -Tpdf -wbreak -rS12 -rU0  <.tmp/man/man2/statmount.2.pdf.troff 2>&1 >.tmp/man/man2/statmount.2.pdf.set) \
	| grep ^ >&2
	.tmp/man/man2/statmount.2:155: warning: table row does not fit on page 2
Quoted 5 lines
Quoted 3 lines
But the PDF looks good to me. Would you mind looking at that page and telling me what you'd do with that diagnostic, and whether I should care about it?
I can't reproduce this diagnostic with groff 1.23.0 or 1.24.0.

Quoted 8 lines

Huh... I'm reproducing it on my server, but not on my desktop. On my server, I have:
	GNU troff (groff) version 1.23.0.1254-19a18
On my desktop, I have:
	GNU grops (groff) version 1.23.0.2695-49927

Hmm. Revision 19a18 is 9 May 2024, and 49927 is 22 December 2024.

Both are a little old. :)

Quoted 2 lines

I'll assume this is a fixed issue, since the newer groff(1) from my desktop doesn't reproduce it.

If I get stuck on the other stuff I'm working on,[1] I'll see if I can reproduce the problem with 1.23.0.1254-19a18 and then bisect it down.

Quoted 9 lines

Quoted 6 lines
Quoted 1 line
.tmp/man/man2/statmount.2:155: warning: table row does not fit on page 2
This warning means that a row of the table might be vertically encroaching into the footer margin; in extreme cases, might overprint the page footer itself; and in really loony cases, format text beyond the bottom of the page.
Yeah, that's what I was expecting, but the page renders fine, and I don't see anything near the margins, which made it weird.

The warning will issue if the table row is overset by as little as 1 "basic unit", which for PostScript and PDF is 1/72,000 of an inch.

That's 0.0000353 centimeters. It's microscopically small.

If there's a way to get the _amount_ of overset into that diagnostic message, it would be helpful to the document maintainer to decide whether it's a problem they want to deal with.

Your other data explains why I couldn't reproduce it. It could have been a bug that got squashed between May and December of 2024.

If I find out it's still there, latent, I'll go at it with guns blazing.

Regards,
Branden

[1] largely https://savannah.gnu.org/bugs/?67978

Bhavik Sachdev

author

2026-03-06 07:51 UTC #25 lore raw

Re: [PATCH v3] man/man2/statmount.2: Document STATMOUNT_BY_FD

Quoted 16 lines · Alejandro Colomar

Hi,
Quoted 13 lines · Alejandro Colomar
Hi Bhavik,
Sorry for the delay; I had an issue with my mail provider. It's now resolved.
Quoted 7 lines · Bhavik Sachdev
STATMOUNT_BY_FD introduces the ability to get information about a mount using a fd on the mount. This functionality is currently in linux-next [1].
Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?h=next-20260126&id=d5bc4e31f2a3f301b4214858bec834c67bb2be5c>

Quoted 8 lines

Quoted 3 lines
Quoted 3 lines
Signed-off-by Bhavik Sachdev <[email protected]>
Message-ID: <57c96336ccfbdc05f60b7875c315a8c1dd0d14b8.1771870334.git.b.sachdev1904@gmail.com>
Message-ID: <7d4b22c595feeadb3be6df8a8781344597120f7e.1771870502.git.b.sachdev1904@gmail.com>
I've applied the patch. I've amended the few formatting issues I reported below. Since the only important issue was the wording one, and Askar confirmed it's good, I took the patch.

Thanks Alex, Askar and Aleksa for taking a look :)

Quoted 142 lines

BTW, Branden, I'm getting a diagnostic from groff:
	TROFF		.tmp/man/man2/statmount.2.pdf.set
	! (troff -mandoc -Tpdf -wbreak -rS12 -rU0  <.tmp/man/man2/statmount.2.pdf.troff 2>&1 >.tmp/man/man2/statmount.2.pdf.set) \
	| grep ^ >&2
	.tmp/man/man2/statmount.2:155: warning: table row does not fit on page 2
But the PDF looks good to me. Would you mind looking at that page and telling me what you'd do with that diagnostic, and whether I should care about it?
Have a lovely day!
Alex
Quoted 127 lines
Quoted 46 lines
---
Hey Alex!
Quoted 3 lines
Also, should we say the same as elsewhere?:
"It is an ORed combination of the following constants"
and then a list which contains only STATMOUNT_BY_FD?
I am not really sure that statmount flags will be a ORed combination in the future i.e (STATMOUNT_BY_FD | STATMOUNT_NEW_FLAG) would be something that is valid.
I think for now, it is better we don't do that.
Thanks,
Bhavik
 man/man2/statmount.2 | 40 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)
man/man2/statmount.2 +6 -2
index 5ac96796c..1556342de 100644
@@ -24,7 +24,10 @@ .SH SYNOPSIS
 .EX
 .B struct mnt_id_req {
 .BR "    __u32  size;" "        /* sizeof(struct mnt_id_req) */"
-.BR "    __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "    union {"
+.BR "           __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
+.BR "           __u32  mnt_fd;" "      /* fd on the mount being queried */"
+.BR "    };"
 .BR "    __u64  mnt_id;" "      /* The mnt_id being queried */"
 .BR "    __u64  param;" "       /* ORed combination of the STATMOUNT_ constants */"
 .BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"
@@ -89,7 +92,8 @@ .SH DESCRIPTION
 .I bufsize
 with the fields filled in as described below.
 .I flags
-must be 0.
+must either be 0 or
+.BR STATMOUNT_BY_FD .
 .P
 (Note that reserved space and padding is omitted.)
 .SS The mnt_id_req structure
man/man2/statmount.2 .SS The mnt_id_req structure · line 118
Quoted 5 lines
man/man2/statmount.2 +2 -0
@@ -110,6 +114,27 @@ .SS The mnt_id_req structure
 .I req.mnt_id
 (since Linux 6.18).
 .P
+.I req.mnt_fd
+is a file descriptor on a mount.
Is this the same as a "mount object file descriptor" as describer in fsopen(2)? If so, we should use the same language, I think.
CC += Aleksa, Askar
man/man2/statmount.2 .SS The mnt_id_req structure · line 125
Quoted 7 lines
man/man2/statmount.2 +7 -0
+If
+.B STATMOUNT_BY_FD
+flag is specified,
+.I req.mnt_id
+and
+.I req.mnt_ns_id
+are zeroed, the function will return information about the mount the fd is on
We always spell "file descriptor", not fd.
Aleksa, Askar, would you mind reviewing this patch? You may have comments on some specific terms used here, as they might relate to fsopen(2).
man/man2/statmount.2 .SS The mnt_id_req structure · line 126
Quoted 1 line
man/man2/statmount.2 +1 -0
+(Since Linux 7.0).
s/Since/since/
man/man2/statmount.2 .SS The mnt_id_req structure · line 133
Quoted 7 lines
man/man2/statmount.2 +7 -0
+.P
+The fd can also be on a mount that has been lazily unmounted (see
+.BR umount2 (2)
+with
+.BR MNT_DETACH ).
+In this case,
+.BR STATMOUNT_MNT_POINT
s/BR/B/
BR is for alternating bold and roman.
Other than the questios/doubts about mounts and file descriptors, and these minor formatting/source issues (which I would have amended otherwise), the patch looks good to me.
Have a lovely day!
Alex
Quoted 28 lines
man/man2/statmount.2 +15 -0
+and
+.BR STATMOUNT_MNT_NS_ID
+will be unset, since an unmounted mount is no longer a part of the filesystem.
+.P
 .I req.mnt_id
 can be obtained from either
 .BR statx (2)
@@ -392,6 +417,17 @@ .SH ERRORS
 .I req.mnt_ns_fd
 were set.
 .TP
+.B EINVAL
+.I req.mnt_id
+or
+.I req.mnt_ns_id
+was specified alongside
+.IR req.mnt_fd .
+.TP
+.B EBADF
+.I req.mnt_fd
+is an invalid file descriptor.
+.TP
 .B E2BIG
 .I req
 is too large.
Signature · 1 line
2.53.0
Signature · 1 line
<https://www.alejandro-colomar.es>

Bhavik Sachdev

author

2026-02-23 14:13 UTC #26 lore raw

PATCH v2 2/4 man/man2/statmount.2: Document STATMOUNT_SUPPORTED_MASK

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=8f6116b5b77b0536d2ad7482ee42bfe58b8fac01>

Document STATMOUNT_SUPPORTED_MASK flag and the corresponding field introduced by it. It is used to determine the flags supported by the current kernel. The text is based on this commit message [1].

Message-ID: <b537e03ab73d8677a53730311b5a3c11ff29a51b.1771855135.git.b.sachdev1904@gmail.com>

---
 man/man2/statmount.2 | 7 +++++++
 1 file changed, 7 insertions(+)

man/man2/statmount.2 +7 -0

index f5a3721eb..843293e43 100644
@@ -60,6 +60,7 @@ .SH SYNOPSIS
 .B "    __u32  mnt_uidmap;"
 .B "    __u32  mnt_gidmap_num;"
 .B "    __u32  mnt_gidmap;"
+.B "    __u64  supported_mask;"
 .B "    char   str[];"
 .B };
 .EE

@@ -126,6 +127,7 @@ .SS The mnt_id_req structure
 STATMOUNT_OPT_SEC_ARRAY	/* Want/got opt_sec_{num,array} */
 STATMOUNT_MNT_UIDMAP	/* Want/got uidmap... */
 STATMOUNT_MNT_GIDMAP	/* Want/got gidmap... */
+STATMOUNT_SUPPORTED_MASK	/* Want/got supported mask flags */
 .TE
 .in
 .P

@@ -340,6 +342,11 @@ .SS The returned information
 .IR smbuf.mnt_gidmap_num .
 The whole range of gid mappings must be resolvable in the user namespace
 of the caller.
+.TP
+.IR smbuf.supported_mask " (since Linux 6.15)"
+The ORed combination of
+.BI STATMOUNT_ *
+flags supported by the current kernel.
 .SH RETURN VALUE
 On success, zero is returned.
 On error, \-1 is returned, and

Signature · 1 line

2.53.0

Bhavik Sachdev

author

2026-02-23 14:13 UTC #27 lore raw

PATCH v2 3/4 man/man2/statmount.2: Document req.mnt_ns_fd

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9020d0d844ad58a051f90b1e5b82ba34123925b9>

Document the new mnt_ns_fd parameter to struct mnt_id_req.

req.mnt_ns_fd can be used to query for a mount in a foreign mount namespace.

The mnt_ns_fd parameter description is based on this commit message [1].

Message-ID: <6577568b31ba8700214af1696d740d3b6aa1869d.1771855135.git.b.sachdev1904@gmail.com>

---
 man/man2/statmount.2 | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

man/man2/statmount.2 +21 -1

index 843293e43..816ddc24b 100644
@@ -24,6 +24,7 @@ .SH SYNOPSIS
 .EX
 .B struct mnt_id_req {
 .BR "    __u32  size;" "        /* sizeof(struct mnt_id_req) */"
+.BR "    __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
 .BR "    __u64  mnt_id;" "      /* The mnt_id being queried */"
 .BR "    __u64  param;" "       /* ORed combination of the STATMOUNT_ constants */"
 .BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"

@@ -74,7 +75,9 @@ .SH DESCRIPTION
 To access a mount's status,
 the caller must have CAP_SYS_ADMIN in the user namespace.
 In case of accessing a mount in a foreign mount namespace (specified via
-.IR req.mnt_ns_id ),
+.I req.mnt_ns_id
+or
+.IR req.mnt_ns_fd ),
 the foreign mount namespace should be child of the current namespace.
 .P
 This function returns information about a mount,

@@ -97,6 +100,16 @@ .SS The mnt_id_req structure
 it should always be set to
 .IR sizeof(struct\~mnt_id_req) .
 .P
+.I req.mnt_ns_fd
+can be obtained from
+.B PIDFD_GET_MNT_NAMESPACE
+.BR ioctl (2)
+operation or by opening a file descriptor to
+.IR /proc/ pid /ns/mnt
+and is used to specify a foreign mount namespace in which to query
+.I req.mnt_id
+(Since Linux 6.18).
+.P
 .I req.mnt_id
 can be obtained from either
 .BR statx (2)

@@ -372,6 +385,13 @@ .SH ERRORS
 .I req
 is of insufficient size to be utilized.
 .TP
+.B EINVAL
+Both
+.I req.mnt_ns_id
+and
+.I req.mnt_ns_fd
+were set.
+.TP
 .B E2BIG
 .I req
 is too large.

Signature · 1 line

2.53.0

Bhavik Sachdev

author

2026-02-23 14:13 UTC #28 lore raw

PATCH v2 4/4 man/man2/statmount.2: Document STATMOUNT_BY_FD

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?h=next-20260126&id=d5bc4e31f2a3f301b4214858bec834c67bb2be5c>

STATMOUNT_BY_FD introduces the ability to get information about a mount using a fd on the mount. This functionality is currently in linux-next [1].