[PATCH v2 4/4] 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>

Signed-off-by: Bhavik Sachdev <[email protected]>
Message-ID: <57c96336ccfbdc05f60b7875c315a8c1dd0d14b8.1771855135.git.b.sachdev1904@gmail.com>
---
 man/man2/statmount.2 | 37 +++++++++++++++++++++++++++++++++++--
 1 file changed, 35 insertions(+), 2 deletions(-)

diff --git a/man/man2/statmount.2 b/man/man2/statmount.2
index 816ddc24b..73faa8ccc 100644
--- a/man/man2/statmount.2
+++ b/man/man2/statmount.2
@@ -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.
-- 
2.53.0