name_to_handle_at.2: Document the AT_HANDLE_CONNECTABLE flag

A flag since v6.13 to indicate that the requested file_handle is
intended to be used for open_by_handle_at(2) to obtain an open file
with a known path.

Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Jeff Layton <jlayton@poochiereds.net>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Alejandro,

Addressed your comments from v1 and added missing documentation for
AT_HANDLE_MNT_ID_UNIQUE from v6.12.

Thanks,
Amir.

 man/man2/open_by_handle_at.2 | 46 +++++++++++++++++++++++++++++++++++-
 1 file changed, 45 insertions(+), 1 deletion(-)

diff --git a/man/man2/open_by_handle_at.2 b/man/man2/open_by_handle_at.2
index 6b9758d42..10af60a76 100644
--- a/man/man2/open_by_handle_at.2
+++ b/man/man2/open_by_handle_at.2
@@ -127,6 +127,7 @@ The
 .I flags
 argument is a bit mask constructed by ORing together zero or more of
 .BR AT_HANDLE_FID ,
+.BR AT_HANDLE_CONNECTABLE,
 .BR AT_EMPTY_PATH ,
 and
 .BR AT_SYMLINK_FOLLOW ,
@@ -147,6 +148,44 @@ with the returned
 .I file_handle
 may fail.
 .P
+When
+.I flags
+contain the
+.BR AT_HANDLE_MNT_ID_UNIQUE " (since Linux 6.12)"
+.\" commit 4356d575ef0f39a3e8e0ce0c40d84ce900ac3b61
+flag, the caller indicates that the size of the
+.I mount_id
+buffer is at least 64bit
+and then the mount id returned in that buffer
+is the unique mount id as the one returned by
+.BR statx (2)
+with the
+.BR STATX_MNT_ID_UNIQUE
+flag.
+.P
+When
+.I flags
+contain the
+.BR AT_HANDLE_CONNECTABLE " (since Linux 6.13)"
+.\" commit a20853ab8296d4a8754482cb5e9adde8ab426a25
+flag, the caller indicates that the returned
+.I file_handle
+is needed to open a file with known path later,
+so it should be expected that a subsequent call to
+.BR open_by_handle_at ()
+with the returned
+.I file_handle
+may fail if the file was moved,
+but otherwise,
+the path of the opened file is expected to be visible
+from the
+.IR /proc/ pid /fd/ *
+magic link.
+This flag can not be used in combination with the flags
+.B AT_HANDLE_FID
+and/or
+.BR AT_EMPTY_PATH .
+.P
 Together, the
 .I pathname
 and
@@ -311,7 +350,7 @@ points outside your accessible address space.
 .TP
 .B EINVAL
 .I flags
-includes an invalid bit value.
+includes an invalid bit value or an invalid bit combination.
 .TP
 .B EINVAL
 .I handle\->handle_bytes
@@ -398,6 +437,11 @@ was acquired using the
 .B AT_HANDLE_FID
 flag and the filesystem does not support
 .BR open_by_handle_at ().
+This error can also occur if the
+.I handle
+was acquired using the
+.B AT_HANDLE_CONNECTABLE
+flag and the file was moved to a different parent.
 .SH VERSIONS
 FreeBSD has a broadly similar pair of system calls in the form of
 .BR getfh ()
-- 
2.34.1