[PATCH] name_to_handle_at.2,fanotify_mark.2: Document the AT_HANDLE_FID flag

Amir Goldstein
2023-09-03T12:04:43Z

A flag to indicate that the requested file_handle is not intended
to be used for open_by_handle_at(2) and may be needed to identify
filesystem objects reported in fanotify events.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Hi Alejandro,

This is a followup on AT_HANDLE_FID feature from v6.5.

Thanks,
Amir.

man2/fanotify_mark.2 | 11 +++++++++--
man2/open_by_handle_at.2 | 42 +++++++++++++++++++++++++++++++++++++---
2 files changed, 48 insertions(+), 5 deletions(-)

diff --git a/man2/fanotify_mark.2 b/man2/fanotify_mark.2
index 3f85deb23..8e885af69 100644
--- a/man2/fanotify_mark.2
+++ b/man2/fanotify_mark.2
@@ -743,10 +743,17 @@ do not specify a directory.
.B EOPNOTSUPP
The object indicated by
.I pathname
-is associated with a filesystem that does not support the encoding of file
-handles.
+is associated with a filesystem
+that does not support the encoding of file handles.
This error can be returned only with an fanotify group that identifies
filesystem objects by file handles.
+Calling
+.BR name_to_handle_at (2)
+with the flag
+.BR AT_HANDLE_FID " (since Linux 6.5)"
+.\" commit 96b2b072ee62be8ae68c8ecf14854c4d0505a8f8
+can be used as a test
+to check if a filesystem supports reporting events with file handles.
.TP
.B EPERM
The operation is not permitted because the caller lacks a required capability.
diff --git a/man2/open_by_handle_at.2 b/man2/open_by_handle_at.2
index 4061faea9..4cfa21d9c 100644
--- a/man2/open_by_handle_at.2
+++ b/man2/open_by_handle_at.2
@@ -109,17 +109,44 @@ structure as an opaque data type: the
.I handle_type
and
.I f_handle
-fields are needed only by a subsequent call to
+fields can be used in a subsequent call to
.BR open_by_handle_at ().
+The caller can also use the opaque
+.I file_handle
+to compare the identity of filesystem objects
+that were queried at different times and possibly
+at different paths.
+The
+.BR fanotify (7)
+subsystem can report events
+with an information record containing a
+.I file_handle
+to identify the filesystem object.
.PP
The
.I flags
argument is a bit mask constructed by ORing together zero or more of
-.B AT_EMPTY_PATH
+.BR AT_HANDLE_FID ,
+.BR AT_EMPTY_PATH ,
and
.BR AT_SYMLINK_FOLLOW ,
described below.
.PP
+When
+.I flags
+contain the
+.BR AT_HANDLE_FID " (since Linux 6.5)"
+.\" commit 96b2b072ee62be8ae68c8ecf14854c4d0505a8f8
+flag, the caller indicates that the returned
+.I file_handle
+is needed to identify the filesystem object,
+and not for opening the file later,
+so it should be expected that a subsequent call to
+.BR open_by_handle_at ()
+with the returned
+.I file_handle
+may fail.
+.PP
Together, the
.I pathname
and
@@ -363,8 +390,14 @@ capability.
.B ESTALE
The specified
.I handle
-is not valid.
+is not valid for opening a file.
This error will occur if, for example, the file has been deleted.
+This error can also occur if the
+.I handle
+was aquired using the
+.B AT_HANDLE_FID
+flag and the filesystem does not support
+.BR open_by_handle_at ().
.SH VERSIONS
FreeBSD has a broadly similar pair of system calls in the form of
.BR getfh ()
@@ -386,6 +419,9 @@ file handles, for example,
.IR /proc ,
.IR /sys ,
and various network filesystems.
+Some filesystem support the translation of pathnames to
+file handles, but do not support using those file handles in
+.BR open_by_handle_at ().
.PP
A file handle may become invalid ("stale") if a file is deleted,
or for other filesystem-specific reasons.
--
2.34.1