[manpages PATCH] open_by_handle_at: clarifications needed due to NFS reexport

[NeilBrown <neilb-IBi9RG/b67k@public.gmane.org>]

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The recent addition of NFS re-export and the possibility of
using name_to_handle_at() on an NFS filesystem raises issues
with name_to_handle_at() which have not been properly documented.

Getting the filehandle for an untriggered automount point is
arguably meaningless and in certainly not supported by NFS.
name_to_handle_at() will return -EOVERFLOW even though the
requested "handle_bytes" is large enough.  This is an unfortunate
overloading of the error code, but is manageable.

So clarify this and also note that the mount_id is returned
when EOVERFLOW is reported.

Thought: it would be nice if mount_id were returned in the
EOPNOTSUPP case too.  I guess it is too late to fix that (?).

Link: https://github.com/systemd/systemd/issues/7082
Signed-off-by: NeilBrown <neilb-IBi9RG/b67k@public.gmane.org>
---
 man2/open_by_handle_at.2 | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/man2/open_by_handle_at.2 b/man2/open_by_handle_at.2
index 91300e80fe24..9a30dd2ed476 100644
--- a/man2/open_by_handle_at.2
+++ b/man2/open_by_handle_at.2
@@ -109,6 +109,12 @@ and
 is set to indicate the required size;
 the caller can then use this information to allocate a structure
 of the correct size (see EXAMPLE below).
+Some care is needed here as
+.BR EOVERFLOW
+can also indicate that no filehandle is available for this particular
+name in a filesystem which does normally support filehandle lookup.
+This case can be detected when EOVERFLOW is returned without
+handle_bytes being increased.
 .PP
 Other than the use of the
 .IR handle_bytes
@@ -193,6 +199,10 @@ Opening the pathname in the fifth field of that record yields a file
 descriptor for the mount point;
 that file descriptor can be used in a subsequent call to
 .BR open_by_handle_at ().
+.I mount_id
+is returned both for a successful call and for a call that results
+in the error
+.BR EOVERFLOW .
 .PP
 By default,
 .BR name_to_handle_at ()
@@ -206,6 +216,21 @@ is specified in
 .I pathname
 is dereferenced if it is a symbolic link
 (so that the call returns a handle for the file referred to by the link).
+.PP
+.BR name_to_handle_at )
+does not trigger a mount when the final component of the path is an
+automount point.  When a filesystem supports both filehandles and
+automount points, a
+.BR name_to_handle_at ()
+call on an automount point will return with error
+.BR EOVERFLOW
+without having increased
+.IR handle_bytes .
+This can happen since Linux 4.13 with NFS when accessing a directory
+which is on a separate filesystem on the server.
+.\" commit 20fa19027286983ab2734b5910c4a687436e0c31
+In this case the automount can be triggered by adding a "/" to the end
+of the path.
 .SS open_by_handle_at()
 The
 .BR open_by_handle_at ()
-- 
2.14.0.rc0.dirty


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