diff mbox

[manpages] open_by_handle_at: clarifications needed due to NFS reexport

Message ID 87wp2j1ggu.fsf@notabene.neil.brown.name
State New, archived
Headers show

Commit Message

NeilBrown Nov. 21, 2017, 9:15 p.m. UTC
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@suse.com>
---
 man2/open_by_handle_at.2 | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

Comments

Michael Kerrisk (man-pages) Nov. 22, 2017, 4:48 p.m. UTC | #1
Hello Neil,

On 11/21/2017 10:15 PM, NeilBrown wrote:
> 
> 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@suse.com>

Thanks! Patch applied.

Cheers,

Michael

PS And meta-thanks for thinking to send me the steady small 
stream of man-pages patches that you do.

> ---
>  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 ()
>
diff mbox

Patch

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 ()