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 <[email protected]>
---
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
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 <[email protected]>
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 ()
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
As hinted in the kernel source, MAX_HANDLE_SZ is a hint
rather than a promise:
/* limit the handle size to NFSv4 handle size now */
#define MAX_HANDLE_SZ 128
Note the "now" (probably should be "for now").
So change the description to make this clear.
Reported-by: Lennart Poettering <[email protected]>
Signed-off-by: NeilBrown <[email protected]>
---
man2/open_by_handle_at.2 | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/man2/open_by_handle_at.2 b/man2/open_by_handle_at.2
index 79987308bf3e..f3473d24fa50 100644
--- a/man2/open_by_handle_at.2
+++ b/man2/open_by_handle_at.2
@@ -91,7 +91,8 @@ field should be initialized to contain the allocated size for
.BR MAX_HANDLE_SZ ,
defined in
.IR <fcntl.h> ,
-specifies the maximum possible size for a file handle.)
+specifies the maximum expected size for a file handle. It is not a
+guaranteed upper limit as future filesystems may require more space.)
Upon successful return, the
.IR handle_bytes
field is updated to contain the number of bytes actually written to
--
2.14.0.rc0.dirty
Hello Neil,
On 24 November 2017 at 00:53, NeilBrown <[email protected]> wrote:
>
> As hinted in the kernel source, MAX_HANDLE_SZ is a hint
> rather than a promise:
>
> /* limit the handle size to NFSv4 handle size now */
> #define MAX_HANDLE_SZ 128
>
> Note the "now" (probably should be "for now").
> So change the description to make this clear.
Thanks! Applied.
Cheers,
Michael
> Reported-by: Lennart Poettering <[email protected]>
> Signed-off-by: NeilBrown <[email protected]>
> ---
> man2/open_by_handle_at.2 | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/man2/open_by_handle_at.2 b/man2/open_by_handle_at.2
> index 79987308bf3e..f3473d24fa50 100644
> --- a/man2/open_by_handle_at.2
> +++ b/man2/open_by_handle_at.2
> @@ -91,7 +91,8 @@ field should be initialized to contain the allocated size for
> .BR MAX_HANDLE_SZ ,
> defined in
> .IR <fcntl.h> ,
> -specifies the maximum possible size for a file handle.)
> +specifies the maximum expected size for a file handle. It is not a
> +guaranteed upper limit as future filesystems may require more space.)
> Upon successful return, the
> .IR handle_bytes
> field is updated to contain the number of bytes actually written to
> --
> 2.14.0.rc0.dirty
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/