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

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

[NeilBrown]

[-- Attachment #1: Type: text/plain, Size: 2843 bytes --]


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


[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 832 bytes --]

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

[Michael Kerrisk (man-pages)]

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-IBi9RG/b67k@public.gmane.org>

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/
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

[manpages PATCH] open_by_handle_at.2: clarify MAX_HANDLE_SZ

[NeilBrown]

[-- Attachment #1: Type: text/plain, Size: 1158 bytes --]


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 <lennart-mdGvqq1h2p+GdvJs77BJ7Q@public.gmane.org>
Signed-off-by: NeilBrown <neilb-IBi9RG/b67k@public.gmane.org>
---
 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


[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 832 bytes --]

Re: [manpages PATCH] open_by_handle_at.2: clarify MAX_HANDLE_SZ

[Michael Kerrisk (man-pages)]

Hello Neil,

On 24 November 2017 at 00:53, NeilBrown <neilb-IBi9RG/b67k@public.gmane.org> 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 <lennart-mdGvqq1h2p+GdvJs77BJ7Q@public.gmane.org>
> Signed-off-by: NeilBrown <neilb-IBi9RG/b67k@public.gmane.org>
> ---
>  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/
--
To unsubscribe from this list: send the line "unsubscribe linux-nfs" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html