Re: [PATCH] nfsd3: Check write permission after checking existence

[Ross Lagerwall <rosslagerwall@gmail.com>]

On Mon, Aug 11, 2014 at 03:08:38PM -0400, J. Bruce Fields wrote:
> On Sat, Aug 09, 2014 at 02:44:00PM +0100, Ross Lagerwall wrote:
> > When creating a file that already exists in a read-only directory with
> > O_EXCL, the NFSv3 server returns EACCES rather than EEXIST (which local
> > files and the NFSv4 server return).  Fix this by checking the MAY_CREATE
> > permission only if the file does not exist.  Since this already happens
> > in do_nfsd_create, the check in nfsd3_proc_create can simply be removed.
> 
> Thanks.
> 
> From a look at the history I believe the server has behaved this way
> since the beginning.  Is this creating a practical problem for you?  How
> did you notice it?

I help maintain GNOME's gvfs and so I have a bunch of test programs
which I run to check for conformance.  I noticed that:
gvfs-save /mnt/dir/file
fails with a permission denied error when file is on an NFSv3 mount and
dir is read-only.  Basically, gvfs-save first tries to create the file.
If it already exists, then it just truncates it.  But on NFSv3, the
first creation generates a permission denied error so the operation is
aborted.

Not really a practical problem, but it is possible to see this with
various real-world programs which do a similar dance when saving like
this:
open(path, O_WRONLY|O_CREAT|O_EXCL)
if EEXIST:
  open(path, O_WRONLY|O_CREAT|O_TRUNC)
else:
  bail()

(For Linux NFS clients, the kernel does its own client-side caching so
if you do:
ls /mnt/dir; gvfs-save /mnt/dir/file
it magically works!)

> 
> Inclined to apply it just for consistency as you suggest.  And because
> it removes some unnecessary code.  But as a low priority: for 3.18 and
> not stable.
> 

OK, your decision.  That is OK with me.

Cheers,
-- 
Ross Lagerwall