[patch] cifs: accidentally creating empty files

Jeff Layton jlayton at samba.org
Fri Oct 28 21:44:06 MDT 2011

On Fri, 28 Oct 2011 10:27:04 -0500
Steve French <smfrench at gmail.com> wrote:

> On Fri, Oct 28, 2011 at 2:20 AM, Dan Carpenter <dan.carpenter at oracle.com> wrote:
> > On Wed, Oct 19, 2011 at 09:14:37PM -0500, Steve French wrote:
> >> Doesn't this force the create to happen later - rather than
> >> at lookup time where it belongs?
> >>
> >> if the issue is just noperm ... we should let this through if the user
> >> has local permissions.  Doubling the cost of a file create to Samba
> >> seems like a bad idea (ie doing aQueryPathInfo AND an NTCreateX
> >> doubles the roundrip, doubles the load on the server etc.) - the whole
> >> point of this is to let us do an "atomic" open or create operation
> >> and not have to split it into multiple requests.  In any case why would
> >> we do the open and then follow it with a create?
> >>
> >> Can we fix this to (at least) narrow the performance penalty.
> >>
> >
> > Yes, it does add another back and forth to file create...  It's hard/
> > impossible to check the permissions first and then decide whether to
> > pass the O_CREAT flag.  Maybe we could add an if statement to check
> > whether noperm was used and the noperm version could use the create
> > on lookup.
> Why is it hard to check local permissions? We have the local mode
> for the parent.
> Also note that the "intent" flags and the atomic create is not just
> about performance, but also making it atomic reduces some of the weird
> failure cases.

We have the local mode for the parent, but we do not have the ownership
and mode for the file that has not yet been created. Because of the
special (ahem) way that cifs handles permissions, it's easily possible
for the ownership of the file not to match the user doing that create.
At that point, later operations on the file can easily fail.

This was the primary reason for the multiuser patch series, and why I
still say that doing permissions checking on the client is a broken

Jeff Layton <jlayton at samba.org>

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