Issues with Linux kernel oplocks

[Simo]

On Tue, 2013-07-23 at 14:53 -0700, Jeremy Allison wrote:
> On Tue, Jul 23, 2013 at 02:12:28PM -0700, Jeremy Allison wrote:
> > On Tue, Jul 23, 2013 at 05:01:15PM -0400, J. Bruce Fields wrote:
> > > On Tue, Jul 23, 2013 at 01:47:30PM -0700, Jeremy Allison wrote:
> > > > On Tue, Jul 23, 2013 at 04:32:16PM -0400, J. Bruce Fields wrote:
> > > > > 
> > > > > Sure, but...
> > > > > 
> > > > > > Neither process has to be privileged, neither
> > > > > > process has to have changed uids.
> > > > > > 
> > > > > > For leases to work this condition:
> > > > > > 
> > > > > > "the real or effective user ID of the sending process must equal
> > > > > > the real or saved set-user-ID of the target process."
> > > > > > 
> > > > > > cannot be correct. Else you could only break leases between
> > > > > > processes who are owned by the same uid - or from a privileged
> > > > > > opener.
> > > > > 
> > > > > ... you're confusing the lease-breaker and the lease-setter.
> > > > > 
> > > > > Note in the first quote above, "where the sending process is the one
> > > > > that employs F_SETOWN".  (Or equivalently, I think, F_SETLEASE.)
> > > > 
> > > > Ok then I'm really confused. Samba always does
> > > > the lease set as the same uid as the open, we
> > > > never change uids between the open and the
> > > > setting of the kernel oplock.
> > > 
> > > Right, so both are done with uid X.  But then later when the lease break
> > > happens the signal is sent to the thread that did the open.  And that
> > > signal is treated as if it's coming from uid X.  The thread may have
> > > changed uid's, and that may not work.
> > > 
> > > (OK, "uid" isn't quite right, see the kill(2) language for the actual
> > > uid's involved.)
> > 
> > Ah - I get it ! Thanks.
> > 
> > Ok, so the fix inside Samba would be to become_root()/unbecome_root()
> > around the kernel oplock set call to ensure that the uid of the
> > sending process is always 0 and will always have the rights to
> > send the signal.
> > 
> > That's actually quite an easy fix - more so than playing
> > games with a receiver thread.
> 
> Ok Ralph, can you test this fix to see if it addresses
> your issue ? It should apply cleanly to 3.6.x and 4.0.x.
> 
> If it works for you then we'll raise a bug and get it
> into 3.6.next and 4.0.next.

Jeremy,
if it fixes the issue, I'd like a comment before the patch's
become_root() call that explains why becoming root is required.

Simo.