Issues with Linux kernel oplocks

Jeremy Allison jra at samba.org
Tue Jul 23 15:53:04 MDT 2013


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.

Cheers,

	Jeremy.
-------------- next part --------------
diff --git a/source3/smbd/oplock_linux.c b/source3/smbd/oplock_linux.c
index 7fa9b7c..2d2b21d 100644
--- a/source3/smbd/oplock_linux.c
+++ b/source3/smbd/oplock_linux.c
@@ -75,26 +75,28 @@ int linux_set_lease_sighandler(int fd)
 int linux_setlease(int fd, int leasetype)
 {
 	int ret;
+	int saved_errno;
+
+	become_root();
 
 	/* First set the signal handler. */
 	if (linux_set_lease_sighandler(fd) == -1) {
-		return -1;
+		saved_errno = errno;
+		ret = -1;
+		goto out;
 	}
 	ret = fcntl(fd, F_SETLEASE, leasetype);
-	if (ret == -1 && errno == EACCES) {
-		set_effective_capability(LEASE_CAPABILITY);
-		/*
-		 * Bug 8974 - work around Linux kernel bug
-		 * https://bugzilla.kernel.org/show_bug.cgi?id=43336.
-		 * "fcntl(F_SETLEASE) resets signal number when
-		 *  called multiple times"
-		 */
-		if (linux_set_lease_sighandler(fd) == -1) {
-			return -1;
-		}
-		ret = fcntl(fd, F_SETLEASE, leasetype);
+	if (ret == -1) {
+		saved_errno = errno;
 	}
 
+  out:
+
+	unbecome_root();
+
+	if (ret == -1) {
+		errno = saved_errno;
+	}
 	return ret;
 }
 


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