Re: inode_permission NULL pointer dereference in 3.13-rc1

Al Viro
2013-11-29T03:59:53Z

On Fri, Nov 29, 2013 at 02:41:21AM +0000, Al Viro wrote:
> On Thu, Nov 28, 2013 at 06:07:27PM -0800, Linus Torvalds wrote:
>
> > HOWEVER. It's certainly *not* valid if "current->fs->root/pwd" points
> > to it. So yeah, there must have been an extra dput() somewhere. Or,
> > more likely, I think, we don't get the refcount to some dentry
> > properly any more.
> >
> > I don't see where, though. You did change where "LOOKUP_RCU" is
> > cleared in unlazy_walk() but you did add that
> >
> > nd->path.dentry = NULL;
> >
> > and that looks like it should be ok. And I don't see what else would care.
>
> *nod*
>
> BTW, vfsmount refcount is 12, so we *definitely* nowhere near the
> final mntput(), etc. and mnt->mnt_root itself should also have
> contributed.
>
> I'm going to try to find out _which_ test buggers the refcount - at
> least that way I'll have something resembling a usable reproducer...

OK, we have a winner. generic/234 drops refcount of root dentry by about
20 (and yes, I should've started with that one, what with Ted's report).
Run it several times (4 should suffice nicely) and the damn thing triggers
right there. Uff... At least that takes under a minute instead of a couple
of hours, which makes debugging that shite much more tolerable...