Re: Q: check_unsafe_exec() races (Was: [PATCH 2/4] fix setuid sometimes doesn't)

Oleg Nesterov
2009-03-30T00:09:23Z

On 03/30, Oleg Nesterov wrote:
>
> On 03/29, Al Viro wrote:
> >
> > On Sun, Mar 29, 2009 at 11:36:35PM +0200, Oleg Nesterov wrote:
> > > > ... or just do that to fs_struct. After finding that there's no outside
> > > > users. Commenst?
> > >
> > > This is even worse. Not only we race with our sub-threads, we race
> > > with CLONE_FS processes.
> > >
> > > We can't mark fs_struct after finding that there's no outside users
> > > lockless. Because we can't know whether this is "after" or not, we
> > > can't trust "atomic_read(fs->count) <= n_fs".
> >
> > We can lock fs_struct in question, go through the threads, then mark
> > or bail out. With cloning a reference to fs_struct protected by the
> > same lock.
>
> Yes, this is what I meant, copy_fs() needs this lock too,
>
> > FWIW, I'm not at all sure that we want atomic_t for refcount in that
> > case...
>
> I think you are right, because exit_fs() should take fs->lock as well.
>
> But, again. What whould we do when check_unsafe_exec() takes fs->lock
> and sees that this ->fs is already marked?

Ah, I am stupid. There is no another process if this flag is set.

Oleg.