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

Al Viro
2009-03-29T22:21:44Z

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.

FWIW, I'm not at all sure that we want atomic_t for refcount in that
case...