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

[Al Viro]

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...
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