Messages in this thread | | | Date | Mon, 30 Mar 2009 01:56:39 +0200 | From | Oleg Nesterov <> | Subject | Re: Q: check_unsafe_exec() races (Was: [PATCH 2/4] fix setuid sometimes doesn't) |
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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?
In that case -EWHATEVER is not very good, it could be another process (not sub-thread) doing exec.
Oleg.
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