Re: check *uaddr==val after queueing - without faulting

[Darren Hart]

Adding a few key folks to the Cc, apologies for the short initial Cc list.

Darren Hart wrote:
> The current futex_wait() code (I'm looking at tip/core/futexes)
> conflicts with a warning in the comments about checking *uaddr==val
> before the futex_q is queued on the hb list.  While userspace is able to
> alter *uaddr at will and should expect to hang in the kernel forever
> should it do so haphazardly, there are legitimate scenarios where the
> futex value might change between the call to futex_wait() and when the
> futex_q gets on the hb list.
>
> For example, glibc protects access to the value of cond.__data.__futex
> via the cond.__data.__lock.  However, before it can issue the syscall it
> has to drop the cond.__data.__lock, leaving a small race window where
> userspace might issue a signal or broadcast, which will modify the value
> of cond.__data.__futex.  As I understand it, this will result in the
> waiter having changed the value of the futex prior to entering the
> kernel, but not enqueuing itself on the hb list until after the waiter
> issues the broadcast that was intended to wake it up.
>
> I was working up a patch to move the test to after the call to
> queue_me(), but in order to do the test we also have to perform the
> get_user() after the queue_me(), which might sleep if we still hold the
> hb->lock.  If we let queue_me() drop the hb->lock before we call
> get_user() then we may see a legitimate change in *uaddr that occured
> after the queue_me() and before the get_user().
>
> I'm at a loss for how to resolve the race without causing the false
> positive inside the kernel.  It might be resolvable in glibc by looking
> at the return code from futex_requeue and checking if the number 
> woken_or_requeued agrees with the number it expected to be sleeping; 
> this likely leaves other gaps for other waking calls, like FUTEX_WAKE.
>
> Any thoughts?  Am I missing something that guards against this race?


--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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