Re: [PATCH] RFC: futex fault handling and futex key references (NOTFOR INCLUSION)

[Darren Hart]

Peter Zijlstra wrote:
> On Thu, 2009-01-08 at 23:52 -0800, Darren Hart wrote:
> > While trying to bend my brain around the various layers of fault handling in
> > futex.c, I think I may have uncovered some logical errors (or at least stale
> > code sections).  I've attached two patches that address the alleged problems
> > against linux-tip/core/futexes.  They are based on the following assumption:
> >
> > Since the uaddr passed to a futex function isn't updated within the function,
> > and the mm doesn't change while we're in there, 
>
> That's not quite true, you _can_ change the memory map by issuing
> concurrent mmap/munmap/mremap etc.. calls from another thread.

Well, what I meant was that the pointer "current->mm" doesn't change, 
since that is what we store in the futex key union.

> The thing is, afaik futexes have never been completely safe wrt
> concurrent mm modifications -- that is, as long as we fail the futex op
> with -EFAULT or similar and not crash the kernel we're good.
>
> > there should never be a need to
> > make repeat calls to futex_get_key().  Even if the queue_lock is dropped, the
> > futex_q might lose it's waiter (requeued) but the key stays the same.
>
> Yes, so when we assume the mmap stable (and fail the op whenever that
> assumption proves false) we can say the futex keys are stable and should
> never need recomputation.

And if I understand correctly, we would catch this scenario any time we 
try to use uaddr and find it faulting (during the cmpxchg* calls for 
example).  If the mmap changes too much, we'll exhaust our fault 
tolerance (3) and exit with -EFAULT back to userspace.

Sound right?

Thanks for the review Peter,

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