Re: Change in functionality of futex() system call.

From: Andrew Lutomirski
Date: Tue Jun 07 2011 - 14:43:30 EST

On Tue, Jun 7, 2011 at 11:58 AM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> Le mardi 07 juin 2011 à 10:44 -0400, Andy Lutomirski a écrit :
>> On 06/06/2011 11:13 PM, Darren Hart wrote:
>> >
>> >
>> > On 06/06/2011 11:11 AM, Eric Dumazet wrote:
>> >> Le lundi 06 juin 2011 à 10:53 -0700, Darren Hart a écrit :
>> >>>
>> >>
>> >>> If I understand the problem correctly, RO private mapping really doesn't
>> >>> make any sense and we should probably explicitly not support it, while
>> >>> special casing the RO shared mapping in support of David's scenario.
>> >>>
>> >>
>> >> We supported them in 2.6.18 kernels, apparently. This might sounds
>> >> stupid but who knows ?
>> >
>> >
>> > I guess this is actually the key point we need to agree on to provide a
>> > solution. This particular case "worked" in 2.6.18 kernels, but that
>> > doesn't necessarily mean it was supported, or even intentional.
>> >
>> > It sounds to me that we agree that we should support RO shared mappings.
>> > The question remains about whether we should introduce deliberate
>> > support of RO private mappings, and if so, if the forced COW approach is
>> > appropriate or not.
>> >
>> I disagree.
>> FUTEX_WAIT has side-effects.  Specifically, it eats one wakeup sent by
>> FUTEX_WAKE.  So if something uses futexes on a file mapping, then a
>> process with only read access could (if the semantics were changed) DoS
>> the other processes by spawning a bunch of threads and FUTEX_WAITing
>> from each of them.
>> If there were a FUTEX_WAIT_NOCONSUME that did not consume a wakeup and
>> worked on RO mappings, I would drop my objection.
> If a group of cooperating processes uses a memory segment to exchange
> critical information, do you really think this memory segment will be
> readable by other unrelated processes on the machine ?

Depends on the design.

I have some software I'm working on that uses shared files and could
easily use futexes. I don't want random read-only processes to
interfere with the futex protocol.

> How is this related to futex code ?

Because this usage is currently safe and would become unsafe with the
proposed change.

> Same problem for legacy IPC (shm, msg, sem) : Appropriate protections
> are needed, obviously.
> BTW, kernel/futex.c uses a global hash table (futex_queues[256]) and a
> very predictable hash_futex(), so its easy to slow down futex users...

There's a difference between slowing down users by abusing a kernel
hash and deadlocking users by eating a wakeup. (If you eat a wakeup
the wakeup won't magically come back later. It's gone.)

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