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

[David Oliver]

On Tue, Jun 7, 2011 at 1:43 PM, Andrew Lutomirski <luto@xxxxxxx> wrote:
> 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 have software which currently uses shared files for a one way
transfer of information, which is modeled precisely by the futex (as
contrasted to the mutex) model. In this case, the number of receivers
is undetermined, so the number of wakeups is set to maxint.

The receivers are minimally trusted: they have read access to the
files, so they cannot accidentally affect other processes use of the
data. Requiring my files to be writeable by all clients would require
a serious increase in the amount of software needing to be trusted.

> I don't want random read-only processes to
> interfere with the futex protocol.
>
If limited wakeups are needed for your application, this can be
prevented by preventing access to the files. This is analogous to
preventing read access to fifos: reading a random unprotected named
pipe would have interesting consequences.

>> How is this related to futex code ?
>
> Because this usage is currently safe and would become unsafe with the
> proposed change.
>
My usage was both safe and useful before the recent kernel 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.)
>
> --Andy
>

-- 
Cheers!

David.


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