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

[Eric Dumazet]

Le mardi 07 juin 2011 Ã 10:44 -0400, Andy Lutomirski a Ãcrit :
> 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 Ã 10:53 -0700, Darren Hart a Ãcrit :
> >>>
> >>
> >>> 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 ?

How is this related to futex code ? 

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...



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