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

[Darren Hart]

On 06/08/2011 08:20 AM, David Oliver wrote:
> On Tue, Jun 7, 2011 at 5:26 PM, Kyle Moffett <kyle@xxxxxxxxxxxxxxx> wrote:
>> On Tue, Jun 7, 2011 at 15:19, Andrew Lutomirski <luto@xxxxxxx> wrote:
>>> On Tue, Jun 7, 2011 at 3:10 PM, David Oliver <david@xxxxxxxxxxxxxxx> wrote:
>>>> 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.
>>>
>>> What's wrong with adding a FUTEX_WAIT_NOCONSUME flag then?  Your
>>> program can use it to get exactly the semantics it wants and my
>>> program can use it or not depending on which semantics it wants.
>>>
>>> Then we can document in the man page that, on kernels newer than
>>> whichever version introduced the regression, read-only mappings of a
>>> file cannot be used to interfere with futexes on that file.
>>
>> Hmm, I would actually call it "FUTEX_POLL", since that better reflects the
>> operation being performed.
>>
>> Certainly you would want to avoid allowing FUTEX_POLL to "steal"
>> limited wakeups from FUTEX_CMP_REQUEUE or whatever, so you
>> also need a new "FUTEX_NOTIFY".  Alternatively I guess you could just
>> special-case the FUTEX_WAKE && wakeups == INTMAX combination to
>> also notify FUTEX_POLL processes.
>>
>> I almost wonder if long-term there might possibly be some decent way
>> to integrate this with eventfds to allow a thread to wait for notifications from
>> any number of memory addresses as well as other event sources.  This
>> would be a similar extension to signalfd, only for futexes.
>>
>> Cheers,
>> Kyle Moffett
>>
> 
> Having a new call is inelegant from a futex(2) user perspective, as
> the need for a change is due to the kernel implementation and/or mutex
> requirements. The futex() system call, as documented, is ideal for a
> single producer to signal multiple receivers of state updates.
> 
> If it is truly necessary to add new variants to futex() to protect
> applications that allow untrusted applications read access to their
> mutexes, I would avoid both the names suggested, as consumption of
> wakeups is not an obvious issue to users, and POLL suggests waiting
> for multiple entities as in poll(2) (which is not provided), or
> returning immediately (which is orthogonally provided by the timeout
> parameter). What is being provided from the user point of view is a
> FUTEX_WAIT per the man page, which doesn't require write access. How
> about FUTEX_WAIT_RDONLY?
> 
> Alternatively, use the current call and document that when process
> performing a FUTEX_WAIT on read-only memory are woken, they do not
> count towards the number reported as being woken.
> 
> Best, IMHO, would be to document that providing read access to mutexes
> to untrusted software is unsafe behavior, and restore read only access
> to readers of futexes.

I'm inclined to agree with this approach.

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Linux Kernel
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