On 1/16/15, 4:56 PM, "Davidlohr Bueso" <dave@xxxxxxxxxxxx> wrote:
>On Fri, 2015-01-16 at 21:54 +0100, Michael Kerrisk (man-pages) wrote:
>> On 01/16/2015 04:20 PM, Thomas Gleixner wrote:
>> > On Fri, 16 Jan 2015, Michael Kerrisk (man-pages) wrote:
>> >
>> >> Hello Thomas,
>> >>
>> >> On 01/15/2015 11:23 PM, Thomas Gleixner wrote:
>> >>> On Thu, 15 Jan 2015, Michael Kerrisk (man-pages) wrote:
>> >>>>> [EINVAL] uaddr equal uaddr2. Requeue to same futex.
>> >>>>
>> >>>> ??? I added this, but does this error not occur only for PI
>>requeues?
>> >>>
>> >>> It's equally wrong for normal futexes. And its actually the same
>>code
>> >>> checking for this for all variants.
>> >>
>> >> I don't understand "equally wrong" in your reply, I'm sorry. Do you
>> >> mean:
>> >>
>> >> a) This error text should be there for both normal and PI requeues
>> >
>> > It is there for both. The requeue code has that check independent of
>> > the requeue type (normal/pi). It never makes sense to requeue
>> > something to itself whether normal or pi futex. We added this for PI,
>> > because there it is harmful, but we did not special case it. So normal
>> > futexes get the same treatment.
>>
>> Hello Thomas,
>>
>> Color me stupid, but I can't see this in futex_requeue(). Where is that
>> check that is "independent of the requeue type (normal/pi)"?
>>
>> When I look through futex_requeue(), all the likely looking sources
>> of EINVAL are governed by a check on the 'requeue_pi' argument.
>
>Yeah, its not very straightforward, I was also scratching my head. First
>we do:
>
> if (requeue_pi) {
> /*
> * Requeue PI only works on two distinct uaddrs. This
> * check is only valid for private futexes. See below.
> */
> if (uaddr1 == uaddr2)
> return -EINVAL;
We check here to abort as early as possible for the usual security reasons.
>
>Then:
>
> /*
> * The check above which compares uaddrs is not sufficient for
> * shared futexes. We need to compare the keys:
> */
> if (requeue_pi && match_futex(&key1, &key2)) {
> ret = -EINVAL;
> goto out_put_keys;
> }
>
>I wonder why we're checking for requeue_pi again, when, at least
>according to the comments, it should be for shared. I guess it would
>make sense depending on the mappings as the keys are the only true way
>of determining if both futexes are the same, so perhaps:
>
> if ((requeue_pi || (flags & FLAGS_SHARED)) && match_futex())
No, the rule only applies to requeue_pi. This check is the for-sure
version of the uaddr comparison above. We could add if flags &
FLAGS_SHARED, but I'm not sure it's worth it.
--
Darren Hart
Intel Open Source Technology Center
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