Re: futex(2) man page update help request

[Darren Hart]

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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