linux-kernel - Re: futex(2) man page update help request

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Message-ID: <D0DEF5CA.B7F02%dvhart@linux.intel.com>
Date:	Fri, 16 Jan 2015 17:11:49 -0800
From:	Darren Hart <dvhart@...ux.intel.com>
To:	Davidlohr Bueso <dave@...olabs.net>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Carlos O'Donell <carlos@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, Jakub Jelinek <jakub@...hat.com>,
	"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux API <linux-api@...r.kernel.org>,
	Torvald Riegel <triegel@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	Darren Hart <dvhart@...radead.org>,
	Anton Blanchard <anton@...ba.org>, Petr Baudis <pasky@...e.cz>,
	Eric Dumazet <edumazet@...gle.com>,
	bill o gallmeister <bgallmeister@...il.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Daniel Wagner <wagi@...om.org>, Rich Felker <dalias@...c.org>
Subject: Re: futex(2) man page update help request

On 1/16/15, 4:56 PM, "Davidlohr Bueso" <dave@...olabs.net> 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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