linux-kernel - RE: [regression] Re: [PATCH 2/3] futex: Sanitize cmpxchg_futex_value

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Date:	Mon, 5 Mar 2012 23:21:53 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Jonathan Nieder <jrnieder@...il.com>,
	"linux-ia64@...r.kernel.org" <linux-ia64@...r.kernel.org>
CC:	Michel Lespinasse <walken@...gle.com>,
	Émeric Maschino <emeric.maschino@...il.com>,
	Patrick Baggett <baggett.patrick@...il.com>,
	Jakub Jelinek <jakub@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [regression] Re: [PATCH 2/3] futex: Sanitize
 cmpxchg_futex_value_locked API

> It doesn't look like the return value (r8) is actually being set beyond
> initialized to 0. If there is some ia64 instruction that modifies it, GCC
> doesn't know about it from the inline assembly (r8 doesn't appear in the
> inputs/outputs list). From looking at the x86 version (agh, inline asm is
> hard to parse), it does modify the return value based on whether the
> comparison was a success or not, and the return value is certainly used by
> the callers.

The commit comment for the change makes it sound like the return value
is an error code (-ENOSYS if the function isn't implemented/configured;
-EFAULT if the user address is bogus) - or zero if nothing bad happened.

Not "the comparison was a success or not".

What's the real answer? The ia64 code is returning 0 regardless of whether the
compare/exchange found the old value or not.  Is this a bad assumption?

-Tony