linux-kernel - Re: [PATCH, take4] FUTEX : new PRIVATE futexes

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Date:	Sat, 7 Apr 2007 12:00:51 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Ulrich Drepper <drepper@...il.com>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <ak@...e.de>,
	Ravikiran G Thirumalai <kiran@...lex86.org>,
	"Shai Fultheim (Shai@...lex86.org)" <shai@...lex86.org>,
	pravin b shelar <pravin.shelar@...softinc.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH, take4] FUTEX : new PRIVATE futexes

On Sat, 07 Apr 2007 19:30:14 +1000
Nick Piggin <nickpiggin@...oo.com.au> wrote:

> Eric Dumazet wrote:
 
> > 
> > - Current mm code have a problem with 64bit futexes, as spoted by Nick :
> > 
> > get_futex_key() does a check against sizeof(u32) regardless of futex being 64bits or not.
> > So it is possible a 64bit futex spans two pages of memory...
> > I had to change get_futex_key() prototype to be able to do a correct test.
> 
> I wonder if it should be encfocing alignment to keep in on 1 page?

I believe I just did that :)

Before the patch :

Alignment was only 4 bytes for all futexes, but some user app could trigger a kernel bug (since one 64bit futex could sit on two different pages, so possible separate vmas, so the inode refcounting was wrong, and access_ok did not a correct check)

After the patch :

Alignment is 8 bytes for 64 bit futexes, 4 bytes for 32bit futexes.
All futexes are contrained to be in one single page.

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