linux-kernel - Re: [PATCH 3/8] i386: bitops: Rectify bogus "+m" constraints

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 24 Jul 2007 10:22:44 +0100
From:	David Howells <dhowells@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Satyam Sharma <ssatyam@....iitk.ac.in>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <nickpiggin@...oo.com.au>, Andi Kleen <ak@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/8] i386: bitops: Rectify bogus "+m" constraints

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > So, using the "+" constraint modifier for memory, like "+m" is bogus.
> > We must simply specify "=m" which handles the case correctly.
> 
> No. This is wrong.

Agreed.

> "=m" means that the new value of the memory location is *set*.
> 
> Which means that gcc will potentially optimize away any previous stores to 
> that memory location.
> 
> And yes, it happens, and yes, we've seen it.

I had a lot of "fun" with this when mucking around with the asm-optimised R/W
semaphores.

David
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/