linux-kernel - Re: [PATCH] sha: prevent removal of memset as dead store in sha1

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] [day] [month] [year] [list]

Date:	Fri, 26 Feb 2010 16:55:53 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Mikael Pettersson <mikpe@...uu.se>
Cc:	Andi Kleen <andi@...stfloor.org>,
	roel kluin <roel.kluin@...il.com>,
	David Miller <davem@...emloft.net>,
	herbert@...dor.apana.org.au, linux-crypto@...r.kernel.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sha: prevent removal of memset as dead store in
	sha1_update()

> Instead of barrier(), this works with gcc-3.2.3 up to gcc-4.4.3
> for the purpose of making the memset() not disappear:
> 
> 	{
> 		struct s { char c[n]; };
> 		asm("" : : "m"(*(struct s *)p));
> 	}
> 
> Every byte in the [p,p+n[ range must be used. If you only use the
> first byte, via e.g. asm("" :: "m"(*(char*)p)), then the compiler
> _will_ skip scrubbing bytes beyond the first.
> 
> An explicit loop that uses each byte individually also works, but
> results in awful code with older compilers.

I suspect it would be worth asking for a __builtin for this.
iirc Visual C already has one.

BTW I remember finding a few more such cases in random user space
code in past code review.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
--
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/