linux-kernel - Re: [RFC] Documentation about unaligned memory access

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]

Message-Id: <BAAE8DE3-B96F-4EA8-B8CC-B2BD252B7259@mac.com>
Date:	Thu, 22 Nov 2007 22:04:34 -0500
From:	Kyle Moffett <mrmacman_g4@....com>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
Cc:	Daniel Drake <dsd@...too.org>, linux-kernel@...r.kernel.org,
	davem@...emloft.net, kune@...ne-taler.de, johannes@...solutions.net
Subject: Re: [RFC] Documentation about unaligned memory access

On Nov 22, 2007, at 20:29:11, Alan Cox wrote:
>> Most architectures are unable to perform unaligned memory  
>> accesses. Any unaligned access causes a processor exception.
>
> Not all. Some simply produce the wrong answer - thats oh so much  
> more exciting.

As one example, the MicroBlaze soft-core processor family designed  
for use on Xilinx FPGAs will (by default) simply forcibly zero the  
lower bits of the unaligned address, such that the following code  
will fail mysteriously:

const char foo[] = { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07 };
printf("0x%08lx 0x%08lx 0x%08lx 0x%08lx\n",
		*((u32 *)(foo+0)),
		*((u32 *)(foo+1)),
		*((u32 *)(foo+2)),
		*((u32 *)(foo+3)));

Instead of outputting:
0x00010203 0x01020304 0x02030405 0x03040506

It will output:
0x00010203 0x00010203 0x00010203 0x00010203

Other embedded architectures have very similar problems.  Some may  
provide an "unaligned data access" exception, but offer insufficient  
information to repair the damage and resume execution.

Cheers,
Kyle Moffett

-
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/