linux-kernel - Re: [patch 5/8] Immediate Values

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, 13 Nov 2007 13:26:06 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@....de>, Chuck Ebbert <cebbert@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [patch 5/8] Immediate Values - x86 Optimization (update)

Mathieu Desnoyers wrote:
> * H. Peter Anvin (hpa@...or.com) wrote:
>> Mathieu Desnoyers wrote:
>>> * H. Peter Anvin (hpa@...or.com) wrote:
>>>> Mathieu Desnoyers wrote:
>>>>>> - Use "=g" constraint for char immediate value inline assembly.
>>>>>>
>>>>>> "=g" is the same as "=rmi" which is inherently bogus.  In your actual 
>>>>>> code you use "=r", the correct constraint is "=q".
>>>>> q
>>>>>     Any register accessible as rl. In 32-bit mode, a, b, c, and d; in 
>>>>> 64-bit mode, any integer register. I am worried that "=q" might exclude 
>>>>> the si and di registers in 32-bit mode.
>>>>> What exactly is wrong with "=r" ?
>>>> For "char" (8-bit) values, sp/bp/si/di are illegal in 32-bit mode.
>>>>
>>>> Hence "=q".
>>>>
>>> Ah! yep, I see, so we say:
>>> 1 byte : "=q"
>>> 2 bytes : "=r"
>>> 4 bytes : "=r"
>>> 8 bytes : "=r"
>>> ? (si and di appear to be legal for 2 and 4 bytes in 32-bit mode)
>> That's right.
>>
>> 	-hpa
> 

Something else to watch out for... in 64-bit mode the lengths most of 
these will depend on which register is used, since whether or not a REX 
prefix is needed will vary.

As far as I can tell, you're assuming fixed length instructions, which 
is wrong unless you manually constrain yourself to only legacy registers.

	-hpa
-
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/