linux-kernel - Re: [PATCH] x86: do not allow to optimize flag_is_changeable

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 for Android: free password hash cracker in your pocket

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

Message-ID: <48E1CD29.8050109@goop.org>
Date:	Mon, 29 Sep 2008 23:54:33 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Yinghai Lu <yinghai@...nel.org>
CC:	Krzysztof Helt <krzysztof.h1@...zta.fm>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: do not allow to optimize flag_is_changeable_p()

Yinghai Lu wrote:
> On Mon, Sep 29, 2008 at 11:14 PM, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>   
>> Krzysztof Helt wrote:
>>     
>>> From: Krzysztof Helt <krzysztof.h1@...pl>
>>>
>>> The flag_is_changeable_p() is used by
>>> has_cpuid_p() which can return different results
>>> in the code sequence below:
>>>
>>>  if (!have_cpuid_p())
>>>       identify_cpu_without_cpuid(c);
>>>
>>>   /* cyrix could have cpuid enabled via c_identify()*/
>>>   if (!have_cpuid_p())
>>>       return;
>>>
>>> Otherwise, the gcc 3.4.6 optimizes these two calls
>>> into one which make the code not working correctly.
>>> Cyrix cpus have the CPUID instruction enabled but
>>> it is not detected due to the gcc optimization.
>>> Thus the ARR registers (mtrr like) are not detected
>>> on such a cpu.
>>>
>>>       
>> If "asm volatile" changes the code and fixes the bug, it seems like
>> you're making use of an undocumented - or at least non-portable - behaviour.
>>
>> Does adding a "memory" clobber also fix the problem?  That would have
>> better defined characteristics.
>>
>>     
>
> how about
>
>         if (!have_cpuid_p()) {
>                 identify_cpu_without_cpuid(c);
>
>                 /* cyrix could have cpuid enabled via c_identify()*/
>                 if (!have_cpuid_p())
>                         return;
>         }
>   

That doesn't help, does it?  If gcc thinks it can get away with
evaluating have_cpuid_p() once, then that's the same as:

	if (!have_cpuid_p()) {
		identify_cpu_without_cpuid(c);

		return;
	}

even though identify_cpu_without_cpuid() can cause the cpu to suddenly
start supporting cpuid.

The trouble is that flag_is_changeable_p() doesn't have any obvious
global dependencies; it just takes a constant argument and returns a
result.   The asm() needs to be updated to have a "memory" constraint as
a stand-in for the specific constraint of "cpu has switched into
cpuid-supporting state".

    J
--
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/