linux-kernel - Re: vm86.c audit_syscall_exit() call trashes registers

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Message-Id: <42824DD7-AD58-4C2C-A15C-C88EBFAC2C56@MIT.EDU>
Date:	Thu, 4 Oct 2007 19:58:37 -0400
From:	William Cattey <wdc@....EDU>
To:	Chuck Ebbert <cebbert@...hat.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Chuck Anderson <cra@....EDU>,
	linux-kernel@...r.kernel.org, Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: vm86.c audit_syscall_exit() call trashes registers

Thanks very much for thinking about this and providing a revised  
candidate patch.

Sadly, the effect of the patch is the same as the most recent  
candidate patch from Jeremy Fitzhardinge:  The EDID transfer still  
comes up all zeros.

This is very perplexing to me.  If I take the code that appears in  
2.6.18's vm86.c, and simply put #if 0 around the call to  
audit_syscall_exit I get good data.

If this is indeed a correct minimal correction to the  
audit_syscall_exit code, then perhaps there's some other condition  
being exercised.  I guess my next step is to take the whole pt_regs  
patch (commit 49d26b6eaa8e970c8cf6e299e6ccba2474191bf5) from  
kernel.org and see if that has a beneficial effect.

-Bill

----

William Cattey
Linux Platform Coordinator
MIT Information Services & Technology

N42-040M, 617-253-0140, wdc@....edu
http://web.mit.edu/wdc/www/


On Oct 2, 2007, at 12:44 PM, Chuck Ebbert wrote:

> On 09/25/2007 07:38 PM, William Cattey wrote:
>>
>> I'd feel a lot more confident we were on the right track if I  
>> could just
>> correctly patch Fitzhardinge's cleanup into the test setup I have  
>> now.
>>
>
> I think you need to zero both registers if you're using 2.6.16, and  
> force
> %eax as the source so it doesn't choose %ebp?
>
> --- a/arch/i386/kernel/vm86.c
> +++ b/arch/i386/kernel/vm86.c
> @@ -306,19 +334,19 @@ static void do_sys_vm86(struct  
> kernel_vm86_struct *info, struct task_struct *tsk
>  	tsk->thread.screen_bitmap = info->screen_bitmap;
>  	if (info->flags & VM86_SCREEN_BITMAP)
>  		mark_screen_rdonly(tsk->mm);
> -	__asm__ __volatile__("xorl %eax,%eax; movl %eax,%fs; movl %eax,%gs 
> \n\t");
> -	__asm__ __volatile__("movl %%eax, %0\n" :"=r"(eax));
>
>  	/*call audit_syscall_exit since we do not exit via the normal  
> paths */
>  	if (unlikely(current->audit_context))
> -		audit_syscall_exit(AUDITSC_RESULT(eax), eax);
> +		audit_syscall_exit(AUDITSC_RESULT(0), 0);
>
>  	__asm__ __volatile__(
>  		"movl %0,%%esp\n\t"
>  		"movl %1,%%ebp\n\t"
> +		"mov  %2, %%fs\n\t"
> +		"mov  %2, %%gs\n\t"
>  		"jmp resume_userspace"
>  		: /* no outputs */
> -		:"r" (&info->regs), "r" (task_thread_info(tsk)));
> +		:"r" (&info->regs), "r" (task_thread_info(tsk)), "a" (0));
>  	/* we never return here */
>  }
>

-
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