linux-kernel - Re: [Xen-devel] Re: [PATCH] xen: Disable stack protector for irq helper

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Date:	Wed, 07 Oct 2009 17:33:10 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Bastian Blank <bastian@...di.eu.org>,
	Ingo Molnar <mingo@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Stable Kernel <stable@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: [Xen-devel] Re: [PATCH] xen: Disable stack protector for irq
 helper

On 10/07/09 09:35, Bastian Blank wrote:
> On Tue, Oct 06, 2009 at 12:01:12PM -0700, Jeremy Fitzhardinge wrote:
>   
>> On 10/05/09 20:30, Bastian Blank wrote:
>>     
>>> The original version saves ecx, but not edx. Both are official
>>> caller-saved registers.
>>>       
>> Hm.  It doesn't save edx because that can be half of a 64-bit return
>> value, and in general both eax and edx are marked clobbered.
>>     
> Then it will be also wrong for functions returning void. They may
> clobber eax but never set it to something correct.
>   

The asm is marked as clobbering eax/edx, so the compiler knows it can't
rely on them being preserved.  void functions are not expected to
preserve them either, so it all works out (or if they are, the compiler
will do the right thing).

Or did you have something else in mind?


>>                                                               Except one
>> place; does the patch below help?
>>     
> Don't you need to remove the complete wrapper setup to get a correct
> result? (And type safety.)
>   

Which wrapper?  Do you mean the callee-save function stuff?  Or
something else?

It compiles cleanly for me and appears to work.  Does it solve the
problem for you?

>>> Well, my call stack say something different. It crashs during early
>>> startup without a console. The modifications to the function pointers is
>>> done much later.
>>>       
>> You're right.  But you're holding out on me; can I see your backtrace? 
>>     
> Well, I'm traveling and it needs some time to recreate a broken kernel.
>
>   
>> And the disassembly of the troublesome code (both the Xen function and
>> the calling function)?
>>     
> That is easy.
>   

OK, I see, thanks.

    J
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