linux-kernel - Re: Definition of BUG on x86

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Date:	Thu, 19 Feb 2009 07:32:45 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Petr Tesarik <ptesarik@...e.cz>, "H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Definition of BUG on x86

Ingo Molnar wrote:
> * Petr Tesarik <ptesarik@...e.cz> wrote:
>
>   
>> Ingo Molnar píše v Čt 19. 02. 2009 v 13:47 +0100:
>>     
>>> * Petr Tesarik <ptesarik@...e.cz> wrote:
>>>
>>>       
>>>> Ingo Molnar píše v Čt 19. 02. 2009 v 13:22 +0100:
>>>>         
>>>>> * Petr Tesarik <ptesarik@...e.cz> wrote:
>>>>>
>>>>>           
>>>>>> Ingo Molnar píše v Čt 19. 02. 2009 v 13:10 +0100:
>>>>>>             
>>>>>>> * Petr Tesarik <ptesarik@...e.cz> wrote:
>>>>>>>
>>>>>>>               
>>>>>>>> So, the only method I could invent was using gas macros. It 
>>>>>>>> works but is quite ugly, because it relies on the actual 
>>>>>>>> assembler instruction which is generated by the compiler. Now, 
>>>>>>>> AFAIK gcc has always translated "for(;;)" into a jump to self, 
>>>>>>>> and that with any conceivable compiler options, but I don't 
>>>>>>>> know anything about Intel cc.
>>>>>>>>                 
>>>>>>>> +static inline __noreturn void discarded_jmp(void)
>>>>>>>> +{
>>>>>>>> +	asm volatile(".macro jmp target\n"
>>>>>>>> +		     "\t.purgem jmp\n"
>>>>>>>> +		     ".endm\n");
>>>>>>>> +	for (;;) ;
>>>>>>>> +}
>>>>>>>>                 
>>>>>>> hm, that's very fragile.
>>>>>>>
>>>>>>> Why not just:
>>>>>>>
>>>>>>>  static inline __noreturn void x86_u2d(void)
>>>>>>>  {
>>>>>>> 	asm volatile("u2d\n");
>>>>>>>  }
>>>>>>>
>>>>>>> If GCC emits a bogus warning about _that_, then it's a bug in 
>>>>>>> the compiler that should be fixed.
>>>>>>>               
>>>>>> I wouldn't call it a bug. The compiler has no idea about what 
>>>>>> the inline assembly actualy does. So it cannot recognize that 
>>>>>> the ud2 instruction does not return (which BTW might not even 
>>>>>> be the case, depending on the implementation of the Invalid 
>>>>>> Opcode exception).
>>>>>>             
>>>>> No, i'm not talking about the inline assembly.
>>>>>
>>>>> I'm talking about the x86_u2d() _inline function_, which has 
>>>>> the __noreturn attribute.
>>>>>
>>>>> Shouldnt that be enough to tell the compiler that it ... wont 
>>>>> return?
>>>>>           
>>>> Nope, that's not how it works.
>>>>
>>>> You _may_ specify a noreturn attribute to any function (and 
>>>> GCC will honour it AFAICS), but if GCC _thinks_ that the 
>>>> function does return, it will issue the above-mentioned 
>>>> warning:
>>>>
>>>> /usr/src/linux-2.6/arch/x86/include/asm/bug.h:10: warning: 'noreturn' function does return
>>>>
>>>> And that's what your function will do. :-(
>>>>
>>>> Yes, I also thinks that this behaviour is counter-intuitive. 
>>>> Besides, I haven't found a gcc switch to turn this warning 
>>>> off, which would be my next recommendation, since the GCC 
>>>> heuristics is broken, of course.
>>>>         
>>> so GCC should be fixed and improved here, on several levels.
>>>       
>> Agree.
>>
>> But it takes some time, even if we start pushing right now. 
>> What's your suggestion for the meantime? Keep the dummy jmp? 
>> And in case anybody is concerned about saving every byte in 
>> the text section, they can apply my dirty patch?
>>
>> Actually, this doesn't sound too bad.
>>     
>
> yeah. Please forward the problem to the appropriate GCC list in 
> any case.
>
>   

I think the official answer for this case is to use __builtin_trap.  But:

 -- Built-in Function: void __builtin_trap (void)
     This function causes the program to exit abnormally.  GCC
     implements this function by using a target-dependent mechanism
     (such as intentionally executing an illegal instruction) or by
     calling `abort'.  ***The mechanism used may vary from release to
     release so you should not rely on any particular implementation.***

which in principle is hard for us to make use of.  In practice I think 
it has always been ud2a on x86.

http://gcc.gnu.org/ml/gcc-patches/2000-01/msg00190.html

    J
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