linux-kernel - Re: Definition of BUG on x86

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Message-Id: <1235048535.15053.52.camel@nathan.suse.cz>
Date:	Thu, 19 Feb 2009 14:02:15 +0100
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Definition of BUG on x86

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.

Petr Tesarik

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