linux-kernel - Re: [PATCH] pop previous section in alternative.c

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Date:	Thu, 10 Apr 2008 08:18:49 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Andy Whitcroft <apw@...dowen.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	akpm@...ux-foundation.org, Rusty Russell <rusty@...tcorp.com.au>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Jan Beulich <jbeulich@...ell.com>,
	Thomas Gleixner <tglx@...utronix.de>, pinskia@....gnu.org
Subject: Re: [PATCH] pop previous section in alternative.c



On Thu, 10 Apr 2008, Andi Kleen wrote:
> 
> A simple way to detect it on the assembler level would be checking
> that the section is the same after #NO_APP as before #APP

That would mark gcc itself as buggy, because gcc will move some things 
into the #APP/#NO_APP thing, and sometimes doesn't end the #APP at all!

Try gcc -S on this this trivial "program"

	asm("Hello world");

and at least I personally get

	        .file   "bug.c"
	#APP
	        Hello world
	        .ident  "GCC: (GNU) 4.1.2 20070925 (Red Hat 4.1.2-33)"
	        .section        .note.GNU-stack,"",@progbits

and nothing else. Note the lack of #NO_APP ;)

(I also swear I've seen code or data move _into_ the #NO_APP - ie gcc did 
eventually close the #APP section, but did it too late, after it had 
emitted other things itself - but maybe I just dreamed it because I can't 
seem to reproduce it now. Or maybe it's just a historical gcc bug that got 
fixed)

			Linus
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