linux-kernel - Re: [RFC PATCH 02/21] stackvalidate: Add C version of STACKVALIDATE_IGNORE

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Date:	Sat, 18 Jul 2015 11:40:59 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Michal Marek <mmarek@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	Andy Lutomirski <luto@...nel.org>,
	Pedro Alves <palves@...hat.com>, live-patching@...r.kernel.org,
	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC PATCH 02/21] stackvalidate: Add C version of
 STACKVALIDATE_IGNORE_INSN

On Sat, Jul 18, 2015 at 09:16:58AM -0700, Linus Torvalds wrote:
> On Jul 17, 2015 09:57, "Josh Poimboeuf" <jpoimboe@...hat.com> wrote:
> >
> > +
> > +#define STACKVALIDATE_IGNORE_INSN                              \
> > +       ".Ltemp" __stringify(__LINE__) ":;"                     \
> 
> This is wrong. It won't work for people who do multiple of these on the
> same line (think macros etc).
> 
> For temporary labels, you should just use numeric labels. Think Pascal
> style ones. Then use "b" or "f" after the number when referring to it to
> say "back" or "forward" reference.
> 
> So the code for an endless loop with a true temporary label looks like:
> 
>    1:   code
>          jmp 1b
> 
> and a branch over would look like
> 
>           jne 7f
>         .. code ...
>     7:   ...
> 
> and now you can have multiple of these truly temp labels without ever
> getting errors from having a label redefined.

Yeah, I think I'll change the C macro to do that.

However for asm macros you have to worry about collisions.  For example:

.macro FOO
	1: ...
.endm

1:	...
	FOO
	jmp 1b

It would jump to "1" in the macro instead of the "1" before the macro.

So for the asm version of the macro I ended up using Andy's idea of using "\@"
to get a unique identifier:

.macro STACKVALIDATE_IGNORE_INSN
	.if CONFIG_STACK_VALIDATION
		.Ltemp_\@:
		.pushsection __stackvalidate_ignore_insn, "a"
		_ASM_ALIGN
		.long .Ltemp_\@ - .
		.popsection
	.endif
.endm


-- 
Josh
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