linux-kernel - Re: [PATCH v6 4/4] stackvalidate: Add ignore macros

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Date:	Tue, 7 Jul 2015 17:59:31 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>, X86 ML <x86@...nel.org>,
	live-patching@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 4/4] stackvalidate: Add ignore macros

On Tue, Jul 07, 2015 at 03:00:38PM -0700, Andy Lutomirski wrote:
> On Tue, Jul 7, 2015 at 7:54 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > Add new stackvalidate ignore macros: STACKVALIDATE_IGNORE_INSN and
> > STACKVALIDATE_IGNORE_FUNC.  These can be used to tell stackvalidate to
> > skip validation of an instruction or a function, respectively.
> >
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > ---
> >  include/linux/stackvalidate.h | 38 ++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 38 insertions(+)
> >  create mode 100644 include/linux/stackvalidate.h
> >
> > diff --git a/include/linux/stackvalidate.h b/include/linux/stackvalidate.h
> > new file mode 100644
> > index 0000000..30d4a60
> > --- /dev/null
> > +++ b/include/linux/stackvalidate.h
> > @@ -0,0 +1,38 @@
> > +#ifndef _LINUX_STACKVALIDATE_H
> > +#define _LINUX_STACKVALIDATE_H
> > +
> > +#ifndef __ASSEMBLY__
> > +
> > +/*
> > + * This C macro tells the stack validation script to ignore the function.  It
> > + * should only be used in special cases where you're 100% sure it won't affect
> > + * the reliability of frame pointers and kernel stack traces.
> > + *
> > + * For more information, see Documentation/stack-validation.txt.
> > + */
> > +#define STACKVALIDATE_IGNORE_FUNC(_func) \
> > +       void __attribute__((section("__stackvalidate_ignore_func,\"ae\"#"))) \
> > +               *__stackvalidate_ignore_func_##_func = _func
> > +
> 
> static?  Otherwise there's some risk that ignoring a static function
> will cause a duplicate symbol.  Alternatively you could generate a
> more unique name.
> 
> Also, should the linker script be updated to discard this section?

It validates per individual object file, so there shouldn't be a risk of
duplicate symbols.  The 'e' flag in ,\"ae\"#" tells the linker to
discard the section.

(The ,\"ae\"# stuff is a horrible hack, but it's the only way I could
figure out how to set the section flags from C code.  The '#' is used to
comment out gcc's default arguments to the .section directive in favor
of the "ae" flags.)

> > +#else /* __ASSEMBLY__ */
> > +
> > +/*
> > + * This asm macro tells the stack validation script to ignore the instruction
> > + * immediately after the macro.  It should only be used in special cases where
> > + * you're 100% sure it won't affect the reliability of frame pointers and
> > + * kernel stack traces.
> > + *
> > + * For more information, see Documentation/stack-validation.txt.
> > + */
> > +.macro STACKVALIDATE_IGNORE_INSN
> > +       .if CONFIG_STACK_VALIDATION
> > +               163:
> 
> Some day we should come up with a better way to do this than using a
> random three-digit number.  I wonder if something like .Ltemp_\@ could
> be used for this purpose.

Agreed.  And .Ltemp_\@ sounds like a good idea.  I'll try it.

-- 
Josh
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