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

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Date:	Tue, 7 Jul 2015 15:00:38 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
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 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?

> +#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.

--Andy
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