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

[Andy Lutomirski]

On Tue, Jul 7, 2015 at 3:59 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
>> > ---
>> >  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.)

Oh, egads.  FWIW, doing it in the linker script would probably be less hackish.

Does the linker discard before noticing duplicate symbols in that 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.
>
> Agreed.  And .Ltemp_\@ sounds like a good idea.  I'll try it.

:)

--Andy
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