linux-kernel - Re: [PATCH 4/6] objtool: Replace STACK_FRAME_NON

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Message-ID: <20190227122059.GO32494@hirez.programming.kicks-ass.net>
Date:   Wed, 27 Feb 2019 13:20:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     torvalds@...ux-foundation.org, tglx@...utronix.de, hpa@...or.com,
        julien.thierry@....com, will.deacon@....com, luto@...capital.net,
        mingo@...nel.org, catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, luto@...nel.org,
        bp@...en8.de, dvlasenk@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] objtool: Replace STACK_FRAME_NON_STANDARD annotation

On Mon, Feb 25, 2019 at 10:11:24AM -0600, Josh Poimboeuf wrote:
> On Mon, Feb 25, 2019 at 01:43:34PM +0100, Peter Zijlstra wrote:

> > -#define STACK_FRAME_NON_STANDARD(func) \
> > -	static void __used __section(.discard.func_stack_frame_non_standard) \
> > -		*__func_stack_frame_non_standard_##func = func
> > +#define STACK_FRAME_NON_STANDARD(func)					\
> > +	asm (".pushsection .discard.nonstd_frame_strtab, \"S\", @3\n\t"	\
> > +	     "999: .ascii \"" #func "\"\n\t"				\
> > +	     "     .byte 0\n\t"						\
> > +	     ".popsection\n\t"						\
> > +	     ".pushsection .discard.nonstd_frame\n\t"			\
> > +	     ".long 999b - .\n\t"					\
> > +	     ".popsection\n\t")
> > +
> 
> I don't think this will work in the case where GCC does an IPA
> optimization and renames the function (e.g., renames func to
> func.isra.1234), right?  That might be a deal breaker...

Or; as has been found by 0day; the whole function gets inlined and
the symbol no longer exists at all.

That's curable with a noinline, but all things considered, I think we
should go back to the old horrible scheme. Andy?