linux-kernel - Re: [PATCH 3/5] objtool: Support Clang non-section symbols in ORC generation

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Message-ID: <20200401190548.rodiauk3iolknvfe@treble>
Date:   Wed, 1 Apr 2020 14:05:48 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Miroslav Benes <mbenes@...e.cz>,
        Julien Thierry <jthierry@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Dmitry Golovin <dima@...ovin.in>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: Re: [PATCH 3/5] objtool: Support Clang non-section symbols in ORC
 generation

On Wed, Apr 01, 2020 at 08:49:53PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 01, 2020 at 01:23:27PM -0500, Josh Poimboeuf wrote:
> 
> > @@ -105,8 +100,32 @@ static int create_orc_entry(struct elf *elf, struct section *u_sec, struct secti
> >  	}
> >  	memset(rela, 0, sizeof(*rela));
> >  
> > -	rela->sym = insn_sec->sym;
> > -	rela->addend = insn_off;
> > +	if (insn_sec->sym) {
> > +		rela->sym = insn_sec->sym;
> > +		rela->addend = insn_off;
> > +	} else {
> > +		/*
> > +		 * The Clang assembler doesn't produce section symbols, so we
> > +		 * have to reference the function symbol instead:
> > +		 */
> > +		rela->sym = find_symbol_containing(insn_sec, insn_off);
> 
> It's a good thing I made that a lot faster I suppose ;-)

:-)

> > +		if (!rela->sym) {
> > +			/*
> > +			 * Hack alert.  This happens when we need to reference
> > +			 * the NOP pad insn immediately after the function.
> > +			 */
> > +			rela->sym = find_symbol_containing(insn_sec,
> > +							   insn_off - 1);
> 
> Urgh, when does that happen? 

It happens naturally in the padding between functions, since objtool
doesn't traverse those instructions.  So they have undefined entries
like

 .text+68: sp:(und) bp:(und) type:call end:0

I suppose those aren't technically necessary.

-- 
Josh