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

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Message-ID: <20200403145812.zdoxkyjhn42veu52@treble>
Date:   Fri, 3 Apr 2020 09:58:12 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        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 Fri, Apr 03, 2020 at 10:58:20AM +0200, Miroslav Benes wrote:
> > +	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);
> > +		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);
> > +		}
> 
> I suppose there is always just one NOP pad insn, right? Anyway, it would 
> be better to get rid of it as you proposed.

There can actually be multiple NOPs because functions are aligned on a
16-byte boundary.  But the undefined ORC entry is always at the first
NOP because objtool merges duplicate entries.

-- 
Josh