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

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Date:   Wed, 1 Apr 2020 14:08:55 -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 02:05:51PM -0500, Josh Poimboeuf wrote:
> 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.

In fact, we could probably get substantial savings in the ORC table if
we skipped those, i.e.

  .text+0: sp:sp+8 bp:(und) type:call end:0
  .text+8: sp:(und) bp:(und) type:call end:0
  .text+10: sp:sp+8 bp:(und) type:call end:0
  .text+17: sp:sp+16 bp:(und) type:call end:0
  .text+18: sp:sp+24 bp:prevsp-24 type:call end:0
  .text+1c: sp:sp+32 bp:prevsp-24 type:call end:0
  .text+5a: sp:sp+24 bp:prevsp-24 type:call end:0
  .text+61: sp:sp+16 bp:(und) type:call end:0
  .text+63: sp:sp+8 bp:(und) type:call end:0
  .text+68: sp:(und) bp:(und) type:call end:0
  .text+70: sp:sp+8 bp:(und) type:call end:0
  .text+8c: sp:(und) bp:(und) type:call end:0
  .text+90: sp:sp+8 bp:(und) type:call end:0
  .text+cd: sp:(und) bp:(und) type:call end:0
  .text+d0: sp:sp+8 bp:(und) type:call end:0

would be compressed to

  .text+0: sp:sp+8 bp:(und) type:call end:0
  .text+17: sp:sp+16 bp:(und) type:call end:0
  .text+18: sp:sp+24 bp:prevsp-24 type:call end:0
  .text+1c: sp:sp+32 bp:prevsp-24 type:call end:0
  .text+5a: sp:sp+24 bp:prevsp-24 type:call end:0
  .text+61: sp:sp+16 bp:(und) type:call end:0
  .text+63: sp:sp+8 bp:(und) type:call end:0

but I can do that in a separate patch, and if it works I can remove this
hack.

-- 
Josh