linux-kernel - Re: [PATCH 6/9] objtool: Add elf_create_undef

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Date:   Wed, 17 Mar 2021 15:39:59 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     x86@...nel.org, jpoimboe@...hat.com, jgross@...e.com,
        linux-kernel@...r.kernel.org, samitolvanen@...gle.com
Subject: Re: [PATCH 6/9] objtool: Add elf_create_undef_symbol()

On Wed, 17 Mar 2021, Peter Zijlstra wrote:

> On Wed, Mar 17, 2021 at 02:52:23PM +0100, Miroslav Benes wrote:
> 
> > > +	if (!elf_symbol_add(elf, sym, SHN_XINDEX)) {
> > > +		WARN("elf_symbol_add");
> > > +		return NULL;
> > > +	}
> > 
> > SHN_XINDEX means that the extended section index is used. Above you seem 
> > to use it in the opposite sense too (assigning to shndx when shndx_data is 
> > NULL). While it makes the code easier to handle, it is a bit confusing 
> > (and maybe I am just confused now). Could you add a comment about that, 
> > please? elf_symbol_add() seems like a good place.
> 
> Yes, that was a horrible thing to do :/ And you understood it right.
> 
> Looking at it again, I'm not sure it is actually correct tho; shouldn't
> elf_create_undef_symbol() also look at gelf_getsymshndx() of symtab ?

Probably yes.
 
> What toolchain generates these extended sections and how? That is, how
> do I test this crud..

Sami might know.

Miroslav