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

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Message-ID: <YFIOl/3NcnnV0jDa@hirez.programming.kicks-ass.net>
Date:   Wed, 17 Mar 2021 15:13:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     x86@...nel.org, jpoimboe@...hat.com, jgross@...e.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/9] objtool: Add elf_create_undef_symbol()

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 ?

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