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

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Date:   Thu, 18 Mar 2021 08:56:21 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Miroslav Benes <mbenes@...e.cz>, x86@...nel.org, 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 07:46:14PM -0500, Josh Poimboeuf wrote:
> On Wed, Mar 17, 2021 at 03:13:43PM +0100, 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 ?
> > 
> > What toolchain generates these extended sections and how? That is, how
> > do I test this crud..
> 
> SHN_XINDEX is basically a special-case extension to original ELF for
> supporting more than 64k sections.

Yeah, I figured it all out again last night; the patch as presented is
hideous but correct(ish). But I've rewritten it and it's all much better
now. Will post later.