linux-kernel - Re: [PATCH 5/9] objtool: Rework rebuild

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Message-ID: <20210318173842.55rwasdbqlfx7a2i@treble>
Date:   Thu, 18 Mar 2021 12:38:42 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, jgross@...e.com, mbenes@...e.cz,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/9] objtool: Rework rebuild_reloc logic

On Thu, Mar 18, 2021 at 06:04:25PM +0100, Peter Zijlstra wrote:
> On Thu, Mar 18, 2021 at 11:36:40AM -0500, Josh Poimboeuf wrote:
> > > I was thinking you could get a section changed without touching
> > > relocations, but while that is theoretically possible, it is exceedingly
> > > unlikely (and objtool doesn't do that).
> > 
> > Hm?  This is a *relocation* section, not a normal one.  So by
> > definition, it only changes when its relocations change.
> 
> The way I read this code:
> 
>  	list_for_each_entry(sec, &elf->sections, list) {
>  		if (sec->changed) {
> +			if (sec->reloc &&
> +			    elf_rebuild_reloc_section(elf, sec->reloc)) {
> +				WARN_ELF("elf_rebuild_reloc_section");
> +				return -1;
> +			}
> 
> is that we iterate the regular sections (which could be dirtied because
> we changed some data), and if that section has a relocation section, we
> rebuild that for good measure (even though it might not have altered
> relocations).
> 
> Or am I just totally confused ?

Ah, you're right.  I'm the one that's confused.  I guess I was also
confused when I wrote that hunk, but it just happens to work anyway.

It would be cleaner to do something like

			if ((is_reloc_sec(sec) &&	
			    elf_rebuild_reloc_section(elf, sec)) {

so we process the changed reloc section directly, instead of relying on
the (most likely) fact that the corresponding text section also changed.

-- 
Josh