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

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Message-ID: <20210319001931.f5uqd42agkvkxlnc@treble>
Date:   Thu, 18 Mar 2021 19:19:31 -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 12:38:42PM -0500, Josh Poimboeuf wrote:
> 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.

i.e., in actual code:

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 66c49c2e20a6..3b3d19a5e626 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -948,8 +948,7 @@ int elf_write(struct elf *elf)
 	/* Update changed relocation sections and section headers: */
 	list_for_each_entry(sec, &elf->sections, list) {
 		if (sec->changed) {
-			if (sec->reloc &&
-			    elf_rebuild_reloc_section(elf, sec->reloc)) {
+			if (sec->base && elf_rebuild_reloc_section(elf, sec)) {
 				WARN_ELF("elf_rebuild_reloc_section");
 				return -1;
 			}