linux-kernel - Re: [PATCH v2 34/39] objtool: Validate IBT assumptions

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Date:   Mon, 28 Feb 2022 10:39:13 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 34/39] objtool: Validate IBT assumptions

On Mon, Feb 28, 2022 at 10:26:07AM +0100, Peter Zijlstra wrote:
> On Sun, Feb 27, 2022 at 06:00:03PM +0100, Peter Zijlstra wrote:
> > On Sat, Feb 26, 2022 at 07:13:48PM -0800, Josh Poimboeuf wrote:
> > > > +static struct instruction *
> > > > +validate_ibt_reloc(struct objtool_file *file, struct reloc *reloc)
> > > > +{
> > > > +	struct instruction *dest;
> > > > +	struct section *sec;
> > > > +	unsigned long off;
> > > > +
> > > > +	sec = reloc->sym->sec;
> > > > +	off = reloc->sym->offset + reloc->addend;
> > > 
> > > This math assumes non-PC-relative.  If it's R_X86_64_PC32 or
> > > R_X86_64_PLT32 then it needs +4 added.
> > 
> > Right; so I actually had that PC32 thing in there for a while, but ran
> > into other trouble. I'll go try and figure it out.
> 
> Things like .rela.initcall*.init use PC32 but don't need the +4. If we
> get that wrong it'll seal all the initcall and boot doesn't get very
> far at all :-)

Ah...

> How do you feel about something like:
> 
> 	sec = reloc->sym->sec;
> 	off = reloc->sym->offset;
> 
> 	if ((reloc->sec->base->sh.sh_flags & SHF_EXECINSTR) &&
> 	    (reloc->type == R_X86_64_PC32 || reloc->type == R_X86_64_PLT32))
> 		off += arch_dest_reloc_offset(reloc->addend);
> 	else
> 		off += reloc->addend;
> 
> 
> hmm ?

Looks good to me.

-- 
Josh