linux-kernel - Re: [PATCH] objtool: Detect non-relocated text references

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Message-ID: <20241004164653.wntzsvsdd27pbcsw@treble>
Date: Fri, 4 Oct 2024 09:46:53 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ard Biesheuvel <ardb@...nel.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] objtool: Detect non-relocated text references

On Fri, Oct 04, 2024 at 10:20:29AM +0200, Peter Zijlstra wrote:
> On Fri, Oct 04, 2024 at 12:28:47AM -0700, Josh Poimboeuf wrote:
> > On Fri, Oct 04, 2024 at 08:54:16AM +0200, Ard Biesheuvel wrote:
> > > On Fri, 4 Oct 2024 at 02:31, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> > > > However there are some "lea xxx(%rip)" cases which don't use relocations
> > > > because they're referencing code in the same translation unit.
> > > 
> > > input section
> > 
> > "in the same translation unit and section" ?
> > 
> > > > However if we enable -fpie, the compiler would start generating them and
> > > > there would definitely be bugs in the IBT sealing.
> > > >
> > > 
> > > -fpie is guaranteed to break things, but even without it, Clang may
> > > issue RIP-relative LEA instructions (or LLD when it performs
> > > relaxations), so this is definitely worth addressing even if we don't
> > > enable -fpie.
> > 
> > I haven't seen this with Clang either.  Also, objtool runs before the
> > linker so LLD relaxations shouldn't matter.
> 
> LTO might have a few more cases, the input sections are bigger there.
> But even there we run before the final link stage.

Clang LTO uses -ffunction-sections so it shouldn't be possible.

-- 
Josh