linux-kernel - Re: [tip: x86/boot] x86/boot: Disable jump tables in PIC code

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Message-ID: <CAMj1kXERamRZW6rmqa8dqQdx81Sc5bSJKijoQpVp76ZyFns3PA@mail.gmail.com>
Date: Wed, 23 Apr 2025 19:02:30 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: tip-bot2 for Ard Biesheuvel <tip-bot2@...utronix.de>, linux-tip-commits@...r.kernel.org, 
	Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, linux-efi@...r.kernel.org, 
	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [tip: x86/boot] x86/boot: Disable jump tables in PIC code

On Wed, 23 Apr 2025 at 18:41, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> On Wed, Apr 23, 2025 at 05:01:42PM +0200, Ard Biesheuvel wrote:
> > > > So let's not bother and disable jump tables for code built with -fPIC
> > > > under arch/x86/boot/startup.
> > >
> > > Hm, does objtool even run on boot code?
> > >
> >
> > This is about startup code, not boot code. This is code that is part
> > of vmlinux, but runs from a different mapping of memory than the one
> > the linker assumes, and so it needs to be built with -fPIC to
> > discourage the compiler and linker from inserting symbol references
> > via the kernel virtual mapping, which may not be up yet when this code
> > runs.
>
> Maybe objtool should ignore .head.text.  It doesn't need ORC, static
> calls, uaccess validation, retpolines, etc.
>

I am trying to get rid of .head.text.

But some of the startup code may still be in use later, so I don't
think we should disable objtool validation entirely unless we really
have to.