linux-kernel - Re: [PATCH] x86/stackprotector: fix build failure with CONFIG

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Message-ID: <CAMj1kXHTLz4onmR5iyowptRE38RCK4jNT3BoURBkq2FoDOMTxQ@mail.gmail.com>
Date: Tue, 11 Mar 2025 11:37:59 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>, Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org, 
	x86@...nel.org, Ingo Molnar <mingo@...nel.org>, "H . Peter Anvin" <hpa@...or.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Uros Bizjak <ubizjak@...il.com>, stable@...r.kernel.org, 
	Fangrui Song <i@...kray.me>, Nathan Chancellor <nathan@...nel.org>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] x86/stackprotector: fix build failure with CONFIG_STACKPROTECTOR=n

On Tue, 11 Mar 2025 at 11:24, Borislav Petkov <bp@...en8.de> wrote:
>
> On Mon, Mar 10, 2025 at 11:19:03PM +0100, Ard Biesheuvel wrote:
> > and no error.
>
> Oh fun.
>
> > Could you capture the output of
> >
> > objdump -dr .tmp_vmlinux2 --section .head.text
> >
> > and share it somewhere please?
>
> See attached.
>
> Now lemme try to bisect it, see what this machine says since it is magically
> toolchain or whatnot-specific. :-\
>

There are many occurrences of

ffffffff8373cb87: 49 c7 c6 20 c0 55 86 mov    $0xffffffff8655c020,%r14
ffffffff8373cb8a:         R_X86_64_32S __ref_stack_chk_guard

whereas the ordinary Clang uses R_X86_64_REX_GOTPCRELX here, which are
relaxed by the linker.

I suspect that Ubuntu's Clang 15 has some additional patches that
trigger this behavior.

We could add __no_stack_protector to __head to work around this.