linux-kernel - Re: [PATCH] x86/sev: Disable GCOV on noinstr object

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Message-ID: <CAMj1kXH8Yg+eeySUxJ7nRTm=nQWx5Tj4XirBjVrqXB-0Qn1=sA@mail.gmail.com>
Date: Mon, 17 Nov 2025 12:40:41 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Nathan Chancellor <nathan@...nel.org>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] x86/sev: Disable GCOV on noinstr object

On Mon, 17 Nov 2025 at 12:11, Brendan Jackman <jackmanb@...gle.com> wrote:
>
> With Debian clang version 19.1.7 (3+build5) there are calls to
> kasan_check_write() from __sev_es_nmi_complete, which violates noinstr.
> Fix it by disabling GCOV for the noinstr object, as has been done for
> previous such instrumentation issues.
>
> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
> ---
> Details:
>
>  - ❯❯  clang --version
>    Debian clang version 19.1.7 (3+build5)
>    Target: x86_64-pc-linux-gnu
>    Thread model: posix
>    InstalledDir: /usr/lib/llvm-19/bin
>
>  - Compiling from tip/master at 6f85aad74a70d
>
>  - Kernel config:
>
>    https://gist.githubusercontent.com/bjackman/bbfdf4ec2e1dfd0e18657174f0537e2c/raw/a88dcc6567d14c69445e7928a7d5dfc23ca9f619/gistfile0.txt
>
> Note I also get this error:
>
> vmlinux.o: warning: objtool: set_ftrace_ops_ro+0x3b: relocation to !ENDBR: machine_kexec_prepare+0x810
>
> That one's a total mystery to me. I guess it's better to "fix" the SEV
> one independently rather than waiting until I know how to fix them both.
> ---
>  arch/x86/coco/sev/Makefile | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/coco/sev/Makefile b/arch/x86/coco/sev/Makefile
> index 3b8ae214a6a64de6bb208eb3b7c8bf12007ccc2c..d2ceae587b6c30b2fb17209a7426e7893dea988c 100644
> --- a/arch/x86/coco/sev/Makefile
> +++ b/arch/x86/coco/sev/Makefile
> @@ -8,3 +8,6 @@ UBSAN_SANITIZE_noinstr.o        := n
>  # GCC may fail to respect __no_sanitize_address or __no_kcsan when inlining
>  KASAN_SANITIZE_noinstr.o       := n
>  KCSAN_SANITIZE_noinstr.o       := n
> +
> +# Clang 19 and older may fail to respect __no_sanitize_address when inlining
> +GCOV_PROFILE_noinstr.o         := n
>

After Thomas dug into this issue a while ago, I meant to follow up
with a fix, or at least something to start the discussion.

TL;DR there is nothing wrong with either compiler (as far as this
issue is concerned)

The issue is that KASAN/KCSAN enabled builds use a version of
set_bit() that unconditionally inserts a call to