linux-kernel - Re: [PATCH 0/2] Noinstr fixes for K[CA]SAN with GCOV

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Message-ID: <CANpmjNNK6vRsyQ6SiD3Uy7fNim-wV+KWgbEokOaxbbd02Wa=ew@mail.gmail.com>
Date: Mon, 8 Dec 2025 10:37:30 +0100
From: Marco Elver <elver@...gle.com>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: Andrey Ryabinin <ryabinin.a.a@...il.com>, Alexander Potapenko <glider@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Vincenzo Frascino <vincenzo.frascino@....com>, Ard Biesheuvel <ardb@...nel.org>, kasan-dev@...glegroups.com, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] Noinstr fixes for K[CA]SAN with GCOV

On Mon, 8 Dec 2025 at 02:35, Brendan Jackman <jackmanb@...gle.com> wrote:
>
> 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
>
>  - 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.
>
> Note I also mentioned other similar errors in [0]. Those errors don't
> exist in Linus' master and I didn't note down where I saw them. Either
> they have since been fixed, or I observed them in Google's internal
> codebase where they were instroduced downstream.
>
> This is a successor to [1] but I haven't called it a v2 because it's a
> totally different solution. Thanks to Ard for the guidance and
> corrections.
>
> [0] https://lore.kernel.org/all/DERNCQGNRITE.139O331ACPKZ9@google.com/
>
> [1] https://lore.kernel.org/all/20251117-b4-sev-gcov-objtool-v1-1-54f7790d54df@google.com/

Why is [1] not the right solution?
The problem is we have lots of "inline" functions, and any one of them
could cause problems in future.

I don't mind turning "inline" into "__always_inline", but it seems
we're playing whack-a-mole here, and just disabling GCOV entirely
would make this noinstr.c file more robust.

> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
> ---
> Brendan Jackman (2):
>       kasan: mark !__SANITIZE_ADDRESS__ stubs __always_inline
>       kcsan: mark !__SANITIZE_THREAD__ stub __always_inline
>
>  include/linux/kasan-checks.h | 4 ++--
>  include/linux/kcsan-checks.h | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> ---
> base-commit: 67a454e6b1c604555c04501c77b7fedc5d98a779
> change-id: 20251208-gcov-inline-noinstr-1550cfee445c
>
> Best regards,
> --
> Brendan Jackman <jackmanb@...gle.com>
>