linux-kernel - Re: [PATCH v2 0/3] Noinstr fixes for K[CA]SAN with GCOV

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Message-ID: <CANpmjNMqJVzXrMhCCHjLF5+nvLk33+wuUG_EQ6jd=nF4W-+uMQ@mail.gmail.com>
Date: Mon, 15 Dec 2025 13:15:01 +0100
From: Marco Elver <elver@...gle.com>
To: Brendan Jackman <jackmanb@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>
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 v2 0/3] Noinstr fixes for K[CA]SAN with GCOV

On Mon, 15 Dec 2025 at 11:12, 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.
>
> As discussed in [2], the GCOV+*SAN issue is attacked from two angles:
> both adding __always_inline to the instrumentation helpers AND disabling
> GCOV for noinstr.c. Only one or the other of these things is needed to
> make the build error go away, but they both make sense in their own
> right and both may serve to prevent other similar errors from cropping
> up in future.
>
> Signed-off-by: Brendan Jackman <jackmanb@...gle.com>
> ---
> Changes in v2:
> - Also disable GCOV for noinstr.c (i.e. squash in [0]).
> - Link to v1: [2]
>
> [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/
> [2] https://lore.kernel.org/r/20251208-gcov-inline-noinstr-v1-0-623c48ca5714@google.com
>
> ---
> Brendan Jackman (3):
>       kasan: mark !__SANITIZE_ADDRESS__ stubs __always_inline
>       kcsan: mark !__SANITIZE_THREAD__ stub __always_inline
>       x86/sev: Disable GCOV on noinstr object

Acked-by: Marco Elver <elver@...gle.com>

But please double check if you missed any __always_inline on stubs
(see my comment in "kcsan: mark !__SANITIZE_THREAD__ stub
__always_inline").

I don't know which tree this should go through, but since it deals
with KASAN and GCOV, perhaps -mm would be appropriate (also for the
KCSAN patch this time). There shouldn't be any conflicts with other
patches AFAIK.

Thanks,
  -- Marco