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

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Message-ID: <DEDHST3BOZVN.O39PP5JRRFTX@google.com>
Date: Thu, 20 Nov 2025 11:41:03 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Brendan Jackman <jackmanb@...gle.com>, Ard Biesheuvel <ardb@...nel.org>
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 Nov 17, 2025 at 12:37 PM UTC, Brendan Jackman wrote:
> On Mon, 17 Nov 2025 at 12:52, Ard Biesheuvel <ardb@...nel.org> wrote:
>>
>> On Mon, 17 Nov 2025 at 12:40, Ard Biesheuvel <ardb@...nel.org> wrote:
>> >
>> > 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
>>
>> instrument_atomic_write(), which calls the KASAN/KCSAN intrinsics
>> directly, and these are usually only called by compiler generated
>> code.
>>
>> This completely defeats the noinstr per-function annotation, given
>> that each compilation unit only incorporates a single version of
>> set_bit(), which is the instrumented version unless instrumentation is
>> disabled for the entire file.
>>
>> For the short term, we could avoid this by using arch___set_bit()
>> directly in the SEV code that triggers this issue today. But for the
>> longer term, we should get write of those explicit calls to
>> instrumentation intrinsics, as this is fundamentally incompatible with
>> per-function overrides.
>>
>> https://lore.kernel.org/all/8734aqulch.ffs@tglx/T/#u
>
> Ah, yes thank you I think you are right. My GCOV "fix" seems to be
> bogus, it probably just hides the issue with incidental changes.

On the other hand, I guess the intermediate workaround of just disabling
it at the compilation unit still makes sense here, right?

i.e. my patch is still dumb but should we start by just doing
K{A,C}ASAN_SANITIZE_noinstr.o := n instead?