linux-kernel - Re: [PATCH] compiler_types: Provide __no_kstack

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Message-ID: <CANpmjNNkYE9dvyR-YH3U18+g4bG4tpgD1pv99xG6xhdRuQYUQA@mail.gmail.com>
Date: Wed, 30 Jul 2025 01:00:39 +0200
From: Marco Elver <elver@...gle.com>
To: Kees Cook <kees@...nel.org>
Cc: Linus Torvalds <torvalds@...uxfoundation.org>, kernel test robot <lkp@...el.com>, 
	syzbot+5245cb609175fb6e8122@...kaller.appspotmail.com, 
	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>, Ard Biesheuvel <ardb@...nel.org>, 
	Hou Wenlong <houwenlong.hwl@...group.com>, "Kirill A . Shutemov" <kas@...nel.org>, 
	Miguel Ojeda <ojeda@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Przemek Kitszel <przemyslaw.kitszel@...el.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Masahiro Yamada <masahiroy@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Wei Yang <richard.weiyang@...il.com>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Arnd Bergmann <arnd@...db.de>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Michael Kelley <mhklinux@...look.com>, 
	Marc Herbert <Marc.Herbert@...ux.intel.com>, Yafang Shao <laoar.shao@...il.com>, 
	Uros Bizjak <ubizjak@...il.com>, Jan Hendrik Farr <kernel@...rr.cc>, linux-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] compiler_types: Provide __no_kstack_erase to disable
 coverage only on Clang

On Wed, 30 Jul 2025 at 00:43, Kees Cook <kees@...nel.org> wrote:
>
> In order to support Clang's stack depth tracking (for Linux's kstack_erase
> feature), the coverage sanitizer needed to be disabled for __init (and
> __head) section code. Doing this universally (i.e. for GCC too), created
> a number of unexpected problems, ranging from changes to inlining logic
> to failures to DCE code on earlier GCC versions.
>
> Since this change is only needed for Clang, specialize it so that GCC
> doesn't see the change as it isn't needed there (the GCC implementation
> of kstack_erase uses a GCC plugin that removes stack depth tracking
> instrumentation from __init sections during a late pass in the IR).
>
> Successful build and boot tested with GCC 12 and Clang 22.
>
> Fixes: 381a38ea53d2 ("init.h: Disable sanitizer coverage for __init and __head")
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202507270258.neWuiXLd-lkp@intel.com/
> Reported-by: syzbot+5245cb609175fb6e8122@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6888d004.a00a0220.26d0e1.0004.GAE@google.com/
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
> Cc: Linus Torvalds <torvalds@...uxfoundation.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: <x86@...nel.org>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Ard Biesheuvel <ardb@...nel.org>
> Cc: Marco Elver <elver@...gle.com>
> Cc: Hou Wenlong <houwenlong.hwl@...group.com>
> Cc: Kirill A. Shutemov <kas@...nel.org>
> Cc: Miguel Ojeda <ojeda@...nel.org>
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: Przemek Kitszel <przemyslaw.kitszel@...el.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Masahiro Yamada <masahiroy@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Wei Yang <richard.weiyang@...il.com>
> Cc: Sami Tolvanen <samitolvanen@...gle.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
>  arch/x86/include/asm/init.h    | 2 +-
>  include/linux/compiler_types.h | 7 +++++++
>  include/linux/init.h           | 2 +-
>  3 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/init.h b/arch/x86/include/asm/init.h
> index 6bfdaeddbae8..5a68e9db6518 100644
> --- a/arch/x86/include/asm/init.h
> +++ b/arch/x86/include/asm/init.h
> @@ -5,7 +5,7 @@
>  #if defined(CONFIG_CC_IS_CLANG) && CONFIG_CLANG_VERSION < 170000
>  #define __head __section(".head.text") __no_sanitize_undefined __no_stack_protector
>  #else
> -#define __head __section(".head.text") __no_sanitize_undefined __no_sanitize_coverage
> +#define __head __section(".head.text") __no_sanitize_undefined __no_kstack_erase
>  #endif
>
>  struct x86_mapping_info {
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 2b77d12e07b2..89e2c01fc8b1 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -378,6 +378,13 @@ struct ftrace_likely_data {
>  # define __signed_wrap
>  #endif
>
> +/* GCC does not like splitting sanitizer coverage across section inlines */
> +#ifdef CC_IS_CLANG
> +#define __no_kstack_erase      __no_sanitize_coverage
> +#else
> +#define __no_kstack_erase
> +#endif

I think this belongs into compiler-clang.h, we've typically refrained
from ifdef CC_IS_CLANG/GCC in the generic headers.
See __nocfi for an example, where compiler_types.h provides a default
empty definition, and compiler-clang.h provides a non-empty
definition.