Re: [PATCH v2] compiler_types: Provide __no_kstack_erase to disable coverage only on Clang

[Nathan Chancellor <nathan@kernel.org>]

On Tue, Jul 29, 2025 at 04:41:00PM -0700, Kees Cook 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).
> 
> Successfully 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@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202507270258.neWuiXLd-lkp@intel.com/
> Reported-by: syzbot+5245cb609175fb6e8122@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/6888d004.a00a0220.26d0e1.0004.GAE@google.com/
> Signed-off-by: Kees Cook <kees@kernel.org>

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  arch/x86/include/asm/init.h    | 2 +-
>  include/linux/compiler-clang.h | 3 +++
>  include/linux/compiler_types.h | 4 ++++
>  include/linux/init.h           | 2 +-
>  4 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-clang.h b/include/linux/compiler-clang.h
> index 4fc8e26914ad..fa4ffe037bc7 100644
> --- a/include/linux/compiler-clang.h
> +++ b/include/linux/compiler-clang.h
> @@ -89,6 +89,9 @@
>  #define __no_sanitize_coverage
>  #endif
>  
> +/* Only Clang needs to disable the coverage sanitizer for kstack_erase. */

IMO, this comment does not add much now that this is separated from the
dummy definition of __no_kstack_erase but it's probably not worth
changing in a new revision if Linus wants to take this directly.

> +#define __no_kstack_erase	__no_sanitize_coverage
> +
>  #if __has_feature(shadow_call_stack)
>  # define __noscs	__attribute__((__no_sanitize__("shadow-call-stack")))
>  #endif
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 2b77d12e07b2..16755431fc11 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -424,6 +424,10 @@ struct ftrace_likely_data {
>  # define randomized_struct_fields_end
>  #endif
>  
> +#ifndef __no_kstack_erase
> +# define __no_kstack_erase
> +#endif
> +
>  #ifndef __noscs
>  # define __noscs
>  #endif
> diff --git a/include/linux/init.h b/include/linux/init.h
> index c65a050d52a7..a60d32d227ee 100644
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -51,7 +51,7 @@
>     discard it in modules) */
>  #define __init		__section(".init.text") __cold __latent_entropy	\
>  						__noinitretpoline	\
> -						__no_sanitize_coverage
> +						__no_kstack_erase
>  #define __initdata	__section(".init.data")
>  #define __initconst	__section(".init.rodata")
>  #define __exitdata	__section(".exit.data")
> -- 
> 2.34.1
>