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Date: Sun, 27 Jul 2025 08:51:08 -0700
From: Nathan Chancellor <nathan@kernel.org>
To: Kees Cook <kees@kernel.org>
Cc: Nick Desaulniers <nick.desaulniers+lkml@gmail.com>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	Ingo Molnar <mingo@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Mike Rapoport (Microsoft)" <rppt@kernel.org>,
	Pratyush Yadav <ptyadav@amazon.de>,
	Jonathan Cameron <Jonathan.Cameron@huawei.com>,
	Coiby Xu <coxu@redhat.com>, Josh Poimboeuf <jpoimboe@kernel.org>,
	Alexander Graf <graf@amazon.com>, linux-kernel@vger.kernel.org,
	llvm@lists.linux.dev, linux-hardening@vger.kernel.org
Subject: Re: [PATCH] kstack_erase: Add -mgeneral-regs-only to silence Clang
 warnings
Message-ID: <20250727155108.GB1183915@ax162>
References: <20250726212615.work.800-kees@kernel.org>
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In-Reply-To: <20250726212615.work.800-kees@kernel.org>

On Sat, Jul 26, 2025 at 02:26:19PM -0700, Kees Cook wrote:
> Once CONFIG_KSTACK_ERASE is enabled with Clang on i386, the build warns:
> 
>   kernel/kstack_erase.c:168:2: warning: function with attribute 'no_caller_saved_registers' should only call a function with attribute 'no_caller_saved_registers' or be compiled with '-mgeneral-regs-only' [-Wexcessive-regsave]
> 
> Add -mgeneral-regs-only for the kstack_erase handler, to make Clang feel
> better (it is effectively a no-op flag for the kernel). No binary
> changes encountered.
> 
> Build & boot tested with Clang 21 on x86_64, and i386.
> Build tested with GCC 14.2.0 on x86_64, i386, arm64, and arm.
> 
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> Closes: https://lore.kernel.org/all/20250726004313.GA3650901@ax162
> Signed-off-by: Kees Cook <kees@kernel.org>

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

> ---
>  kernel/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/Makefile b/kernel/Makefile
> index e4f01f1d4d0c..0ee9afd8b7cf 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -140,6 +140,7 @@ obj-$(CONFIG_RESOURCE_KUNIT_TEST) += resource_kunit.o
>  obj-$(CONFIG_SYSCTL_KUNIT_TEST) += sysctl-test.o
>  
>  CFLAGS_kstack_erase.o += $(DISABLE_KSTACK_ERASE)
> +CFLAGS_kstack_erase.o += $(call cc-option,-mgeneral-regs-only)
>  obj-$(CONFIG_KSTACK_ERASE) += kstack_erase.o
>  KASAN_SANITIZE_kstack_erase.o := n
>  KCSAN_SANITIZE_kstack_erase.o := n
> -- 
> 2.34.1
>