linux-kernel - [PATCH] kstack_erase: Add -mgeneral-regs-only to silence Clang warnings

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Message-Id: <20250726212615.work.800-kees@kernel.org>
Date: Sat, 26 Jul 2025 14:26:19 -0700
From: Kees Cook <kees@...nel.org>
To: Nathan Chancellor <nathan@...nel.org>
Cc: Kees Cook <kees@...nel.org>,
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Mike Rapoport (Microsoft)" <rppt@...nel.org>,
	Pratyush Yadav <ptyadav@...zon.de>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Coiby Xu <coxu@...hat.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Alexander Graf <graf@...zon.com>,
	linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev,
	linux-hardening@...r.kernel.org
Subject: [PATCH] kstack_erase: Add -mgeneral-regs-only to silence Clang warnings

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@...nel.org>
Closes: https://lore.kernel.org/all/20250726004313.GA3650901@ax162
Signed-off-by: Kees Cook <kees@...nel.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