linux-kernel - [PATCH] x86/asm: Make asm export of __ref_stack_chk

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Message-ID: <20250320213238.4451-2-ardb@kernel.org>
Date: Thu, 20 Mar 2025 22:32:39 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
	samitolvanen@...gle.com,
	brgerst@...il.com,
	Ard Biesheuvel <ardb@...nel.org>
Subject: [PATCH] x86/asm: Make asm export of __ref_stack_chk_guard unconditional

Clang does not tolerate the use of non-TLS symbols for the per-CPU stack
protector very well, and to work around this limitation, the symbol
passed via the -mstack-protector-guard-symbol= option is never defined
in C code, but only in the linker script, and it is exported from an
assembly file. This is necessary because Clang will fail to generate the
correct %GS based references in a compilation unit that includes a
non-TLS definition of the guard symbol being used to store the stack
cookie.

This problem is only triggered by symbol definitions, not by
declarations, but nonetheless, the declaration in <asm/asm-prototypes.h>
is conditional on __GENKSYMS__ being #define'd, so that only genksyms
will observe it, but for ordinary compilation, it will be invisible.

This is causing problems with the genksyms alternative gendwarfksyms,
which does not #define __GENKSYMS__, does not observe the symbol
declaration, and therefore lacks the information it needs to version it.
Adding the #define creates problems in other places, so that is not a
straight-forward solution. So take the easy way out, and drop the
conditional on __GENKSYMS__, as this is not really needed to begin with.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 arch/x86/include/asm/asm-prototypes.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/asm-prototypes.h b/arch/x86/include/asm/asm-prototypes.h
index 8d9e62725202..11c6fecc3ad7 100644
--- a/arch/x86/include/asm/asm-prototypes.h
+++ b/arch/x86/include/asm/asm-prototypes.h
@@ -20,6 +20,6 @@
 extern void cmpxchg8b_emu(void);
 #endif

-#if defined(__GENKSYMS__) && defined(CONFIG_STACKPROTECTOR)
+#ifdef CONFIG_STACKPROTECTOR
 extern unsigned long __ref_stack_chk_guard;
 #endif
-- 
2.47.2