linux-kernel - [PATCH] arm64: Do not include __READ

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Date:   Wed,  9 Mar 2022 12:16:34 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, Nathan Chancellor <nathan@...nel.org>,
        stable@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>
Subject: [PATCH] arm64: Do not include __READ_ONCE() block in assembly files

When building arm64 defconfig + CONFIG_LTO_CLANG_{FULL,THIN}=y after
commit 558c303c9734 ("arm64: Mitigate spectre style branch history side
channels"), the following error occurs:

  <instantiation>:4:2: error: invalid fixup for movz/movk instruction
   mov w0, #ARM_SMCCC_ARCH_WORKAROUND_3
   ^

Marc figured out that moving "#include <linux/init.h>" in
include/linux/arm-smccc.h into a !__ASSEMBLY__ block resolves it. The
full include chain with CONFIG_LTO=y from include/linux/arm-smccc.h:

include/linux/init.h
include/linux/compiler.h
arch/arm64/include/asm/rwonce.h
arch/arm64/include/asm/alternative-macros.h
arch/arm64/include/asm/assembler.h

The asm/alternative-macros.h include in asm/rwonce.h only happens when
CONFIG_LTO is set, which ultimately casues asm/assembler.h to be
included before the definition of ARM_SMCCC_ARCH_WORKAROUND_3. As a
result, the preprocessor does not expand ARM_SMCCC_ARCH_WORKAROUND_3 in
__mitigate_spectre_bhb_fw, which results in the error above.

Avoid this problem by just avoiding the CONFIG_LTO=y __READ_ONCE() block
in asm/rwonce.h with assembly files, as nothing in that block is useful
to assembly files, which allows ARM_SMCCC_ARCH_WORKAROUND_3 to be
properly expanded with CONFIG_LTO=y builds.

Cc: stable@...r.kernel.org
Fixes: e35123d83ee3 ("arm64: lto: Strengthen READ_ONCE() to acquire when CONFIG_LTO=y")
Link: https://lore.kernel.org/r/20220309155716.3988480-1-maz@kernel.org/
Reported-by: Marc Zyngier <maz@...nel.org>
Acked-by: James Morse <james.morse@....com>
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---

This is based on current mainline; if it should be based on a specific
arm64 branch, please let me know.

As 558c303c9734 is going to stable, I marked this for stable as well to
avoid breaking Android. I used e35123d83ee3 for the fixes tag to make it
clear to the stable team this should only go where that commit is
present. If a different fixes tag should be used, please feel free to
substitute.

 arch/arm64/include/asm/rwonce.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
index 1bce62fa908a..56f7b1d4d54b 100644
--- a/arch/arm64/include/asm/rwonce.h
+++ b/arch/arm64/include/asm/rwonce.h
@@ -5,7 +5,7 @@
 #ifndef __ASM_RWONCE_H
 #define __ASM_RWONCE_H

-#ifdef CONFIG_LTO
+#if defined(CONFIG_LTO) && !defined(__ASSEMBLY__)

 #include <linux/compiler_types.h>
 #include <asm/alternative-macros.h>
@@ -66,7 +66,7 @@
 })

 #endif	/* !BUILD_VDSO */
-#endif	/* CONFIG_LTO */
+#endif	/* CONFIG_LTO && !__ASSEMBLY__ */

 #include <asm-generic/rwonce.h>

base-commit: 330f4c53d3c2d8b11d86ec03a964b86dc81452f5
-- 
2.35.1