linux-kernel - [PATCH] powerpc/vdso: Filter clang's auto var init zero enabler when linking

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Message-Id: <20230123-qunused-argument-auto-var-init-v1-1-ec06051f0d87@kernel.org>
Date:   Tue, 24 Jan 2023 09:19:28 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     masahiroy@...nel.org
Cc:     ndesaulniers@...gle.com, nicolas@...sle.eu, mpe@...erman.id.au,
        npiggin@...il.com, christophe.leroy@...roup.eu, trix@...hat.com,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, llvm@...ts.linux.dev,
        Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH] powerpc/vdso: Filter clang's auto var init zero enabler
 when linking

After commit 7bbf02b875b5 ("kbuild: Stop using '-Qunused-arguments' with
clang"), the PowerPC vDSO shows the following error with clang-13 and
older when CONFIG_INIT_STACK_ALL_ZERO is enabled:

  clang: error: argument unused during compilation: '-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang' [-Werror,-Wunused-command-line-argument]

clang-14 added a change to make sure this flag never triggers
-Wunused-command-line-argument, so it is fixed with newer releases. For
older releases that the kernel still supports building with, just filter
out this flag, as has been done for other flags.

Fixes: b174f4c26aa3 ("powerpc/vdso: Improve linker flags")
Fixes: 7bbf02b875b5 ("kbuild: Stop using '-Qunused-arguments' with clang")
Link: https://github.com/llvm/llvm-project/commit/ca6d5813d17598cd180995fb3bdfca00f364475f
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
This should be the last flag that needs to be filtered (famous last
words...) but if any more come up, we should really just explore
switching the PowerPC vDSO to linking with $(LD) like every other part
of the kernel; for now, I hope this is fine.

Cheers,
Nathan
---
 Makefile                          | 4 +++-
 arch/powerpc/kernel/vdso/Makefile | 2 +-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index cf30a8b6463e..0406f37fd7a2 100644
--- a/Makefile
+++ b/Makefile
@@ -912,7 +912,9 @@ ifdef CONFIG_INIT_STACK_ALL_ZERO
 KBUILD_CFLAGS	+= -ftrivial-auto-var-init=zero
 ifdef CONFIG_CC_HAS_AUTO_VAR_INIT_ZERO_ENABLER
 # https://github.com/llvm/llvm-project/issues/44842
-KBUILD_CFLAGS	+= -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
+CC_AUTO_VAR_INIT_ZERO_ENABLER := -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
+export CC_AUTO_VAR_INIT_ZERO_ENABLER
+KBUILD_CFLAGS	+= $(CC_AUTO_VAR_INIT_ZERO_ENABLER)
 endif
 endif
 
diff --git a/arch/powerpc/kernel/vdso/Makefile b/arch/powerpc/kernel/vdso/Makefile
index 7e69e87fbf74..d2a1dc0956d5 100644
--- a/arch/powerpc/kernel/vdso/Makefile
+++ b/arch/powerpc/kernel/vdso/Makefile
@@ -56,7 +56,7 @@ ccflags-y := -fno-common -fno-builtin
 ldflags-y := -Wl,--hash-style=both -nostdlib -shared -z noexecstack
 ldflags-$(CONFIG_LD_IS_LLD) += $(call cc-option,--ld-path=$(LD),-fuse-ld=lld)
 # Filter flags that clang will warn are unused for linking
-ldflags-y += $(filter-out $(CC_FLAGS_FTRACE) -Wa$(comma)%, $(KBUILD_CFLAGS))
+ldflags-y += $(filter-out $(CC_AUTO_VAR_INIT_ZERO_ENABLER) $(CC_FLAGS_FTRACE) -Wa$(comma)%, $(KBUILD_CFLAGS))
 
 CC32FLAGS := -m32
 LD32FLAGS := -Wl,-soname=linux-vdso32.so.1

---
base-commit: a778c9dd138b4f4410779705b444d58ce6f8fc44
change-id: 20230123-qunused-argument-auto-var-init-8ae79ddfd34f

Best regards,
-- 
Nathan Chancellor <nathan@...nel.org>