linux-kernel - [PATCH] arm64: Don't unconditionally add -Wno-psabi to KBUILD

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Message-Id: <20190607161201.73430-1-natechancellor@gmail.com>
Date:   Fri,  7 Jun 2019 09:12:01 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Dave Martin <Dave.Martin@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        Qian Cai <cai@....pw>
Subject: [PATCH] arm64: Don't unconditionally add -Wno-psabi to KBUILD_CFLAGS

This is a GCC only option, which warns about ABI changes within GCC, so
unconditionally adding breaks Clang with tons of:

warning: unknown warning option '-Wno-psabi' [-Wunknown-warning-option]

and link time failures:

ld.lld: error: undefined symbol: __efistub___stack_chk_guard
>>> referenced by arm-stub.c:73
(/home/nathan/cbl/linux/drivers/firmware/efi/libstub/arm-stub.c:73)
>>>               arm-stub.stub.o:(__efistub_install_memreserve_table)
in archive ./drivers/firmware/efi/libstub/lib.a

I suspect the link time failure comes from some flags not being added
via cc-option, which will always fail when an unknown flag is
unconditionally added to KBUILD_CFLAGS because -Werror is added after
commit c3f0d0bc5b01 ("kbuild, LLVMLinux: Add -Werror to cc-option to
support clang").

$ echo "int main() { return 0; }" | clang -Wno-psabi -o /dev/null -x c -
warning: unknown warning option '-Wno-psabi' [-Wunknown-warning-option]
1 warning generated.

$ echo $?
0

$ echo "int main() { return 0; }" | clang -Werror -Wno-psabi -o /dev/null -x c -
error: unknown warning option '-Wno-psabi' [-Werror,-Wunknown-warning-option]

$ echo $?
1

This side effect is user visible (aside from the inordinate amount of
-Wunknown-warning-option and build failure), as some warnings that are
normally disabled like -Waddress-of-packed-member or
-Wunused-const-variable show up.

Use cc-disable-warning so that it gets disabled for GCC and does nothing
for Clang.

Fixes: ebcc5928c5d9 ("arm64: Silence gcc warnings about arch ABI drift")
Link: https://github.com/ClangBuiltLinux/linux/issues/511
Reported-by: Qian Cai <cai@....pw>
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
---
 arch/arm64/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 8fbd583b18e1..e9d2e578cbe6 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -51,7 +51,7 @@ endif
 
 KBUILD_CFLAGS	+= -mgeneral-regs-only $(lseinstr) $(brokengasinst)
 KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
-KBUILD_CFLAGS	+= -Wno-psabi
+KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)
 KBUILD_AFLAGS	+= $(lseinstr) $(brokengasinst)
 
 KBUILD_CFLAGS	+= $(call cc-option,-mabi=lp64)
-- 
2.22.0.rc3