[PATCH v2] arm64: Don't unconditionally add -Wno-psabi to KBUILD_CFLAGS

From: Nathan Chancellor
Date: Tue Jun 11 2019 - 13:25:48 EST

This is a GCC only option, which warns about ABI changes within GCC, so
unconditionally adding it 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
>>> arm-stub.stub.o:(__efistub_install_memreserve_table)
in archive ./drivers/firmware/efi/libstub/lib.a

These failures come from the lack of -fno-stack-protector, which is
added via cc-option in drivers/firmware/efi/libstub/Makefile. When an
unknown flag is added to KBUILD_CFLAGS, clang will noisily warn that it
is ignoring the option like above, unlike gcc, who will just error.

$ echo "int main() { return 0; }" > tmp.c

$ clang -Wno-psabi tmp.c; echo $?
warning: unknown warning option '-Wno-psabi' [-Wunknown-warning-option]
1 warning generated.

$ gcc -Wsometimes-uninitialized tmp.c; echo $?
gcc: error: unrecognized command line option
â-Wsometimes-uninitializedâ; did you mean â-Wmaybe-uninitializedâ?

For cc-option to work properly with clang and behave like gcc, -Werror
is needed, which was done in commit c3f0d0bc5b01 ("kbuild, LLVMLinux:
Add -Werror to cc-option to support clang").

$ clang -Werror -Wno-psabi tmp.c; echo $?
error: unknown warning option '-Wno-psabi'

As a consequence of this, when an unknown flag is unconditionally added
to KBUILD_CFLAGS, it will cause cc-option to always fail and those flags
will never get added:

$ clang -Werror -Wno-psabi -fno-stack-protector tmp.c; echo $?
error: unknown warning option '-Wno-psabi'

This can be seen when compiling the whole kernel as some warnings that
are normally disabled (see below) show up. The full list of flags
missing from drivers/firmware/efi/libstub are the following (gathered
from diffing .arm64-stub.o.cmd):


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@xxxxxx>
Acked-by: Dave Martin <Dave.Martin@xxxxxxx>
Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>

v1 -> v2:

* Improve commit message explanation, I wasn't entirely happy with the
first one; let me know if there are any issues/questions.

* Carry forward Dave's ack and Nick's review (let me know if you
disagree with the commit messasge rewording).

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)