From: Nathan Chancellor natechancellor@gmail.com
commit fa63da2ab046b885a7f70291aafc4e8ce015429b upstream.
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
(/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
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. 0
$ gcc -Wsometimes-uninitialized tmp.c; echo $? gcc: error: unrecognized command line option ‘-Wsometimes-uninitialized’; did you mean ‘-Wmaybe-uninitialized’? 1
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' [-Werror,-Wunknown-warning-option] 1
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' [-Werror,-Wunknown-warning-option] 1
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):
-fno-delete-null-pointer-checks -Wno-address-of-packed-member -Wframe-larger-than=2048 -Wno-unused-const-variable -fno-strict-overflow -fno-merge-all-constants -fno-stack-check -Werror=date-time -Werror=incompatible-pointer-types -ffreestanding -fno-stack-protector
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@lca.pw Acked-by: Dave Martin Dave.Martin@arm.com Reviewed-by: Nick Desaulniers ndesaulniers@google.com Signed-off-by: Nathan Chancellor natechancellor@gmail.com Signed-off-by: Will Deacon will.deacon@arm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/arm64/Makefile | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- 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)