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 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 c3f0d0bc ("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: ebcc5928 ("arm64: Silence gcc warnings about arch ABI drift")
Link: https://github.com/ClangBuiltLinux/linux/issues/511Reported-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>
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