• Nathan Chancellor's avatar
    arm64: Don't unconditionally add -Wno-psabi to KBUILD_CFLAGS · fa63da2a
    Nathan Chancellor authored
    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: default avatarQian Cai <cai@lca.pw>
    Acked-by: default avatarDave Martin <Dave.Martin@arm.com>
    Reviewed-by: default avatarNick Desaulniers <ndesaulniers@google.com>
    Signed-off-by: default avatarNathan Chancellor <natechancellor@gmail.com>
    Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
    fa63da2a
Makefile 5.81 KB