powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp

commit 8dcd71b45df34d9b903450fab147ee8c1e6c16b5 upstream.

LLVM revision r374662 gives LLVM the ability to convert certain loops
into a reference to bcmp as an optimization; this breaks
prom_init_check.sh:

    CALL    arch/powerpc/kernel/prom_init_check.sh
  Error: External symbol 'bcmp' referenced from prom_init.c
  make[2]: *** [arch/powerpc/kernel/Makefile:196: prom_init_check] Error 1

bcmp is defined in lib/string.c as a wrapper for memcmp so this could
be added to the whitelist. However, commit
450e7dd4001f ("powerpc/prom_init: don't use string functions from
lib/") copied memcmp as prom_memcmp to avoid KASAN instrumentation so
having bcmp be resolved to regular memcmp would break that assumption.
Furthermore, because the compiler is the one that inserted bcmp, we
cannot provide something like prom_bcmp.

To prevent LLVM from being clever with optimizations like this, use
-ffreestanding to tell LLVM we are not hosted so it is not free to
make transformations like this.

Reviewed-by: Nick Desaulneris <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191119045712.39633-4-natechancellor@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 5819a577d267af97c226b0bde90fa4d226d2f053..c127304b26c959b42da0a044c9fc032b8cbf3a1f 100644 (file)
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -21,6 +21,7 @@ CFLAGS_prom.o += $(DISABLE_LATENT_ENTROPY_PLUGIN)
 
 CFLAGS_prom_init.o += $(call cc-option, -fno-stack-protector)
 CFLAGS_prom_init.o += -DDISABLE_BRANCH_PROFILING
+CFLAGS_prom_init.o += -ffreestanding
 
 ifdef CONFIG_FUNCTION_TRACER
 # Do not trace early boot code