[PATCH v2 4/5] x86/boot: use -fno-builtin-bcmp

[Nick Desaulniers]

We're reverting
commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")
in favor of -fno-builtin-bcmp. Remove the additional definition here,
too.

arch/x86/purgatory/Makefile uses -ffreestanding, so there's no risk of
this libcall optimization occurring for arch/x86/boot/purgatory.ro.

arch/x86/boot/Makefile resets KBUILD_CFLAGS, so make sure to reset this
flag that was set for the top level Makefile.

Fixes: 4ce97317f41d ("x86/purgatory: Do not use __builtin_memcpy and __builtin_memset")
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
 arch/x86/boot/Makefile | 1 +
 arch/x86/boot/string.c | 8 --------
 2 files changed, 1 insertion(+), 8 deletions(-)

diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
index fe605205b4ce..ef7f15bfceab 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -70,6 +70,7 @@ KBUILD_CFLAGS	:= $(REALMODE_CFLAGS) -D_SETUP
 KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
 KBUILD_CFLAGS	+= $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
 KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
+KBUILD_CFLAGS	+= -fno-builtin-bcmp
 GCOV_PROFILE := n
 UBSAN_SANITIZE := n
 
diff --git a/arch/x86/boot/string.c b/arch/x86/boot/string.c
index 8a3fff9128bb..23d91aa7691e 100644
--- a/arch/x86/boot/string.c
+++ b/arch/x86/boot/string.c
@@ -37,14 +37,6 @@ int memcmp(const void *s1, const void *s2, size_t len)
 	return diff;
 }
 
-/*
- * Clang may lower `memcmp == 0` to `bcmp == 0`.
- */
-int bcmp(const void *s1, const void *s2, size_t len)
-{
-	return memcmp(s1, s2, len);
-}
-
 int strcmp(const char *str1, const char *str2)
 {
 	const unsigned char *s1 = (const unsigned char *)str1;
-- 
2.28.0.297.g1956fa8f8d-goog