[PATCH v5 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp

[Nathan Chancellor]

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.

Link: https://github.com/ClangBuiltLinux/linux/issues/647
Link: https://github.com/llvm/llvm-project/commit/76cdcf25b883751d83402baea6316772aa73865c
Reviewed-by: Nick Desaulneris <ndesaulniers@xxxxxxxxxx>
Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
---

v1 -> v3:

* New patch in the series

v3 -> v4:

* Rebase on v5.4-rc3.

* Add Nick's reviewed-by tag.

* Update the LLVM commit reference to the latest applied version (r374662)
  as it was originally committed as r370454, reverted in r370788, and
  reapplied as r374662.

v4 -> v5:

* Rebase on next-20191118 to avoid a conflict with commit
  6266a4dadb1d ("powerpc/64s: Always disable branch profiling for prom_init.o")

 arch/powerpc/kernel/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 3c113ae0de2b..82170c155cb6 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -23,6 +23,7 @@ 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
-- 
2.24.0