[PATCH v2 5/5] x86: don't build CONFIG_X86_32 as -ffreestanding

[Nick Desaulniers]

-ffreestanding typically inhibits "libcall optimizations" where calls to
certain library functions can be replaced by the compiler in certain
cases to calls to other library functions that may be more efficient.
This can be problematic for embedded targets that don't provide full
libc implementations.

-ffreestanding inhibits all optimizations, which
is the safe choice, but generally we want the optimizations that are
performed. The Linux kernel does implement a fair amount of libc
routines. Instead of -ffreestanding (which makes more sense in smaller
images like kexec's purgatory image), prefer -fno-builtin-* flags to
disable the compiler from emitting calls to functions which may not be
defined.

If you see a linkage failure due to a missing symbol that's typically
defined in a libc, and not explicitly called from the source code, then
the compiler may have done such a transform.  You can either implement
such a function (ie. in lib/string.c) or disable the transform outright
via -fno-builtin-* flag (where * is the name of the library routine, ie.
-fno-builtin-bcmp).

i386_defconfig build+boot tested with GCC and Clang. Removes a pretty
old TODO from the codebase.

Fixes: 6edfba1b33c7 ("x86_64: Don't define string functions to builtin")
Suggested-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
Might be nice to have
https://lore.kernel.org/lkml/CAKwvOdn-mv1D1GEk3pWnPYsyzQRRk5qZFhSi0CYn6tRDo1O_iw@xxxxxxxxxxxxxx/T/#u
first.

 arch/x86/Makefile | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 4346ffb2e39f..2383a96cf4fd 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -80,9 +80,6 @@ ifeq ($(CONFIG_X86_32),y)
         # CPU-specific tuning. Anything which can be shared with UML should go here.
         include arch/x86/Makefile_32.cpu
         KBUILD_CFLAGS += $(cflags-y)
-
-        # temporary until string.h is fixed
-        KBUILD_CFLAGS += -ffreestanding
 else
         BITS := 64
         UTS_MACHINE := x86_64
-- 
2.28.0.297.g1956fa8f8d-goog