Re: [PATCH 4/4] x86: don't build CONFIG_X86_32 as -ffreestanding

[Fangrui Song]

On 2020-08-17, Nick Desaulniers wrote:
> -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 such 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>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> 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
> --
> 2.28.0.220.ged08abb693-goog

Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>

But dropping -ffreestanding causes compiler produced declarations which
require
https://lore.kernel.org/lkml/20210107001739.1321725-1-maskray@xxxxxxxxxx/
"x86: Treat R_386_PLT32 as R_386_PC32" as a prerequisite
to build with trunk Clang https://github.com/ClangBuiltLinux/linux/issues/1210

Since there have been more than 4 months, it seems that something else
regressed the non -ffreestanding build. Maybe another -fno-builtin-* is
needed somewhere.