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

[Kees Cook]

On Mon, Aug 17, 2020 at 03:02:12PM -0700, 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>

-- 
Kees Cook