Re: [tip: x86/build] x86/build: Don't build CONFIG_X86_32 as -ffreestanding

[Nick Desaulniers <ndesaulniers@google.com>]

On Thu, Apr 7, 2022 at 8:34 AM tip-bot2 for Nick Desaulniers
<tip-bot2@linutronix.de> wrote:
>
> The following commit has been merged into the x86/build branch of tip:
>
> Commit-ID:     9b2687f29bc1a050ffd63b425129aa9db987e4f3
> Gitweb:        https://git.kernel.org/tip/9b2687f29bc1a050ffd63b425129aa9db987e4f3
> Author:        Nick Desaulniers <ndesaulniers@google.com>
> AuthorDate:    Thu, 03 Feb 2022 12:40:25 -08:00
> Committer:     Borislav Petkov <bp@suse.de>
> CommitterDate: Thu, 07 Apr 2022 11:55:42 +02:00
>
> x86/build: Don't build CONFIG_X86_32 as -ffreestanding
>
> -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 (i.e. in lib/string.c) or disable the transform outright
> via -fno-builtin-* flag (where * is the name of the library routine,
> i.e. -fno-builtin-bcmp).
>
> i386_defconfig build+boot tested with GCC and Clang. Removes a pretty
> old TODO from the code base.
>
> [kees: These libcall optimizations are specifically needed to allow Clang
> to correctly optimize the string functions under CONFIG_FORTIFY_SOURCE.]

Right, but...
I think we found that doing so leads to a boot regression for i386
when built w/ clang because:
https://github.com/ClangBuiltLinux/linux/issues/1583
https://github.com/llvm/llvm-project/issues/53645
TL;DR
In doing such libcall optimizations, LLVM drops the -mregparm=3
calling convention...on the caller's side and not the callee's.

Boris, Can I send you a patch to replace this one (with a guard for
clang) or a patch on top? i.e. what base would you prefer me to use.

Using mainline:

```
diff --git a/arch/x86/Makefile b/arch/x86/Makefile
index 63d50f65b828..c94de779e334 100644
--- a/arch/x86/Makefile
+++ b/arch/x86/Makefile
@@ -100,8 +100,13 @@ ifeq ($(CONFIG_X86_32),y)
         include $(srctree)/arch/x86/Makefile_32.cpu
         KBUILD_CFLAGS += $(cflags-y)

-        # temporary until string.h is fixed
+        # LLVM is dropping -mregparm=3 from callers when doing libcall
+        # optimization.
+        # https://github.com/ClangBuiltLinux/linux/issues/1583
+        # https://github.com/llvm/llvm-project/issues/53645
+        ifndef CONFIG_CC_IS_CLANG
         KBUILD_CFLAGS += -ffreestanding
+        endif

        ifeq ($(CONFIG_STACKPROTECTOR),y)
                ifeq ($(CONFIG_SMP),y)
```

but happy to rebase onto whichever and put a commit message on that.
We'll bump up the priority on getting that fixed.

>
> Fixes: 6edfba1b33c7 ("[PATCH] x86_64: Don't define string functions to builtin")
> Suggested-by: Arvind Sankar <nivedita@alum.mit.edu>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Borislav Petkov <bp@suse.de>
> Reviewed-by: Fangrui Song <maskray@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Link: https://lore.kernel.org/r/20200817220212.338670-5-ndesaulniers@google.com
> Link: https://lore.kernel.org/r/20220203204025.1153397-1-keescook@chromium.org
> ---
>  arch/x86/Makefile | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> index 1abd7cc..670fe40 100644
> --- a/arch/x86/Makefile
> +++ b/arch/x86/Makefile
> @@ -100,9 +100,6 @@ ifeq ($(CONFIG_X86_32),y)
>          include $(srctree)/arch/x86/Makefile_32.cpu
>          KBUILD_CFLAGS += $(cflags-y)
>
> -        # temporary until string.h is fixed
> -        KBUILD_CFLAGS += -ffreestanding
> -
>         ifeq ($(CONFIG_STACKPROTECTOR),y)
>                 ifeq ($(CONFIG_SMP),y)
>                         KBUILD_CFLAGS += -mstack-protector-guard-reg=fs -mstack-protector-guard-symbol=__stack_chk_guard



-- 
Thanks,
~Nick Desaulniers