linux-kernel - Re: [PATCH 4/4] x86: don't build CONFIG_X86

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Date:   Wed, 6 Jan 2021 16:27:34 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        Tony Luck <tony.luck@...el.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Joe Perches <joe@...ches.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Daniel Axtens <dja@...ens.net>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Yury Norov <yury.norov@...il.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Bruce Ashfield <bruce.ashfield@...il.com>,
        Marco Elver <elver@...gle.com>,
        Vamshi K Sthambamkadi <vamshi.k.sthambamkadi@...il.com>,
        Andi Kleen <ak@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        Eli Friedman <efriedma@...cinc.com>
Subject: Re: [PATCH 4/4] x86: don't build CONFIG_X86_32 as -ffreestanding

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@...m.mit.edu>
>Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>Reviewed-by: Kees Cook <keescook@...omium.org>
>---
> 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@...gle.com>

But dropping -ffreestanding causes compiler produced declarations which
require
https://lore.kernel.org/lkml/20210107001739.1321725-1-maskray@google.com/
"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.