From: Fangrui Song <maskray@google.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: "Masahiro Yamada" <masahiroy@kernel.org>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Ingo Molnar" <mingo@redhat.com>,
"Borislav Petkov" <bp@alien8.de>,
"Michal Marek" <michal.lkml@markovi.net>,
linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org,
"Kees Cook" <keescook@chromium.org>,
"Tony Luck" <tony.luck@intel.com>,
"Dmitry Vyukov" <dvyukov@google.com>,
"Michael Ellerman" <mpe@ellerman.id.au>,
"Joe Perches" <joe@perches.com>,
"Joel Fernandes" <joel@joelfernandes.org>,
"Daniel Axtens" <dja@axtens.net>,
"Arvind Sankar" <nivedita@alum.mit.edu>,
"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
"Alexandru Ardelean" <alexandru.ardelean@analog.com>,
"Yury Norov" <yury.norov@gmail.com>,
x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
"Ard Biesheuvel" <ardb@kernel.org>,
"Paul E . McKenney" <paulmck@kernel.org>,
"Daniel Kiper" <daniel.kiper@oracle.com>,
"Bruce Ashfield" <bruce.ashfield@gmail.com>,
"Marco Elver" <elver@google.com>,
"Vamshi K Sthambamkadi" <vamshi.k.sthambamkadi@gmail.com>,
"Andi Kleen" <ak@suse.de>,
"Linus Torvalds" <torvalds@linux-foundation.org>,
"Dávid Bolvanský" <david.bolvansky@gmail.com>,
"Eli Friedman" <efriedma@quicinc.com>
Subject: Re: [PATCH 4/4] x86: don't build CONFIG_X86_32 as -ffreestanding
Date: Wed, 6 Jan 2021 16:27:34 -0800 [thread overview]
Message-ID: <20210107002734.p62fsyhuo75psta4@google.com> (raw)
In-Reply-To: <20200817220212.338670-5-ndesaulniers@google.com>
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@alum.mit.edu>
>Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
>Reviewed-by: Kees Cook <keescook@chromium.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@google.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.
next prev parent reply other threads:[~2021-01-07 0:28 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-17 22:02 [PATCH 0/4] -ffreestanding/-fno-builtin-* patches Nick Desaulniers
2020-08-17 22:02 ` [PATCH 1/4] Makefile: add -fno-builtin-stpcpy Nick Desaulniers
2020-08-17 22:31 ` H. Peter Anvin
2020-08-17 23:36 ` Nick Desaulniers
2020-08-18 19:21 ` Kees Cook
2020-08-18 7:10 ` Ard Biesheuvel
2020-08-18 7:25 ` Greg KH
2020-08-18 7:29 ` Ard Biesheuvel
2020-08-18 7:34 ` Greg KH
2020-08-18 19:23 ` Kees Cook
2020-08-17 22:02 ` [PATCH 2/4] Revert "lib/string.c: implement a basic bcmp" Nick Desaulniers
2020-08-18 5:44 ` Nathan Chancellor
2020-08-18 18:00 ` Nick Desaulniers
2020-08-18 19:24 ` Kees Cook
2020-08-17 22:02 ` [PATCH 3/4] x86/boot: use -fno-builtin-bcmp Nick Desaulniers
2020-08-18 19:24 ` Kees Cook
2020-08-17 22:02 ` [PATCH 4/4] x86: don't build CONFIG_X86_32 as -ffreestanding Nick Desaulniers
2020-08-18 19:24 ` Kees Cook
2021-01-07 0:27 ` Fangrui Song [this message]
2022-04-07 15:34 ` [tip: x86/build] x86/build: Don't " tip-bot2 for Nick Desaulniers
2022-04-07 17:01 ` Nick Desaulniers
2022-04-07 22:28 ` Borislav Petkov
2020-08-17 22:44 ` [PATCH 0/4] -ffreestanding/-fno-builtin-* patches H. Peter Anvin
2020-08-18 17:56 ` Nick Desaulniers
2020-08-18 19:02 ` H. Peter Anvin
2020-08-18 19:13 ` Linus Torvalds
2020-08-18 19:25 ` Nick Desaulniers
2020-08-18 19:58 ` Nick Desaulniers
2020-08-19 12:19 ` Clement Courbet
2020-08-18 20:24 ` Arvind Sankar
2020-08-18 20:27 ` Nick Desaulniers
2020-08-18 20:58 ` Nick Desaulniers
2020-08-18 21:41 ` Arvind Sankar
2020-08-18 21:51 ` Dávid Bolvanský
2020-08-18 21:59 ` Nick Desaulniers
2020-08-18 22:05 ` Dávid Bolvanský
2020-08-18 23:22 ` Nick Desaulniers
2020-08-20 14:56 ` Rasmus Villemoes
2020-08-20 17:56 ` Arvind Sankar
2020-08-20 18:05 ` Dávid Bolvanský
2020-08-20 23:33 ` Linus Torvalds
2020-08-21 17:29 ` Arvind Sankar
2020-08-21 17:54 ` Linus Torvalds
2020-08-21 18:02 ` Linus Torvalds
2020-08-21 19:14 ` Arvind Sankar
2020-08-21 19:23 ` Linus Torvalds
2020-08-21 19:57 ` Arvind Sankar
2020-08-21 20:03 ` Peter Zijlstra
2020-08-21 21:39 ` Linus Torvalds
2020-08-22 0:12 ` Nick Desaulniers
2020-08-22 12:20 ` David Laight
2020-08-21 6:45 ` Rasmus Villemoes
2020-08-24 15:57 ` Masahiro Yamada
2020-08-24 17:34 ` Arvind Sankar
2020-08-25 7:10 ` Nick Desaulniers
2020-08-25 7:31 ` Nick Desaulniers
2020-08-25 12:28 ` Masahiro Yamada
2020-08-25 14:02 ` Nick Desaulniers
2020-08-26 13:28 ` Masahiro Yamada
2020-08-18 21:53 ` David Laight
2020-08-20 22:41 ` H. Peter Anvin
2020-08-20 23:17 ` Arvind Sankar
2020-08-18 19:35 ` Nick Desaulniers
2020-08-18 22:25 ` Arvind Sankar
2020-08-18 22:59 ` Nick Desaulniers
2020-08-18 23:51 ` Arvind Sankar
2020-08-19 0:20 ` Arvind Sankar
2020-08-19 8:26 ` David Laight
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