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From: Nick Desaulniers <ndesaulniers@google.com>
To: 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>
Cc: "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>,
	"Nick Desaulniers" <ndesaulniers@google.com>
Subject: [PATCH 4/4] x86: don't build CONFIG_X86_32 as -ffreestanding
Date: Mon, 17 Aug 2020 15:02:12 -0700	[thread overview]
Message-ID: <20200817220212.338670-5-ndesaulniers@google.com> (raw)
In-Reply-To: <20200817220212.338670-1-ndesaulniers@google.com>

-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>
---
 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


  parent reply	other threads:[~2020-08-17 22:02 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 ` Nick Desaulniers [this message]
2020-08-18 19:24   ` [PATCH 4/4] x86: don't build CONFIG_X86_32 as -ffreestanding Kees Cook
2021-01-07  0:27   ` Fangrui Song
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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