linux-kernel - [PATCH v2 2/5] Makefile: add -fno-builtin-bcmp

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Message-Id: <20200819191654.1130563-3-ndesaulniers@google.com>
Date:   Wed, 19 Aug 2020 12:16:51 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     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>
Cc:     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>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>
Subject: [PATCH v2 2/5] Makefile: add -fno-builtin-bcmp

The issue with using `-fno-builtin-*` flags was that they were not
retained during an LTO link with LLVM.  This was fixed in clang-11 by
https://reviews.llvm.org/D71193
(0508c994f0b14144041f2cfd3ba9f9a80f03de08), which is also the minimum
supported version of clang for LTO.  Use `-fno-builtin-bcmp` instead.

With this applid, we can cleanly revert
commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")

Reviewed-by: Kees Cook <keescook@...omium.org>
Suggested-by: Nathan Chancellor <natechancellor@...il.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
 Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Makefile b/Makefile
index e523dc8d30e0..def590b743a9 100644
--- a/Makefile
+++ b/Makefile
@@ -579,6 +579,7 @@ CLANG_FLAGS	+= -no-integrated-as
 endif
 CLANG_FLAGS	+= -Werror=unknown-warning-option
 CLANG_FLAGS	+= -fno-builtin-stpcpy
+CLANG_FLAGS	+= -fno-builtin-bcmp
 KBUILD_CFLAGS	+= $(CLANG_FLAGS)
 KBUILD_AFLAGS	+= $(CLANG_FLAGS)
 export CLANG_FLAGS
-- 
2.28.0.297.g1956fa8f8d-goog