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Date:   Tue, 18 Aug 2020 12:24:57 -0700
From:   Kees Cook <keescook@chromium.org>
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,
        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>,
        =?iso-8859-1?Q?D=E1vid_Bolvansk=FD?= <david.bolvansky@gmail.com>,
        Eli Friedman <efriedma@quicinc.com>
Subject: Re: [PATCH 4/4] x86: don't build CONFIG_X86_32 as -ffreestanding
Message-ID: <202008181224.096C2926E@keescook>
References: <20200817220212.338670-1-ndesaulniers@google.com>
 <20200817220212.338670-5-ndesaulniers@google.com>
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On Mon, Aug 17, 2020 at 03:02:12PM -0700, 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>

-- 
Kees Cook