linux-kernel - Re: [PATCH] Makefile: Add '-fno-builtin-bcmp' to CLANG

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Date:   Wed, 13 Mar 2019 06:44:47 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        James Y Knight <jyknight@...gle.com>,
        clang-built-linux@...glegroups.com, stable@...r.kernel.org
Subject: Re: [PATCH] Makefile: Add '-fno-builtin-bcmp' to CLANG_FLAGS

On Wed, Mar 13, 2019 at 09:13:11AM +0100, Rasmus Villemoes wrote:
> On 12/03/2019 22.52, Nathan Chancellor wrote:
> > After LLVM revision r355672 [1], all known working kernel configurations
> > fail to link [2]:
> > 
> > ld: init/do_mounts.o: in function `prepare_namespace':
> > do_mounts.c:(.init.text+0x5ca): undefined reference to `bcmp'
> > ld: do_mounts.c:(.init.text+0x5e6): undefined reference to `bcmp'
> > ld: init/initramfs.o: in function `do_header':
> > initramfs.c:(.init.text+0x6e0): undefined reference to `bcmp'
> > ld: initramfs.c:(.init.text+0x6f8): undefined reference to `bcmp'
> > ld: arch/x86/kernel/setup.o: in function `setup_arch':
> > setup.c:(.init.text+0x21d): undefined reference to `bcmp'
> > 
> > Commit 6edfba1b33c7 ("[PATCH] x86_64: Don't define string functions to
> > builtin") removed '-ffreestanding' globally and the kernel doesn't
> > provide a bcmp definition so the linker cannot find a reference to it.
> >
> 
> > Fix this by explicitly telling LLVM through Clang not to emit bcmp
> > references. This flag does not need to be behind 'cc-option' because all
> > working versions of Clang support this flag.
> 
> Wouldn't it be better to just define bcmp as an alias for memcmp? They
> seem to have compatible prototypes, and then somebody might someday sit
> down and implement some word-at-a-time version of bcmp making use of the
> weaker guarantees about the return value to gain some performance. But I
> suppose that can also be done later.
> 
> Rasmus

Hi Rasmus,

Thank you much for the review, I didn't even realize this was possible :)

I'd certainly like to explore it as that is what glibc does. How would
you suggest going about it here?

Thanks,
Nathan