linux-kernel - Re: [PATCH v3 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp

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Date:   Wed, 11 Sep 2019 22:43:04 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v3 3/3] powerpc/prom_init: Use -ffreestanding to avoid a
 reference to bcmp

On Wed, Sep 11, 2019 at 02:01:59PM -0700, Nick Desaulniers wrote:
> On Wed, Sep 11, 2019 at 11:21 AM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > r370454 gives LLVM the ability to convert certain loops into a reference
> > to bcmp as an optimization; this breaks prom_init_check.sh:
> >
> >   CALL    arch/powerpc/kernel/prom_init_check.sh
> > Error: External symbol 'bcmp' referenced from prom_init.c
> > make[2]: *** [arch/powerpc/kernel/Makefile:196: prom_init_check] Error 1
> >
> > bcmp is defined in lib/string.c as a wrapper for memcmp so this could be
> > added to the whitelist. However, commit 450e7dd4001f ("powerpc/prom_init:
> > don't use string functions from lib/") copied memcmp as prom_memcmp to
> > avoid KASAN instrumentation so having bcmp be resolved to regular memcmp
> > would break that assumption. Furthermore, because the compiler is the
> > one that inserted bcmp, we cannot provide something like prom_bcmp.
> >
> > To prevent LLVM from being clever with optimizations like this, use
> > -ffreestanding to tell LLVM we are not hosted so it is not free to make
> > transformations like this.
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/647
> > Link: https://github.com/llvm/llvm-project/commit/5c9f3cfec78f9e9ae013de9a0d092a68e3e79e002
> 
> The above link doesn't work for me (HTTP 404).  PEBKAC?
> https://github.com/llvm/llvm-project/commit/5c9f3cfec78f9e9ae013de9a0d092a68e3e79e002

Not really sure how an extra 2 got added on the end of that... Must have
screwed up in vim somehow.

Link: https://github.com/llvm/llvm-project/commit/5c9f3cfec78f9e9ae013de9a0d092a68e3e79e00

I can resend unless the maintainer is able to fix that up when it gets
applied.

Cheers,
Nathan