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

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Date:   Thu, 12 Sep 2019 10:30:57 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        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 10:43 PM Nathan Chancellor
<natechancellor@...il.com> wrote:
>
> 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

That looks better.  Assuming Michael doesn't mind amending the link
when applying:
Reviewed-by: Nick Desaulneris <ndesaulniers@...gle.com>

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



-- 
Thanks,
~Nick Desaulniers