Date:   Tue, 18 Aug 2020 12:24:21 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        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>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.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>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <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>
Subject: Re: [PATCH 2/4] Revert "lib/string.c: implement a basic bcmp"

On Tue, Aug 18, 2020 at 11:00:01AM -0700, Nick Desaulniers wrote:
> On Mon, Aug 17, 2020 at 10:44 PM Nathan Chancellor
> <natechancellor@...il.com> wrote:
> >
> > On Mon, Aug 17, 2020 at 03:02:10PM -0700, Nick Desaulniers wrote:
> > > This reverts commit 5f074f3e192f10c9fade898b9b3b8812e3d83342.
> > >
> > > Use `-fno-builtin-bcmp` instead.
> > >
> > > 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.
> > >
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > > ---
> > >  Makefile               |  1 +
> > >  include/linux/string.h |  3 ---
> > >  lib/string.c           | 20 --------------------
> > >  3 files changed, 1 insertion(+), 23 deletions(-)
> > >
> > > diff --git a/Makefile b/Makefile
> > > index 211a1b6f6478..722ff5864275 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -964,6 +964,7 @@ endif
> > >  # to provide implementations of these routines, then prevent the compiler from
> > >  # emitting calls to what will be undefined symbols.
> > >  KBUILD_CFLAGS        += -fno-builtin-stpcpy
> > > +KBUILD_CFLAGS        += -fno-builtin-bcmp
> >
> > I personally think that this hunk should be its own patch before this
> > one then have this patch just be the revert, that way there is no
> > regression across a bisect (if one were to ever occur) and so the revert
> > is a straight 'git revert', rather than have something else mixed in
> > that requires reading the actual changelog text.
> >
> > No objections if you disagree though.
> 
> That's a great idea.  I considered it before sending, but I think it
> would be interesting to divorce the KBUILD changes which can be picked
> up quickly from the latter changes.  Will send a V2.

Yeah, I had the same thoughts as Nathan. With that change:

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> 
> >
> > >  # include additional Makefiles when needed
> > >  include-y                    := scripts/Makefile.extrawarn
> > > diff --git a/include/linux/string.h b/include/linux/string.h
> > > index b1f3894a0a3e..f3bdb74bc230 100644
> > > --- a/include/linux/string.h
> > > +++ b/include/linux/string.h
> > > @@ -155,9 +155,6 @@ extern void * memscan(void *,int,__kernel_size_t);
> > >  #ifndef __HAVE_ARCH_MEMCMP
> > >  extern int memcmp(const void *,const void *,__kernel_size_t);
> > >  #endif
> > > -#ifndef __HAVE_ARCH_BCMP
> > > -extern int bcmp(const void *,const void *,__kernel_size_t);
> > > -#endif
> > >  #ifndef __HAVE_ARCH_MEMCHR
> > >  extern void * memchr(const void *,int,__kernel_size_t);
> > >  #endif
> > > diff --git a/lib/string.c b/lib/string.c
> > > index 6012c385fb31..69328b8353e1 100644
> > > --- a/lib/string.c
> > > +++ b/lib/string.c
> > > @@ -922,26 +922,6 @@ __visible int memcmp(const void *cs, const void *ct, size_t count)
> > >  EXPORT_SYMBOL(memcmp);
> > >  #endif
> > >
> > > -#ifndef __HAVE_ARCH_BCMP
> > > -/**
> > > - * bcmp - returns 0 if and only if the buffers have identical contents.
> > > - * @a: pointer to first buffer.
> > > - * @b: pointer to second buffer.
> > > - * @len: size of buffers.
> > > - *
> > > - * The sign or magnitude of a non-zero return value has no particular
> > > - * meaning, and architectures may implement their own more efficient bcmp(). So
> > > - * while this particular implementation is a simple (tail) call to memcmp, do
> > > - * not rely on anything but whether the return value is zero or non-zero.
> > > - */
> > > -#undef bcmp
> > > -int bcmp(const void *a, const void *b, size_t len)
> > > -{
> > > -     return memcmp(a, b, len);
> > > -}
> > > -EXPORT_SYMBOL(bcmp);
> > > -#endif
> > > -
> > >  #ifndef __HAVE_ARCH_MEMSCAN
> > >  /**
> > >   * memscan - Find a character in an area of memory.
> > > --
> > > 2.28.0.220.ged08abb693-goog
> > >
> >
> > Cheers,
> > Nathan
> 
> 
> 
> -- 
> Thanks,
> ~Nick Desaulniers

-- 
Kees Cook

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