linux-kernel - Re: [PATCH 1/4] Makefile: add -fno-builtin-stpcpy

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Message-ID: <20200818073408.GA12514@kroah.com>
Date:   Tue, 18 Aug 2020 09:34:08 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.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@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.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>, X86 ML <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        "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>,
        "# 3.4.x" <stable@...r.kernel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH 1/4] Makefile: add -fno-builtin-stpcpy

On Tue, Aug 18, 2020 at 09:29:39AM +0200, Ard Biesheuvel wrote:
> On Tue, 18 Aug 2020 at 09:25, Greg KH <gregkh@...uxfoundation.org> wrote:
> >
> > On Tue, Aug 18, 2020 at 09:10:01AM +0200, Ard Biesheuvel wrote:
> > > On Tue, 18 Aug 2020 at 00:02, Nick Desaulniers <ndesaulniers@...gle.com> wrote:
> > > >
> > > > LLVM implemented a recent "libcall optimization" that lowers calls to
> > > > `sprintf(dest, "%s", str)` where the return value is used to
> > > > `stpcpy(dest, str) - dest`. This generally avoids the machinery involved
> > > > in parsing format strings. This optimization was introduced into
> > > > clang-12. Because the kernel does not provide an implementation of
> > > > stpcpy, we observe linkage failures for almost all targets when building
> > > > with ToT clang.
> > > >
> > > > The interface is unsafe as it does not perform any bounds checking.
> > > > Disable this "libcall optimization" via `-fno-builtin-stpcpy`.
> > > >
> > > > Unlike
> > > > commit 5f074f3e192f ("lib/string.c: implement a basic bcmp")
> > > > which cited failures with `-fno-builtin-*` flags being retained in LLVM
> > > > LTO, that bug seems to have been fixed by
> > > > https://reviews.llvm.org/D71193, so the above sha can now be reverted in
> > > > favor of `-fno-builtin-bcmp`.
> > > >
> > > > Cc: stable@...r.kernel.org # 4.4
> > >
> > > Why does a fix for Clang-12 have to be backported all the way to v4.4?
> > > How does that meet the requirements for stable patches?
> >
> > Because people like to build older kernels with new compliler versions.
> >
> > And those "people" include me, who doesn't want to keep around old
> > compilers just because my distro moved to the latest one...
> >
> > We've been doing this for the past 4+ years, for new versions of gcc,
> > keeping 4.4.y building properly with the bleeding edge of that compiler,
> > why is clang any different here?
> >
> 
> Fair enough. I am just struggling to match stable-kernel-rules.rst
> with the actual practices - perhaps it is time to update that
> document?

The rules are tiny and simple for 99% of the issues involved.  Stuff
like "add patches to fix build failures and warnings for newer compiler
versions" are so rare (they only happen every 2 years or so), it's not
worth it.

thanks,

greg k-h