Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20200817220212.338670-1-ndesaulniers@google.com>
 <20200817220212.338670-2-ndesaulniers@google.com> <CAMj1kXH0gRCaoF0NziC870=eSEy0ghi8b0b6A+LMu8PMd58C0Q@mail.gmail.com>
 <20200818072531.GC9254@kroah.com>
In-Reply-To: <20200818072531.GC9254@kroah.com>
From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Tue, 18 Aug 2020 09:29:39 +0200
Message-ID: <CAMj1kXF_RhV7D8D8J_fwTruiKWbHapeGe-omwyBoR8t4gRv7QA@mail.gmail.com>
Subject: Re: [PATCH 1/4] Makefile: add -fno-builtin-stpcpy
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Michal Marek <michal.lkml@markovi.net>,
        linux-kbuild@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Tony Luck <tony.luck@intel.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Joe Perches <joe@perches.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Daniel Axtens <dja@axtens.net>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Alexandru Ardelean <alexandru.ardelean@analog.com>,
        Yury Norov <yury.norov@gmail.com>, X86 ML <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Daniel Kiper <daniel.kiper@oracle.com>,
        Bruce Ashfield <bruce.ashfield@gmail.com>,
        Marco Elver <elver@google.com>,
        Vamshi K Sthambamkadi <vamshi.k.sthambamkadi@gmail.com>,
        Andi Kleen <ak@suse.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        =?UTF-8?B?RMOhdmlkIEJvbHZhbnNrw70=?= <david.bolvansky@gmail.com>,
        Eli Friedman <efriedma@quicinc.com>,
        "# 3.4.x" <stable@vger.kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, 18 Aug 2020 at 09:25, Greg KH <gregkh@linuxfoundation.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@google.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@vger.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?