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

From: Greg KH <gregkh@linuxfoundation.org>
To: Ard Biesheuvel <ardb@kernel.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>,
	"Dávid Bolvanský" <david.bolvansky@gmail.com>,
	"Eli Friedman" <efriedma@quicinc.com>,
	"# 3.4.x" <stable@vger.kernel.org>,
	"Sami Tolvanen" <samitolvanen@google.com>
Subject: Re: [PATCH 1/4] Makefile: add -fno-builtin-stpcpy
Date: Tue, 18 Aug 2020 09:34:08 +0200	[thread overview]
Message-ID: <20200818073408.GA12514@kroah.com> (raw)
In-Reply-To: <CAMj1kXF_RhV7D8D8J_fwTruiKWbHapeGe-omwyBoR8t4gRv7QA@mail.gmail.com>

On Tue, Aug 18, 2020 at 09:29:39AM +0200, Ard Biesheuvel wrote:
> 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?

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