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

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Date:   Tue, 18 Aug 2020 09:25:31 +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 09:10:01AM +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?

thanks,

greg k-h