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

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Message-ID: <CAKwvOdma_n8D5iERGiSSMc7o2bc-rTZN_KhqPZ+rAOP2sGw5uA@mail.gmail.com>
Date:   Mon, 17 Aug 2020 16:36:28 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     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>,
        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>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        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>,
        "# 3.4.x" <stable@...r.kernel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH 1/4] Makefile: add -fno-builtin-stpcpy

On Mon, Aug 17, 2020 at 3:31 PM H. Peter Anvin <hpa@...or.com> wrote:
>
> On 2020-08-17 15:02, Nick Desaulniers 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`.
> >
>
> stpcpy() and (to a lesser degree) mempcpy() are fairly useful routines
> in general. Perhaps we *should* provide them?

Sorry, I forgot to provide context of the previous thread, which is
worth a read.  To answer this question specifically (or at least for
stpcpy), the answer from the previous thread was (via Kees): "No;
please no more unbounded string.h routines":
https://lore.kernel.org/lkml/202008150921.B70721A359@keescook/
-- 
Thanks,
~Nick Desaulniers