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

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Message-ID: <202008181214.5C736E7@keescook>
Date:   Tue, 18 Aug 2020 12:21:51 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "H. Peter Anvin" <hpa@...or.com>
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@...r.kernel.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@...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>, 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 03:31:26PM -0700, H. Peter Anvin 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?

As Nick mentioned, I really don't want to expand the already bad
interfaces from libc. We have enough messes to clean up already, and I
don't want to add more. The kernel already uses a subset of C, we have
(several) separate non-libc memory allocators, we're using strscpy() and
scnprintf() widely in favor of their buggy libc counterparts, etc. We
don't need to match the libc string interfaces especially when they're
arguably bug-prone foot-guns. :)

-- 
Kees Cook