linux-kernel - Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

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Date:   Tue, 25 Aug 2020 07:02:28 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        Eli Friedman <efriedma@...cinc.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        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>,
        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>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches

On Tue, Aug 25, 2020 at 5:30 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Tue, Aug 25, 2020 at 4:10 PM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > Masahiro, are you implying that we shouldn't take the
> > -fno-builtin-stpcpy patch, because Clang is inconsistent? (That can be
> > fixed.) Even though -fno-builtin-stpcpy works here as intended?
> > https://lore.kernel.org/lkml/20200817220212.338670-2-ndesaulniers@google.com/
> >
> > Otherwise we need to provide an implementation of this symbol in the kernel.
> > https://lore.kernel.org/lkml/20200815020946.1538085-1-ndesaulniers@google.com/
> >
> > Please, pick your poison.
>
>
>
> I am not a compiler expert.
>
> Nor am I sure if I am the right person who makes this decision.
> But, if so, I would choose the latter.
> (implement stpcpy() in the kernel)
>
> I was addressed last night, so
> I should write up my thoughts.
>
> I do not think -fno-builtin-stpcpy is a
> general solution.
>
> -fno-builtin-stpcpy will work for now
> because only Clang implements the transformation
> from 'sprintf(dest, "%s", str)' into
> 'stpcpy(dest, str) - dest'.
>
> If GCC implements it some day,
> we would run into a problem because
> in GCC, it is not -fno-builtin-stpcpy, but
> -fno-builtin-sprintf that disables that optimization.
>
> In this regard, 'KBUILD_CFLAGS += -fno-builtin-sprintf'
> would be more future-proof, but it is potentially
> an overkill.
> We want to disable optimization from sprintf() to stpcpy(),
> but we may still benefit from the optimization from
> sprintf() into something else.
>
>
> Linus is uncomfortable with this kind of compiler magic.
> If we take compiler's freedom away,
> -ffreestanding is a big hammer to solve this problem.
>
> If we welcome the compiler's optimization,
> we should implement stpcpy(), bcmp(), and whatever
> until we solve all link errors.

Speculating that -ffreestanding is untenable, submitted v3:
https://lore.kernel.org/lkml/20200825135838.2938771-1-ndesaulniers@google.com/T/#u
-- 
Thanks,
~Nick Desaulniers