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

From: Nick Desaulniers <ndesaulniers@google.com>
To: Masahiro Yamada <masahiroy@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Arvind Sankar" <nivedita@alum.mit.edu>,
	"Dávid Bolvanský" <david.bolvansky@gmail.com>,
	"Eli Friedman" <efriedma@quicinc.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"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 mailing list" <linux-kbuild@vger.kernel.org>,
	LKML <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>,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Alexandru Ardelean" <alexandru.ardelean@analog.com>,
	"Yury Norov" <yury.norov@gmail.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, "Ard Biesheuvel" <ardb@kernel.org>,
	"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>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches
Date: Tue, 25 Aug 2020 07:02:28 -0700	[thread overview]
Message-ID: <CAKwvOdk9gpKNmEAHLf9KtzmzxnSdmDdutu5Mpp39XEaxMywMFA@mail.gmail.com> (raw)
In-Reply-To: <CAK7LNAQynuqqjhL1rqMf0YnRpQ0jsWrJ7fy51FM7d0xdxws5wQ@mail.gmail.com>

On Tue, Aug 25, 2020 at 5:30 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Tue, Aug 25, 2020 at 4:10 PM Nick Desaulniers
> <ndesaulniers@google.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