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

From: Nick Desaulniers <ndesaulniers@google.com>
To: Arvind Sankar <nivedita@alum.mit.edu>,
	Masahiro Yamada <masahiroy@kernel.org>
Cc: "Dávid Bolvanský" <david.bolvansky@gmail.com>,
	"Eli Friedman" <efriedma@quicinc.com>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"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 00:10:37 -0700	[thread overview]
Message-ID: <CAKwvOd=BEwuHFeuskJ4gPOGLoXm98oXA18U=tTw981g+HdVz-w@mail.gmail.com> (raw)
In-Reply-To: <20200824173450.GA4157679@rani.riverdale.lan>

On Mon, Aug 24, 2020 at 10:34 AM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Tue, Aug 25, 2020 at 12:57:22AM +0900, Masahiro Yamada wrote:
> >
> >
> > To prevent transformation from foo() into bar(),
> > there are two ways in Clang to do that;
> > -fno-builtin-foo, and -fno-builtin-bar.
> > There is only one in GCC; -fno-buitin-foo.
> >
> > Is this correct?
> >
>
> It looked that way from previous experimentation, but...
>
> >
> >
> > I just played the optimization
> > from printf("helloworld\n") to puts("helloworld").
> >
> > https://godbolt.org/z/5s4ded
> >
> >
> > -fno-builtin-puts cannot prevent clang
> > from emitting puts.
> > Is it because clang does not support
> > -fno-builtin-puts?
>
> Ugh. clang doesn't have __builtin_puts() but it optimizes printf() into
> puts(). It doesn't have __builtin_putchar() but will optimize
> printf("c") into putchar('c').

Bah, merely a <strikethrough>flesh
wound</strikethrough><strikethrough>compiler bug</strikethrough>rather
long TODO in the compiler.
https://github.com/llvm/llvm-project/blob/be2bc7d4cef2edd66c7fb74b70adf62fc68754db/clang/include/clang/Basic/Builtins.def#L943

Anyways, give me a week and I'll hack through the rest of them
https://reviews.llvm.org/D86508.  Certainly made HPA's point hit home,
that's a lot of functionality to implement or disable in an
environment.

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.
-- 
Thanks,
~Nick Desaulniers