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

From: Nick Desaulniers <ndesaulniers@google.com>
To: Arvind Sankar <nivedita@alum.mit.edu>,
	Eli Friedman <efriedma@quicinc.com>
Cc: "Masahiro Yamada" <masahiroy@kernel.org>,
	"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>, "H . Peter Anvin" <hpa@zytor.com>,
	"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>,
	"Andi Kleen" <ak@suse.de>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Dávid Bolvanský" <david.bolvansky@gmail.com>
Subject: Re: [PATCH 0/4] -ffreestanding/-fno-builtin-* patches
Date: Tue, 18 Aug 2020 15:59:45 -0700	[thread overview]
Message-ID: <CAKwvOdmfiD1TNqRvFuX07BqonYzh1eKFE9mFmOpaSyrbR0d5Lw@mail.gmail.com> (raw)
In-Reply-To: <20200818222542.GA3254379@rani.riverdale.lan>

On Tue, Aug 18, 2020 at 3:25 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> Another thing that needs to be fixed is that at least lib/string.c needs
> to be compiled with -ffreestanding.
>
> gcc-10 optimizes the generic memset implementation in there into a call
> to memset. Now that's on x86 which doesn't use the generic
> implementation, but this is just waiting to bite us.
>
> https://godbolt.org/z/6EhG15

I'll let you send the patch for that this time.  (It's too bad godbolt
doesn't have newer versions of GCC for cross compilation...cant test
aarch64 gcc-10, for example.)  It would be interesting for sure to see
resulting differences in disassembly observed in lib/string.o with
-ffreestanding.

But, oof, that's not good.  Certainly impressive and powerful loop
idiom recognition, but wouldn't you consider it a bug that this
optimization should probably first check that it's not replacing part
of a loop with a potentially recursive call to itself?

Admittedly, we've had the same shenanigans with memcpy implemented in
terms of calls to __builtin_memcpy being lowered to infinitely
recursive calls...which feels like the same kind of bug.  ("You wanted
infinite recursion in the kexec purgatory image, right?" "No,
compiler, I did not.")  example: https://godbolt.org/z/MzrTaM
(probably should fix this in both implementations; at the least I feel
like Clang's -Winfinite-recursion should try to help us out here).

Feels almost like it may be difficult to provide an implementation of
memset without stepping on a landmine.  One thing I'd be curious about
is whether all of lib/string.c would need -ffreestanding, or if you
could move just memset to its own TU then use -ffreestanding on that.
A free standing environment must always provide a core set of
functions like memset, memcpy, memcmp, memmove, according to
https://gcc.gnu.org/onlinedocs/gcc/Standards.html.  Maybe those four
should be in a separate TU compiled as -ffreestanding, so that they
can never be lowered to calls to themselves (potentially infinitely
recursive)?
-- 
Thanks,
~Nick Desaulniers