From: Arvind Sankar <nivedita@alum.mit.edu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Arvind Sankar" <nivedita@alum.mit.edu>,
"Rasmus Villemoes" <linux@rasmusvillemoes.dk>,
"Nick Desaulniers" <ndesaulniers@google.com>,
"Dávid Bolvanský" <david.bolvansky@gmail.com>,
"Eli Friedman" <efriedma@quicinc.com>,
"H. Peter Anvin" <hpa@zytor.com>,
"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>, "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: Fri, 21 Aug 2020 15:57:12 -0400 [thread overview]
Message-ID: <20200821195712.GB1475504@rani.riverdale.lan> (raw)
In-Reply-To: <CAHk-=wi8vdb+wo-DACDpSijYfAbCs135YcnnAbRhGJcU+A=-+Q@mail.gmail.com>
On Fri, Aug 21, 2020 at 10:54:57AM -0700, Linus Torvalds wrote:
> On Fri, Aug 21, 2020 at 10:29 AM Arvind Sankar <nivedita@alum.mit.edu> wrote:
> >
> > This one is slightly different from the previous one. The first case is
> > really a call to __builtin_free().
>
> No, the first case is a disgrace and a compiler bug.
>
> We've had a situation where gcc complained about a static function
> called "free()", without any header file inclusion, and then
> complained about it not matching its idea of what "free()" is.
>
> Which is pure and utter garbage.
>
> It's like you have a local variable "int free", and the compiler says
> "hey, this doesn't match the prototype that I know this name should
> have". It's BS. You just saw the user not just *use* that name, but
> *define* it, and do it in a scope where the complaint is irrelevant
> and stupid, and when we hadn't even included the header that would
> have resulted in conflicts.
>
> IOW, it's an example of a compiler that thinks "it knows better".
>
> It's a broken compiler. And it's an example of the kind of breakage
> that compilers absolutely shouldn't do.
That's -Wbuiltin-declaration-mismatch, and can be turned off, and it
won't warn if you have -fno-builtin-free. I don't completely agree with
you, though warning for static functions is a bit overzealous. For an
external function, especially something more obscure like stpcpy(), I
appreciate the warning.
>
> The second example is from clang doesn't something horribly horribly stupid.
Calm down man :)
>
> > This one is turning something that wasn't a function call into
> > __builtin_bzero(), and I would hope that no-builtin-bzero would stop it
> > as well. OTOH, the compiler is free to turn it into memset(), just like
> > it could for structure/array initializers.
>
> The whole "the compiler is free to do X" argument is pure BS. Stop
> repeating that bogus argument.
>
> Of COURSE a compiler can do whatever the hell it wants.
>
> That doesn't change the fact that certain things are broken beyond
> words and utterly stupid, and a compiler that does them is a *BAD*
> compiler.
>
> Turning four stores into a memset() is garbage. Just admit it, instead
> of trying to say that it's allowed.
>
Look, four stores into memset(), yeah that's a bit weird. I didn't think
you meant "four" literally. But in any case, that has nothing to do with
the topic at hand. It would be just as bad if it was a 16-byte structure
being initialized with an out-of-line memset() call.
But coming back to the actual topic: it is fine if the compiler turns
four stores into __builtin_memset(). A size-16 or -32 __builtin_memset()
will get inlined anyway. There's a lot of garbage here if you look
closely: check out what gcc does to initialize a 7-character array with
zeros at -Os.
next prev parent reply other threads:[~2020-08-21 19:57 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-17 22:02 [PATCH 0/4] -ffreestanding/-fno-builtin-* patches Nick Desaulniers
2020-08-17 22:02 ` [PATCH 1/4] Makefile: add -fno-builtin-stpcpy Nick Desaulniers
2020-08-17 22:31 ` H. Peter Anvin
2020-08-17 23:36 ` Nick Desaulniers
2020-08-18 19:21 ` Kees Cook
2020-08-18 7:10 ` Ard Biesheuvel
2020-08-18 7:25 ` Greg KH
2020-08-18 7:29 ` Ard Biesheuvel
2020-08-18 7:34 ` Greg KH
2020-08-18 19:23 ` Kees Cook
2020-08-17 22:02 ` [PATCH 2/4] Revert "lib/string.c: implement a basic bcmp" Nick Desaulniers
2020-08-18 5:44 ` Nathan Chancellor
2020-08-18 18:00 ` Nick Desaulniers
2020-08-18 19:24 ` Kees Cook
2020-08-17 22:02 ` [PATCH 3/4] x86/boot: use -fno-builtin-bcmp Nick Desaulniers
2020-08-18 19:24 ` Kees Cook
2020-08-17 22:02 ` [PATCH 4/4] x86: don't build CONFIG_X86_32 as -ffreestanding Nick Desaulniers
2020-08-18 19:24 ` Kees Cook
2021-01-07 0:27 ` Fangrui Song
2022-04-07 15:34 ` [tip: x86/build] x86/build: Don't " tip-bot2 for Nick Desaulniers
2022-04-07 17:01 ` Nick Desaulniers
2022-04-07 22:28 ` Borislav Petkov
2020-08-17 22:44 ` [PATCH 0/4] -ffreestanding/-fno-builtin-* patches H. Peter Anvin
2020-08-18 17:56 ` Nick Desaulniers
2020-08-18 19:02 ` H. Peter Anvin
2020-08-18 19:13 ` Linus Torvalds
2020-08-18 19:25 ` Nick Desaulniers
2020-08-18 19:58 ` Nick Desaulniers
2020-08-19 12:19 ` Clement Courbet
2020-08-18 20:24 ` Arvind Sankar
2020-08-18 20:27 ` Nick Desaulniers
2020-08-18 20:58 ` Nick Desaulniers
2020-08-18 21:41 ` Arvind Sankar
2020-08-18 21:51 ` Dávid Bolvanský
2020-08-18 21:59 ` Nick Desaulniers
2020-08-18 22:05 ` Dávid Bolvanský
2020-08-18 23:22 ` Nick Desaulniers
2020-08-20 14:56 ` Rasmus Villemoes
2020-08-20 17:56 ` Arvind Sankar
2020-08-20 18:05 ` Dávid Bolvanský
2020-08-20 23:33 ` Linus Torvalds
2020-08-21 17:29 ` Arvind Sankar
2020-08-21 17:54 ` Linus Torvalds
2020-08-21 18:02 ` Linus Torvalds
2020-08-21 19:14 ` Arvind Sankar
2020-08-21 19:23 ` Linus Torvalds
2020-08-21 19:57 ` Arvind Sankar [this message]
2020-08-21 20:03 ` Peter Zijlstra
2020-08-21 21:39 ` Linus Torvalds
2020-08-22 0:12 ` Nick Desaulniers
2020-08-22 12:20 ` David Laight
2020-08-21 6:45 ` Rasmus Villemoes
2020-08-24 15:57 ` Masahiro Yamada
2020-08-24 17:34 ` Arvind Sankar
2020-08-25 7:10 ` Nick Desaulniers
2020-08-25 7:31 ` Nick Desaulniers
2020-08-25 12:28 ` Masahiro Yamada
2020-08-25 14:02 ` Nick Desaulniers
2020-08-26 13:28 ` Masahiro Yamada
2020-08-18 21:53 ` David Laight
2020-08-20 22:41 ` H. Peter Anvin
2020-08-20 23:17 ` Arvind Sankar
2020-08-18 19:35 ` Nick Desaulniers
2020-08-18 22:25 ` Arvind Sankar
2020-08-18 22:59 ` Nick Desaulniers
2020-08-18 23:51 ` Arvind Sankar
2020-08-19 0:20 ` Arvind Sankar
2020-08-19 8:26 ` David Laight
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