linux-kernel - Re: [PATCH v3] Many pages: Document fixed-width types with ISO C naming

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Date:   Thu, 25 Aug 2022 08:41:41 +0200
From:   Florian Weimer <fweimer@...hat.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Alejandro Colomar <alx.manpages@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Alex Colomar <alx@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        linux-man <linux-man@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Zack Weinberg <zackw@...ix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        glibc <libc-alpha@...rceware.org>, GCC <gcc-patches@....gnu.org>,
        bpf <bpf@...r.kernel.org>, LTP List <ltp@...ts.linux.it>,
        Linux API <linux-api@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        David Laight <David.Laight@...lab.com>,
        Joseph Myers <joseph@...esourcery.com>,
        Cyril Hrubis <chrubis@...e.cz>,
        David Howells <dhowells@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, Rich Felker <dalias@...c.org>,
        Adhemerval Zanella <adhemerval.zanella@...aro.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3] Many pages: Document fixed-width types with ISO C
 naming

* Greg Kroah-Hartman:

> On Thu, Aug 25, 2022 at 01:36:10AM +0200, Alejandro Colomar wrote:
>> But from your side what do we have?  Just direct NAKs without much
>> explanation.  The only one who gave some explanation was Greg, and he
>> vaguely pointed to Linus's comments about it in the past, with no precise
>> pointer to it.  I investigated a lot before v2, and could not find anything
>> strong enough to recommend using kernel types in user space, so I pushed v2,
>> and the discussion was kept.
>
> So despite me saying that "this is not ok", and many other maintainers
> saying "this is not ok", you applied a patch with our objections on it?
> That is very odd and a bit rude.

The justifications brought forward are just regurgitating previous
misinformation.  If you do that, it's hard to take you seriously.

There is actually a good reason for using __u64: it's always based on
long long, so the format strings are no longer architecture-specific,
and those ugly macro hacks are not needed to achieve portability.  But
that's really the only reason I'm aware of.  Admittedly, it's a pretty
good reason.

>> I would like that if you still oppose to the patch, at least were able to
>> provide some facts to this discussion.
>
> The fact is that the kernel can not use the namespace that userspace has
> with ISO C names.  It's that simple as the ISO standard does NOT
> describe the variable types for an ABI that can cross the user/kernel
> boundry.

You cannot avoid using certain ISO C names with current GCC or Clang,
however hard you try.  But currently, the kernel does not try at all,
not really: it is not using -ffreestanding and -fno-builtin, at least
not consistently.  This means that if the compiler sees a known function
(with the right name and a compatible prototype), it will optimize based
on that.  What kind of headers you use does not matter.

<stdarg.h>, <stddef.h>, <stdint.h> are compiler-provided headers that
are designed to be safe to use for bare-metal contexts (like in
kernels).  Avoiding them is not necessary per se.  However, <stdint.h>
is not particularly useful if you want to use your own printf-style
functions with the usual format specifiers (see above for __u64).  But
on its own, it's perfectly safe to use.  You have problems with
<stdint.h> *because* you use well-known, standard facilities in kernel
space (the printf format specifiers), not because you avoid them.  So
exactly the opposite of what you say.

> But until then, we have to stick to our variable name types,
> just like all other operating systems have to (we are not alone here.)

FreeBSD uses <stdint.h> and the <inttypes.h> formatting macros in kernel
space.  I don't think that's unusual at all for current kernels.  It's
particularly safe for FreeBSD because they use a monorepo and toolchain
variance among developers is greatly reduced.  Linux would need to
provide its own <inttypes.h> equivalent for the formatting macros
(as it's not a compiler header; FreeBSD has <machine/_inttypes.h>).

At this point and with the current ABIs we have for Linux, it makes
equal (maybe more) sense to avoid the <stdint.h> types altogether and
use Linux-specific typedefs with have architecture-independent format
strings.

Thanks,
Florian