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

[Florian Weimer]

* 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