Re: On time64 and Large File Support

[Sam James <sam@gentoo.org>]

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> On 12 Nov 2022, at 04:20, Wookey <wookey@wookware.org> wrote:
> 
> On 2022-11-11 10:19 +0100, Florian Weimer wrote:
> 
> Hi. I've started looking into the 64-bit time_t transition for 32-bit armhf
> in Debian.  We are currently doing a preliminary bootstrap to see what
> breaks. We strongly suspect that only a wholesale rebuild for the new
> ABI (i.e a new Debian architecture) is practical, but have not yet
> entirely ruled out attempting a migration within the existing armhf
> arch.
> 
> [snip]
> 
>> * Sam James
>> 
>>> In Gentoo, we've been planning out what we should do for time64 on
>>> glibc [0] and concluded that we need some support in glibc for a newer
>>> option. I'll outline why below.
>>> 
>>> Proposal: glibc gains two new build-time configure options:
>>> * --enable-hard-time64
>>> * --enable-hard-lfs
> 
> I don't quite follow the logic of this. glibc already has build-time macros to set these two things:
> _TIME_BITS=64
> _FILE_OFFSET_BITS=64
> 
> why do we need configure options too?

How do you make sure that every program built uses it? Not every
program respects CPPFLAGS and even in CFLAGS, it's a bit
of a nuisance.

If you patch GCC, you don't cover Clang. If you patch system
compilers, that's messy but also doesn't help with custom-built programs.

Of course, we could just patch glibc and cheerily jam it in the headers,
but we run into the kind of problems that Joseph Myers mentions then,
I think (basically I'd want to make sure we do it right.)

> 
>>> We're now (possibly) on the eve of an autoconf 2.72 release which contains two changes
>>> of note [2][3]
>>> 1. addition of a new AC_SYS_YEAR2038 macro;
>>> 2. making AC_SYS_LARGEFILE change behaviour to imply AC_SYS_YEAR2038.
> 
> Which is the opposite way round to glibc, where _TIME_BITS=64 requires
> _FILE_OFFSET_BITS=64, but not the other way round
> (_FILE_OFFSET_BITS=64, can be set on its own). Am I misunderstanding something here?
> 

I wonder the same. I don't think it's obvious, and it may not be obvious
to people writing software using autoconf either...

> It doesn't seem right to me that AC_SYS_LARGEFILE should imply
> AC_SYS_YEAR2038. What is the reasoning behind that?
> 
>> I really wish the rest of GNU would talk to glibc maintainers before
>> overriding glibc maintainer decisions.  If we cannot revert this in
>> autoconf (and gnulib), this will very much endanger the Fedora i386
>> port.  Debian will probably be impacted in the same way.
> 
> I need to read around all this as I have only just become aware that
> the LFS thing is entangled with the timet_64 thing. Is there a good
> place to read _why_ one implies the other? It definitely complicates
> matters.

time64 has to imply LFS because of some structures like stat including
both off_t (LFS) and st_atim (time64), I think. Some of it is internal too.

Or do you mean LFS => time64? I have no idea for why that's
entangled in autoconf and gnulib.

> 
>>> On reflection and after extensive discussion within Gentoo (although
>>> I don't seek to speak for everybody there) - with special thanks to
>>> David Seifert and Arsen Arsenović for tolerating my bikesheds on this,
>>> we don't think it's feasible to handle this in a piecemeal fashion -
>>> at the very least not without spending a significant & for some,
>>> undesirable amount of time on supporting "obsolete" 32-bit platforms.
> 
> Distros need to co-ordinate on this. If there are going to be new
> triplets for the 'LFS and 64_bit timet' ABI(s) then we should agree on
> them and use them. If distros are happy to migrate to these ABIs
> within the existing arm-linux-gnueabihf and i386-linux-gnu (or
> i686-linux-gnu) then we should do that.
> 
> If half the distros migrate within the existing triplet and the rest use
> a new one, that sounds like a recipie for much confusion.
> 

100%. And also on sharing patches and known problems
and experience with the migration. All of it!

> I could write more, but I'll swot up a bit first :-)

It's not easy to find much about all of this! I almost
felt like I was missing something at first. :)

Best,
sam

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