Re: On time64 and Large File Support

[Florian Weimer <fweimer@redhat.com>]

* Sam James:

>> On 11 Nov 2022, at 09:19, Florian Weimer <fweimer@redhat.com> wrote:
>> 
>> * 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
>> 
>> We should define new target triplets for this if it's really required.
>
> I hadn't considered that option. I'll reflect on it. Please let me know
> if you have further thoughts on this.
>
> But that said, these binaries are broken anyway in 2038?

No, I expect users to run them in time-shifted VMs or containers.
Wrong dates are better than no ability to run anything at all.

And whoever can recompile to switch to time64 can just recompile for a
64-bit target.  There are some embedded users that stick to 32-bit for
cost savings, but I think the cost allocation is quite wrong: They save
a bit on per-unit costs, but they do not really contribute back to GNU
(and most don't even use glibc, although there is some use out there).

>> We need to support legacy binaries on i386.  Few libraries are
>> explicitly dual-ABI.  Whether it's safe to switch libraries above glibc
>> to LFS or time64 needs to be evaluated on a per-library basis.  For most
>> distributions, no one is going to do that work, and we have to stick to
>> whathever we are building today.
>
> While I agree, I don't think it's as well-known that it should be that
> these are ABI breaking and require inspection. It's being done ad-hoc
> or in many cases, not at all.
>
> Part of the use of this thread is, if nothing else, we can show upstreams
> and other distros It if they're confused.
>
> It's very easy to miss that a package has started enabling LFS
> and then your opportunity to catch the ABI breakage is gone.
>
> It doesn't help that I (and I suspect most distribution maintainers)
> do all development on x86_64 and hence even ABI diffing isn't
> going to notice. You have to specifically diff the build system, which I
> do, but it's not easy if it's buried deep within gnulib or something.

I really assumed that setting the default in glibc would solve this for
everyone: binary distributions keep using time32, and source-based
embedded distributions can switch to time64 if they want to. *sigh*

I mean, we have things like more compact stack usage through certain
ABI-breaking GCC options.  The kernel can use those safely, but few
people attempt to apply them to userspace.  There, having the right
default in the toolchain is sufficient.  I didn't expect time64 to be
different.

Thanks,
Florian