Re: time64 / Large File Support: 2) default time64 breaks legacy 32bit binaries

[Sam James <sam@gentoo.org>]

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> On 26 Jan 2023, at 13:21, Adhemerval Zanella Netto via Libc-alpha <libc-alpha@sourceware.org> wrote:
> 
> 
> 
> On 26/01/23 01:13, Paul Eggert wrote:
>> On 1/25/23 15:59, Andreas K. Huettel via Libc-alpha wrote:
>> 
>>> This was discussed already in the previous thread on this list [1], with reactions
>>> ranging from "need new triplet" via "need new libdir" to "meh"....
>>> [1] https://sourceware.org/pipermail/libc-alpha/2022-November/143386.html
>> 
>> One thing new since that November email is that in bleeding-edge Autoconf we've scaled back AC_SYS_LARGEFILE so it no longer widens time_t by default. Instead, you need to pass a new option --enable-year2038 to 'configure' if you want 64-bit time_t on 32-bit glibc x86 and ARM platforms, which as I understand it are the only platforms that have this problem. If a package author wants --enable-year2038 to be the default, they need to use Autoconf's new AC_SYS_YEAR2038 macro. This change has also percolated into Gnulib so source packages using recent Gnulib will need to use the new Gnulib module year2038 if they want --enable-year2038 to be the default.
>> 
>> This change was done out of concern that although AC_SYS_LARGEFILE has long tweaked blkcnt_t, dev_t, ino_t, fsblkcnt_t, fsfilcnt_t and rlim_t (in addition to off_t of course), having it also tweak time_t was a compatibility bridge too far.
>> 
>>> Proposal: glibc gains two new build-time configure options:
>>> * --enable-hard-time64
>>> * --enable-hard-lfs
>> 
>> This sort of thing sounds like a good way to go. However, I suggest simplifying things, by having just one option (say, --enable-hard-sys-types64) that does both at once, because --enable-hard-time64 and --enable-hard-lfs would not be orthogonal and this would be confusing, and anyway nobody sane will want to use one option without also using the other - who wants the agony of *two* conversions?
> 
> I agree a single option make sense, there is no good reason to add LFS-only
> with 64-bit support. It also simplify build systems that are not autoconf
> based.
> 

Single option is fine with me and I agree it makes more sense.

> A minor problem, which is for all configure switch, it adds another build
> permutation that incur in more testing requirements and maintenance.
> 
> However it does not help with the rest of plumbing that a system will need
> to do for correct set library selection, since ldconfig will see both 32 and
> 64 bit time_t shared library essentially being the same ABI.  A mixed
> environment with legacy binaries/libraries will still incur in similar
> issue, albeit in a different direction.  So to run old binaries one will
> need to either setup LD_PRELOAD/LD_LIBRARY_PATH/RUNPATH or run it in an
> isolated environment (which itself has its own issues).
> 

My feeling was that anyone who continues to need 32-bit time_t would just
run a system without such a glibc built and wouldn't contaminate it with
64-bit time_t binaries.

> And the configure switch also adds a kind of fragmentation, but it is also
> we already have when a projects enables time64_t anyway.
> 

Yeah, I think the ship has more-or-less sailed, but my hope is that we'd
all agree to do this as distros around the same time with new ABI
names to indicate it.

> So although I am not quite against --enable-hard-sys-types64, I personally
> think we should do something more drastically (which not all other glibc
> developers agree) and flip the switch to enable 64-bit time_t *as default*
> and document 32-bit is opt-in. If Fedora or any distro wants to keep the
> *broken* non-LFS / 32 bit time_t, it is up to them to patch glibc to do so.

Right, it seems RH has some needs due to supporting existing customers,
but I don't think this should unduly affect what glibc upstream does if there's
one clear technical path forward. Nobody seems to actually dispute that
the end-game here is a hard switch at some point. Just about when.

But I'm a bit less bothered about this if we're saying that we only need
to wait 2 years or so. I could live with that if we really have to. Not ideal,
but my hands are full at the moment, so...


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