time_t is 32-bit and is subject to the Y2038 problem. glibc 2.34 onwards supports opting-in to a 64-bit
time_t which allows handling time beyond Y2038.
This page discusses how to handle the migration for (32-bit) systems affected by this, including non-glibc environments. 64-bit systems are not affected.
time_t and 64-bit
time_t is a recipe for disaster where
time_t is exposed in a public API and the consumer/provider are mismatched e.g. net-misc/wget with net-libs/gnutls in bug #828001. This is actually
a problem with Large File Support (LFS) too but it seems to happen far less frequently there, or it was minimised by LARGEFILE_SOURCE allowing dual ABI in packages.
For 32-bit arches, we need to coordinate flipping glibc systems over to use 64-bit
CPPFLAGS="-D_TIME_BITS=64" (note that not all packages may support/respect
CPPFLAGS). Again, 64-bit arches are already fine.
- Libraries using time_t directly or indirectly needs to be switched at the same time. Applications using any such libraries must be switched at the same time too.
- Software might be using
off_t, needing Large File Support (LFS)) without it being obvious. For example, it might be handling a
statstruct which contains both
- Software needs to provide a way to stick with 32-bit
time_tfor now to avoid making the problem worse, but allow optionally using 64-bit
- We likely have to complete Modern C porting first to remove any instances of
-Wimplicit-function-declarationotherwise the redirects in glibc for e.g. time->time64 won't actually work. We might be able to avoid this with some hacks like removing the 32-bit symbols entirely from glibc so we get linker errors.
The core issues are:
- Auditing packages piecemeal is an unfeasibly large amount of work
- glibc has no mechanism to hard-switch yet (and discussion has stalled )
- Acting alone in a hard-switch (or at all, really) risks incompatibility with other distributions, as all of this obviously affects ABI
time_t-dependent types in a library's API is not always straightforward to see
Misc other notes:
- sam summarised the situation on libc-alpha in November in "On time64 and Large File Support" which mentions the problems we face
- dilfridge rebooted the discussion in January in "The time64 and Large File Support mess"
- Support is new in glibc 2.34, stabled in April 2022 in bug #833191
gnulib automatically enables time64 support if the system supports it, breaking ABI(edit: gnulib reverted this). We set
gl_cv_type_time_t_bits_macro=no(and may need to set
ac_cv_type_time_t_y2038=no) for now (for software released with the intermediate "bad" versions of gnulib).
- See older discussion on libc-alpha .
- Our friends in opensuse are on it too (https://www.reddit.com/r/linux/comments/xjtf3q/in_the_year_2038/)
- Gentoo's tracker bug: bug #876883
- Push for an autoconf release which both fixes some Modern C porting issues and contains the needed macros for Y2038 support. The needed fixes are already in git.
- Agree on a way with the upstream glibc + distro community on how to hard-switch glibc.
- Coordinate hard-switching once (plenty of!) testing has been done
- Achieve consensus on new tuple names for time64'd ABIs
Gentoo-specific TODO items, but they may affect other distros too:
- Implement LFS and time64 QA check for Portage
- bug #549092
- Rebased version of LFS check at https://github.com/thesamesam/portage/blob/wip/bin/install-qa-check.d/10large-file-support
- Draft version of time64 check at https://github.com/thesamesam/portage/blob/wip/bin/install-qa-check.d/95time64_t
Backport autoconf commit from master to make(this got yanked upstream so no need to just accept it now)
AC_SYS_LARGEFILEimply 64-bit time_t
- How do we safely recommend rebuilding?
- Start with (some subset of?) @system then @world?
- How do we identify mismatched systems (possibly musl)? Will they just need to follow the glibc procedure we decide on (sans flags)?
- musl upstream already migrated to pure 64-bit
- Upstream have a detailed set of notes  on the problem
Gentoo specific notes:
- Need to check how/if at all we handled it in Gentoo
- Check whether there's more work to be done to ensure user systems are in a consistent state
- 1.2.0 was added on 2020-02-25 and 1.2.1 (first of 1.2.x) was stabled on 2020-08-20.
- Possible user systems are in an inconsistent state given we've not rebuilt / news item'd for this?
We may want to compare with how Large File Support (LFS) /
FILE_OFFSET_BITS has been handled. Much more mature support overall in the ecosystem, although it wasn't generally treated like the ABI break it was.
- For glibc, time64 needs LFS (example of why: see e.g. stat struct).
- Meson already adds this as required
- autoconf has
AC_SYS_LARGEFILEwhich things should be using (but may not be!)
Gentoo specific notes:
- bug #471102 tracks progress in Gentoo overall.
- We have
filter-lfs-flags) in flag-o-matic.eclass
Rebuild after profile change
Not everyone is using autoconf or even passing flags correctly. A safer, though probably more controversial way would be to force changed defaults in gcc and glibc.
- We need to agree on names for these new ABIs, ideally using new tuple names, and do so with other distributions
- In current profiles, set
gl_cv_type_time_t_bits_macro=noto prevent gnulib autoconf macro from automatically adding
-D_TIME_BITS=64(done, note that gnulib reverted the change (see above))
- Ditto for
- Ditto for
- In new profile, add
CPPFLAGS="-D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64"(do CFLAGS instead given insufficient packages respect CPPFLAGS).
- Advise users to adjust custom
CPPFLAGSif necessary, and rebuild @world after switching profiles.
This option is considered far too much work and doesn't achieve ABI compatibility with other distros, so is unlikely to be pursued.
- In packages where
time_tis utilized, revbump and add
append-cppflags -D_FILE_OFFSET_BITS=64 -D_TIME_BITS=64.
- This doesn't ensure that libraries and their consumers get rebuilt together, but it should work out in the end.
- Presumably, we'd have a large package.mask for this.
time64 USE flag
Not likely to be pursued for the same reasons as "Revbump ebuilds".
- May need lots of eautoreconf (after patching autoconf)
- Lots of --enable-year2038 added to econfs
- Unmask on to-be-created features/time64 profile
- Allows expressing dependencies on time64ness to help rebuild order
- Allows adding machinery to ebuilds at our own pace, then create new profiles when ready
- Doesn't cause churn to 64-bit arches
|Solution||Consistency during migration||Minimises disruption to bystander/unaffected users (64-bit arches)||Amount of manual labor needed||Avoids flag day|
|Rebuild after profile change (just changes cache variables)||Provides no guarantees of correct rebuild order and may require several rebuilds to ensure consistent ABI.||The change is invisible to these users as they already have 64-bit support and won't be changing profiles.||Not much at all wrt packaging minutiae (of course, there's still bug fixing.)||No|
|Revbump ebuilds to add time64 CFLAGS||Provides no guarantees of correct rebuild order and may require several rebuilds to ensure consistent ABI.||All users, even those unaffected, have to rebuild.||Lots of ebuild churn.||We can make the changes gradually, by traversing leaf packages, and building up to libraries.|
|time64 USE flag||If dependencies are expressed correctly, we can ensure libraries gain time64 support before consumers, and we can use use= deps to enforce correctness.|| The USE flag will be masked on irrelevant profiles so only
||A lot of work modifying many ebuilds, especially if we're going to get the dependencies right, which is the advantage from this option...||We'd need a flag day to eventually remove the time64 flag, but not to add it.|
- Preparing for Y2038 (Already?!)
- glibc is still not Y2038 compliant by default
- Debian discusses how to handle 2038 (from 2020, outdated wrt glibc)
- Debian wiki: 64bit-time
- Sam James. On time64 and Large File Support, libc-alpha mailing list, November 11th, 2022. Retrieved on January 19th, 2023.
- Kaz Kylheku. Best practices in regard to -D_TIME_BITS=64, libc-alpha mailing list, January 5th, 2022. Retrieved on January 19th, 2023.
- Rich Felker. musl time64 Release Notes, musl libc project, February 20th, 2020. Retrieved on January 19th, 2023.