Re: utmp 64 bit time_t support

[Paul Eggert <eggert@cs.ucla.edu>]

On 2023-08-01 13:28, Adhemerval Zanella Netto via Libc-alpha wrote:
> The gnulib developers has reported that the lack of 64-bit support on utmp
> records has created some real issues [1]. This issue is problematic because
> now some 32-bit architectures when built with 64-bit time_t support have
> a different utmp struct size, which essentially breaks __WORDSIZE_TIME64_COMPAT32
> support. The simple straightforward fix would to make utmp/utmpx always 32-bit
> for theses architecture (mips, mips64n32, riscv32, sparcv9, and i686) but this
> would be an ABI break.

Isn't there a less-simple but still reasonably-straightforward fix that 
wouldn't break the ABI? On these traditionally-32-bit time_t platforms 
when _TIME_BITS=64, do what Gnulib is doing[8], namely, convert from 
file format to internal format while reading the utmp data.

Obviously this Gnulib hack won't work after the year 2038, but it's 
still a win because struct utmpx doesn't work today. That is, with 
current glibc, utmpx-using programs fail today if compiled with 
_TIME_BITS=64 on 32-bit platforms. This is why I filed glibc bug 30701. 
I suspect this bug will need to be fixed regardless of whether utmpx 
becomes deprecated in the next glibc release.

> [1] https://sourceware.org/bugzilla/show_bug.cgi?id=30701
> [2] https://github.com/thkukuk/utmpx/blob/main/Y2038.md
> [3] https://sourceware.org/bugzilla/show_bug.cgi?id=28146
> [4] https://sourceware.org/bugzilla/show_bug.cgi?id=17470
> [5] https://sourceware.org/bugzilla/show_bug.cgi?id=24492
> [6] https://sourceware.org/pipermail/libc-alpha/2020-July/116742.html
> [7] https://sourceware.org/pipermail/libc-alpha/2020-July/116764.html

[8]: https://lists.gnu.org/r/bug-gnulib/2023-07/msg00159.html