Re: 64 bit time_t on riscv32

[Rich Felker <dalias@libc.org>]

On Tue, Jan 16, 2024 at 05:22:55PM +0100, Florian Weimer wrote:
> * Rich Felker:
> 
> > On Tue, Jan 16, 2024 at 04:46:17PM +0100, Florian Weimer wrote:
> >> * Adhemerval Zanella Netto:
> >> 
> >> > My understanding was __USE_TIME_BITS64 would be required only for ABIs
> >> > that actually would multiple time_t sizes and the glibc 64 bit time_t 
> >> > supported was added on this assumption.  However, the design document 
> >> > does state __USE_TIME_BITS64 would be defined even for architectures 
> >> > where is does not require it.  I it was not really caught in development
> >> > because the design creator was not really much involved at the time.
> >> >
> >> > I think it should be feasible to fix it, although backporting it would
> >> > generate a quite large patch.
> >> 
> >> It will break applications that use __USE_TIME_BITS64 with glibc's
> >> current semantics centered on time64 redirects (incorrectly, of
> >> course—it's an internal macro).  I think the use in the kernel is
> >> probably easier to address, and it has to be changed for compatibility
> >> with other libcs anyway.
> >
> > If there are applications assuming __USE_TIME_BITS64 implies redirects
> > and doing some hack around that, they really need to be fixed not to
> > do that. It will break on musl with (not yet merged) rv32 port as well
> > as any future 32-bit ports, which won't have any redirects because
> > there is no legacy 32-bit time_t ABI to support.
> 
> That's actually not a problem because __USE_TIME_BITS64 is not defined
> on rv32 et al. (hence the complaint about what the UAPI headers are
> doing).

It is a problem for musl users because we *do* define
__USE_TIME_BITS64 and do so unconditionally, on all archs, including
future rv32. Even if we could opt not to define it on rv32, that would
not help with future 32-bit ports like sparc where there will be no
redirection but __USE_TIME_BITS64 is needed to get the kernel headers
to do the right thing. So if any applications are really misusing
__USE_TIME_BITS64 as an indicator of having symbol redirections, they
need to stop/get fixed.

Rich