Re: [PATCH 00/25] Change time_t and clock_t to 64 bit

Arnd Bergmann
2014-05-15T20:39:14Z

On Thursday 15 May 2014 20:10:05 Joseph S. Myers wrote:
> On Thu, 15 May 2014, Arnd Bergmann wrote:
>
> > Earlier in the thread there seemed to be a rough consensus that
> > _TIME_BITS=64 wouldn't be a good idea because we wouldn't get everything
> > changed to use it. For _FILE_OFFSET_BITS=64 that's ok because most
> > user space doesn't ever want to deal with large files.
>
> Well, I'm coming into this in the middle since it isn't on linux-api and
> noone has tried to work out on libc-alpha what things should look like
> from the glibc side. _TIME_BITS seemed to make sense when I thought about
> this previously, however.
>
> > Can you elaborate on how the switch to the new default would work?
>
> At some appropriate release (probably after _TIME_BITS=64 is widely used
> in distributions), the glibc headers would change so that _TIME_BITS=64 is
> the default and _TIME_BITS=32 can be set to get the old interfaces. At
> some later point _TIME_BITS=32 API support might be removed, leaving the
> old symbols as compat symbols for existing binaries.

Ok.

> > If it's easy, why hasn't it been done for _FILE_OFFSET_BITS already
> > and what's stopping us from changing the default as soon as the interfaces
> > are there? If it's hard, what would need to happen before the default
> > time_t can be set?
>
> The distribution side of the change for _FILE_OFFSET_BITS (i.e., moving to
> building libraries that way so a glibc change to the default wouldn't
> cause issues for other libraries' ABIs) has gradually been done. The
> discussion in March on libc-alpha about changing the default tailed off.
> This is something that needs someone to take the lead with a *careful and
> detailed analysis of the information from the previous discussion* in
> order to present a properly reasoned proposal for a change to the default
> - not scattergun patches, not patches with brief or no analysis of the
> environment in which glibc is used, not dismissing concerns, but a
> properly reasoned argument for why the change should be made, along with
> details of how distributions can determine whether ABI issues would arise
> from rebuilding a particular library against newer glibc.

Ok, I see. I wasn't aware that distributions actually set _FILE_OFFSET_BITS
globally for building packages. I guess the effect (from the distro point
of view) of that is similar to having a configure option when building glibc
as I expected to be the normal way to do it.

> > > Obviously 64-bit time_t syscalls would be an appropriately narrow set of
> > > syscalls like those in the generic ABI (so glibc would implement stat for
> > > _TIME_BITS=64 using fstatat64_time64 or whatever the syscall is called,
> > > just as the stat functions for generic ABI architectures are implemented
> > > with newfstatat / fstatat64 rather than lots of separate syscalls.
> >
> > This assumes that we'd leave the kernel time_t/timespec/timeval using 'long'
> > and introduce a new timespec64 using a signed 64-bit type, rather than
> > changing the kernel headers to the new syscalls and data structures with
> > new names for the existing ones, right?
>
> Yes. I consider it simply common sense that new kernel headers should
> continue to work with much older glibc, meaning that the API (syscall
> names etc.) presented by the headers from headers_install should not
> change incompatibly.

Right. we have done it both ways in the past, but it seems that renaming
syscalls hasn't been done in some time. I can only find definitions for
oldfstat, oldlstat, oldolduname, olduname, oldumount, vm86old and oldwait4.
It's possible they all predate libc6.

> (64-bit type only for time_t, of course. There's no need for a 64-bit
> type for nanoseconds and tv_nsec is explicitly "long" in POSIX, meaning
> that if the kernel uses a 64-bit type for nanoseconds on systems where
> "long" is 32-bit in userspace, either it needs to treat the high word as
> padding or glibc needs to wrap all interfaces passing a struct timespec
> into the kernel so they clear the padding field. There's even less need
> for a 64-bit type for microseconds.)

For practical purposes in the kernel, we may still want to use 64-bit
nanoseconds: if we use a 96 bit struct timespec, that would be incompatible
with the native type on 64-bit kernels, thus complicating the syscall
emulation layer.

I don't know why timespec on x32 uses 'long tv_nsec', it does seem
problematic.

What could work is a type that has explicit padding:

struct timespec {
__s64 tv_sec;
#ifdef BIG_ENDIAN_32BIT
u32 __pad;
#endif
long tv_nsec;
#ifdef LITTLE_ENDIAN_32BIT
u32 __pad;
#endif
};

For timeval, I think we don't care about the padding, because we wouldn't
use it on new interfaces when the kernel uses nanosecond resolution
internally.

Arnd