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Re: Summary of the 2038 BoF at DC17




On Sat, Sep 02, 2017 at 05:41:09PM +0100, Ben Hutchings wrote:
> On Sat, 2017-09-02 at 04:04 +0200, Adam Borowski wrote:
> > On Sat, Sep 02, 2017 at 12:58:54AM +0100, Steve McIntyre wrote:
> > > UNIX time_t is 31 bits (signed), counting seconds since Jan 1,
> > > 1970. It's going to wrap.. It's used *everywhere* in UNIX-based
> > > systems. Imagine the effects of Y2K, but worse.
> > > Glibc is the next obvious piece of the puzzle - almost everything
> > > depends on it. Planning is ongoing at
> > > 
> > >   https://sourceware.org/glibc/wiki/Y2038ProofnessDesign
> > > 
> > > to provide 64-bit time_t support without breaking the existing 32-bit
> > > code.
> > 
> > I find it strange that you don't mention x32 anywhere.  Dealing with
> > assumptions that time_t = long was the majority of work with this port; a
> > lot of software still either did not apply submitted patches or accepted
> > only dumb casts to (long) instead of a proper y2038-proof solution.
> 
> AFAIK, various compat ioctl implementations assume 32-bit time_t and so
> don't work properly for x32.  The work being done to add 64-bit time
> support for older 32-bit architectures will work for both i386 and x32.
> So x32 doesn't solve any problems.

Parts of the userland↔kernel interface indeed fail.  The obvious parts (such
as off_t and friends) are cleanly 64-bit but many ioctls were/are broken,
and don't match uapi headers.  Most of those have been fixed since, usually
on the kernel side, but indeed a concerted effort to get everything right
can't hurt.

But the section that I responded to was about userland API/ABI.  And on x32
all of the following types are 64-bit:
    time_t
    struct timeval (both tv_sec and tv_usec)
    struct timespec (both tv_sec and tv_nsec)
Sub-second fields are pointlessly wide (both usec and nsec fits within 31
bits fine).  This is not a problem on amd64 where it matches "long" but is a
frequent cause of unjoy on x32 where it's longer than long.

Usually the FTBFS comes from printf("%l", t) where it causes a warning which
in turns triggers -Werror (used by way too many packages).

In other cases, a pointer to time_t or a struct time{val,spec} field is
taken for a pointer to long.  #874018 is an example I filed less that 22
hours ago -- a library that package uses (libstartupnotification) has the
following prototype:
    sn_startup_sequence_get_last_active_time (SnStartupSequence *sequence,
                                          long              *tv_sec,
                                          long              *tv_usec);
which will accept pointers to timespec.tv_{,sec} on every architecture but
x32 -- until this planned y2038 change when every 32-bit arch will follow.


Thus, there are good news and bad news:
* good: userspace time_t↔long mismatches are almost all fixed
* bad: in many cases they were cast into (long) as there's no PRId for
  time_t, which makes your work harder: instead of a nice warning/error,
  the y2038 bug is silent.


The latter doesn't affect cases like libstartupnotification -- once that
library's API changes from long to the new type, users will nicely FTBFS.
There's nothing wrong with tv_{u,n}sec as no one plans to extend a second
above 1000000μs or 1000000000ns, but casts on time_t are not nice.


Meow!
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