• y2038

The year 2038 problem

All 32-bit kernels to date use a signed 32-bit time_t type, which can only represent time until January 2038. Since embedded systems running 32-bit Linux are going to survive beyond that date, we have to change all current uses, in a backwards compatible way.

User space interfaces

We will likely keep the 32-bit time_t in all user space interfaces that currently use it, but add new interfaces with a 64-bit timespec or another type that can represent later times. Most importantly that impacts system calls, but also specific ioctl commands and a few other interfaces. User space programs have to be recompiled to use the new interfaces, and the policy whether to use the old or the time time is left to the C library. While that policy is a complex topic itself, we don't cover it here.

System calls

interfaces that uses relative time_t/timespec/timeval

These can stay compatible, but we'd have to use a different type if we change time_t.

• nanosleep
• select/pselect/poll/ppoll/epoll
• getrusage
• sched_rr_get_interval
• sigtimedwait
• alarm

interfaces that don't make sense for times in the past

Here, we are relatively free to change the start of the epoch in the kernel but convert to something else on the user space boundary. One possibility is to scale them to boot time and use ktime_t in the kernel.

• getitimer/setitimer
• timer_settime/timer_gettime
• gettimeofday/settimeofday
• adjtimex
• clock_gettime/clock_settime/clock_adjtime/clock_nanosleep
• time/stime
• sysv ipc (msg, sem, shm)
• sysinfo

interfaces that require absolute times

These absolutely have to use something better than time_t both in user space and in the kernel so we can deal with old files. A lot of file systems need to be fixed as well so we can actually store the times, regardless of whether we are running a 32 or 64 bit kernel.

• stat/lstat/fstatat/
• utime/utimes/futimesat

ioctl

There are numerous ioctl commands using a time argument. This list is incomplete

• audio time stamps
• v4l time stamps
• input event time stamps
• socket time stamps
• ...

memory mapped packet sockets

Socket timestamps are exported to user space using a memory mapped interface defined in include/uapi/linux/if_packet.h. There are currently three versions of this interface, all use a 32-bit time type. We will likely need a version 4 to solve this.

File systems

Each file system stores its file modification times in its own format on disk, and a lot of them have the same problem.