• 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 (+ wait4?)
• sched_rr_get_interval
• sigtimedwait
• alarm
• Also: futex, recvmmsg, io_getevents?

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.

[Does checkpoint/restore have any implications here wrt to how freely we can change the start of the epoch? E.g., when freezing/restoring processed from different systems that have timer_settime() timers?]

• getitimer/setitimer
• timer_settime/timer_gettime(+timerfd_settime, timerfd_gettime?)
• gettimeofday/settimeofday
• adjtimex
• clock_gettime/clock_settime/clock_adjtime/clock_nanosleep(? what about clock_nanosleep TIMER_ABSTIME?)
• 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/futimesa/utimensat

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.

Audit of include/uapi for time_t impact

Structure and IOCTL dependency:

time_t
        struct msqid64_ds (has 2038 padding!)
        struct semid64_ds (has 2038 padding!)
        struct cyclades_idle_stats
        struct video_event
                VIDEO_GET_EVENT
        struct msqid_ds
        struct ppp_idle
                PPPIOCGIDLE
        struct semid_ds
                union semun
        struct timespec
                SIOCGSTAMPNS
                struct coda_vattr
                        ...
                struct scm_timestamping
                struct som_hdr
                struct itimerspec
                struct v4l2_event
                        VIDIOC_DQEVENT
                struct snd_pcm_status
                        SNDRV_PCM_IOCTL_STATUS
                struct snd_pcm_mmap_status
                        struct snd_pcm_sync_ptr
                                SNDRV_PCM_IOCTL_SYNC_PTR
                struct snd_rawmidi_status
                        SNDRV_RAWMIDI_IOCTL_STATUS
                struct snd_timer_status
                        SNDRV_TIMER_IOCTL_STATUS
                struct snd_timer_tread
                struct snd_ctl_elem_value
                        SNDRV_CTL_IOCTL_ELEM_READ
                        SNDRV_CTL_IOCTL_ELEM_WRITE
        struct timeval
                SIOCGSTAMP
                struct zatm_t_hist
                struct bcm_msg_head
                struct elf_prstatus
                struct input_event
                struct omap3isp_stat_data
                        VIDIOC_OMAP3ISP_STAT_REQ
                PPGETTIME
                PPSETTIME
                struct rusage
                struct itimerval
                struct timex
                struct v4l2_buffer
                        VIDIOC_QUERYBUF
                        VIDIOC_QBUF
                        VIDIOC_DQBUF
                        VIDIOC_PREPARE_BUF
        struct utimbuf

Syscalls affected by the above types:

        sys_time
        sys_stime
        sys_nanosleep
        sys_clock_settime
        sys_clock_gettime
        sys_clock_getres
        sys_clock_nanosleep
        sys_sched_rr_get_interval
        sys_futex
        sys_rt_sigtimedwait
        sys_io_getevents
        sys_recvmmsg
        sys_semtimedop
        sys_mq_timedsend
        sys_mq_timedreceive
        sys_utimensat
        sys_pselect6
        sys_ppoll
        sys_gettimeofday
        sys_settimeofday
        sys_utimes
        sys_select
        sys_futimesat
        sys_utime
        sys_timer_gettime
        sys_timer_settime
        sys_timerfd_settime
        sys_timerfd_gettime
        sys_wait4
        sys_waitid
        sys_getrusage
        sys_getitimer
        sys_setitimer
        sys_adjtimex
        sys_clock_adjtime

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.

Tasks

The task list is for people that want to get involved, there will be many more tasks over time, so this is just a starting point. In the end, we should remove all instances of 'time_t', 'timespec' and 'timeval' from the kernel.

Small tasks

• Find a driver using time_t/timespec/timeval internally and convert it to ktime_t/timespec64, examples:
• * security/keys/gc.c (time_t)
• * kernel/cpuset.c (time_t)
• * fs/reiserfs/journal.c (time_t)
• * drivers/scsi/ips.c (time_t)
• * sound/pci/es1968.c (timeval)
• * kernel/power/hibernate.c (timeval)
• * drivers/s390/net/ctcm_fsms.c (timespec)
• * drivers/power/ab8500_fg.c (timespec)

Medium tasks

• Modify an ioctl interface in a driver to support both 32- and 64-bit time interfaces, examples:
• * include/uapi/linux/atm_zatm.h (zatm_t_hist/timeval)
• * include/uapi/linux/videodev2.h (v4l2_buffer/timespec)
• Convert the internal timekeeping in fs/nfsd
• Convert all 'ptp' users in the kernel

• Introduce known unsafe types (possibly like kernel_time32_t, kernel_compat_time_t etc) so we can annotate interfaces that are known to use a fixed size and cannot be changed to new types.

Advanced tasks

• Introduce a new system call family to replace one or more of the problematic calls listed above.
• Change the on-disk layout of a broken file system to optionally support longer time stamps
• Port a small C library (uClibc, newlib, ...) to optionally use 64-bit time_t and build an embedded distribution (openembedded, openwrt, buildroot, ...) with this.

Tasks later in the project

• Hook up all 32-bit architectures to use the new system calls
• Introduce a Kconfig symbol to disable all code that has not yet been converted at compile time.