Differences between revisions 3 and 4

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.

file system	time type	expiration year
9p	u32 seconds	2106
adfs	40-bit cs since 1900	2248
affs	u32 days/mins/(secs/50)	11760870
afs	s32 seconds (bug)	2038
autofs4	internal
befs	48-bit seconds	never
bfs	u32 seconds	2106
btrfs	64-bit seconds, 32-bit ns	never
cachefiles	internal
ceph	unsigned 32-bit second/ns	2106
cifs (smb)	7-bit years since 1980	2107
cifs (modern)	64-bit 100ns since 1601	30328
coda	timespec ioctl	2038
configfs	internal
cramfs	fixed	1970
debugfs	internal
devpts	internal
dlm	internal
ecryptfs	internal
efivarfs	internal
efs	u32 seconds	2106
exofs	s32 seconds	2038
exportfs	internal
ext2	u32 seconds	2106
ext3	u32 seconds	2106
ext4	34 bit seconds / 30-bit ns	2514
f2fs	64-bit seconds / 32-bit ns	never
fat	7-bit years since 1980, 2s resolution	2107
freevxfs	u32 seconds/u32 microseconds	2106
fscache	internal
fuse	64-bit second/32-bit ns	never
gfs2	u64 seconds/u32 ns	never
hfs	u32 seconds since 1904	2040
hfsplus	u32 seconds since 1904	2040
hostfs	timespec	2038
hpfs	u32 seconds	2106
hppfs	internal
hugetlbfs	internal
isofs	'char' year since 1900	2028 (!)
jbd	internal
jbd2	internal
jffs2	u32 seconds	2106
jfs	u32 seconds/ns	2106
kernfs	internal
lockd	internal
logfs	u64 ns	2554
minix	u32 seconds	2106
ncpfs	7-bit year since 1980	2107
nfsv2,v3	u32 seconds/ns	2106
nfsv4	u64 seconds/u32 ns	never
nfs_common	internal
nfsd	u32 seconds/ns	2106
nilfs2	u64 seconds/u32 ns	never
nls	internal
notify	internal
ntfs	64-bit 100ns since 1601	30828
ocfs2	34-bit seconds/30-bit ns	2514
omfs	64-bit milliseconds	never
openpromfs	internal
proc	internal
pstore	internal
qnx4	u32 seconds	2106
qnx6	u32 seconds	2106
quota	internal
ramfs	internal
reiserfs	u32 seconds	2106
romfs	fixed	1970
squashfs	u32 seconds	2106
sysfs	internal
sysv	u32 seconds	2106
ubifs	u64 second/u32 ns	never
udf	u16 year	2038
ufs1 ufs2	u32 seconds u64 seconds/u32 nanoseconds	2106 never
xfs	u32 seconds/ns	2106

-  ⇤ ← Revision 3 as of 2014-05-28 14:25:19 → 
  Size: 5160
  Editor: arnd
  Comment: add more file system info
+   ← Revision 4 as of 2014-05-28 14:55:21 → ⇥
  Size: 5531
  Editor: arnd
  Comment: fill out the remaining file systems
-Deletions are marked like this.
+Additions are marked like this.
 Line 2:
-Line 6:
+Line 5:
-Line 10:
+Line 8:
+==== 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.
 Line 11:
-==== 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
+ * nanosleep
 * select/pselect/poll/ppoll/epoll
 * getrusage
 * sched_rr_get_interval
 * sigtimedwait
 * alarm
-Line 24:
+Line 19:
-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.
+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.
-Line 29:
+Line 21:
-   * 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
+ * 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
-Line 39:
+Line 31:
-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.
+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.
-Line 45:
+Line 33:
-   * stat/lstat/fstatat/
   * utime/utimes/futimesat
+ * stat/lstat/fstatat/
 * utime/utimes/futimesat
-Line 49:
+Line 37:
-Line 52:
+Line 39:
-   * audio time stamps
   * v4l time stamps
   * input event time stamps
   * socket time stamps
   * ...
+ * audio time stamps
 * v4l time stamps
 * input event time stamps
 * socket time stamps
 * ...
-Line 59:
+Line 46:
-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.
+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.
-Line 66:
+Line 49:
-Each file system stores its file modification times in its own format on disk,
and a lot of them have the same problem.
+Each file system stores its file modification times in its own format on disk, and a lot of them have the same problem.
-Line 74:
+Line 55:
-|| afs || || ||
|| autofs4 || internal || ||
|| befs || || ||
|| bfs || || ||
|| btrfs || 64-bit second, 32-bit ns || never ||
|| cachefiles || internal || ||
|| ceph || unsigned 32-bit second/ns || 2106||
+|| afs ||  s32 seconds (bug)||2038||
|| autofs4 || internal ||  ||
|| befs ||  48-bit seconds||  never||
|| bfs ||  u32 seconds||  2106||
|| btrfs || 64-bit seconds, 32-bit ns || never ||
|| cachefiles || internal ||  ||
|| ceph || unsigned 32-bit second/ns || 2106 ||
-Line 84:
+Line 65:
-|| configfs || internal || ||
+|| configfs || internal ||  ||
-Line 86:
+Line 67:
-|| debugfs || internal || ||
|| devpts || internal || ||
|| dlm || internal || ||
|| ecryptfs || internal || ||
|| efivarfs || internal || ||
+|| debugfs || internal ||  ||
|| devpts || internal ||  ||
|| dlm || internal ||  ||
|| ecryptfs || internal ||  ||
|| efivarfs || internal ||  ||
-Line 93:
+Line 74:
-|| exportfs || internal || ||
+|| exportfs || internal ||  ||
-Line 96:
+Line 77:
-|| ext4 || 34 bit seconds / 30-bit ns || 2514||
+|| ext4 || 34 bit seconds / 30-bit ns || 2514 ||
-Line 99:
+Line 80:
-|| freevxfs || || ||
|| fscache || internal || ||
+|| freevxfs ||  u32 seconds/u32 microseconds||  2106||
|| fscache || internal ||  ||
-Line 102:
+Line 83:
-|| gfs2 || || ||
+|| gfs2 ||  u64 seconds/u32 ns||  never||
-Line 105:
+Line 86:
-|| hostfs || || ||
|| hpfs || || ||
|| hppfs || || ||
|| hugetlbfs || internal || ||
|| isofs || || ||
|| jbd || internal || ||
|| jbd2 || internal || ||
+|| hostfs ||  timespec||  2038||
|| hpfs ||  u32 seconds||  2106||
|| hppfs ||  internal||  ||
|| hugetlbfs || internal ||  ||
|| isofs || 'char' year since 1900||  2028 (!)||
|| jbd || internal ||  ||
|| jbd2 || internal ||  ||
-Line 113:
+Line 94:
-|| jfs || || ||
|| kernfs || internal || ||
|| lockd || internal || ||
+|| jfs ||  u32 seconds/ns||  2106||
|| kernfs || internal ||  ||
|| lockd || internal ||  ||
-Line 117:
+Line 98:
-|| minix || || ||
|| ncpfs || || ||
+|| minix ||  u32 seconds||  2106||
|| ncpfs ||  7-bit year since 1980||  2107||
-Line 120:
+Line 101:
-|| nfsv4 || u64 seconds/u32 ns|| never ||
|| nfs_common || internal || ||
+|| nfsv4 || u64 seconds/u32 ns || never ||
|| nfs_common || internal ||  ||
-Line 123:
+Line 104:
-|| nilfs2 || || ||
|| nls || internal || ||
|| notify || internal || ||
+|| nilfs2 ||  u64 seconds/u32 ns||  never||
|| nls || internal ||  ||
|| notify || internal ||  ||
-Line 128:
+Line 109:
-|| omfs || || ||
|| openpromfs || internal || ||
|| proc || internal || ||
|| pstore || internal || ||
|| qnx4 || || ||
|| qnx6 || || ||
|| quota || internal || ||
|| ramfs || internal || ||
+|| omfs ||  64-bit milliseconds||  never||
|| openpromfs || internal ||  ||
|| proc || internal ||  ||
|| pstore || internal ||  ||
|| qnx4 ||  u32 seconds||  2106||
|| qnx6 ||  u32 seconds||  2106||
|| quota || internal ||  ||
|| ramfs || internal ||  ||
-Line 139:
+Line 120:
-|| sysfs || internal || ||
|| sysv || || ||
+|| sysfs || internal ||  ||
|| sysv ||  u32 seconds||  2106||
-Line 143:
+Line 124:
-|| ufs || || ||
+|| ufs1 ufs2 ||u32 seconds u64 seconds/u32 nanoseconds ||2106 never ||