Differences between revisions 8 and 9

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.

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 (9P2000)	unsigned 32-bit seconds	2106
9p (9P2000.L)	signed 64-bit seconds, ns	never
adfs	40-bit cs since 1900	2248
affs	u32 days/mins/(secs/50)	11760870
afs	signed 32-bit seconds	2038
befs	unsigned 48-bit seconds	never
bfs	unsigned 32-bit seconds	2106
btrfs	signed 64-bit seconds, 32-bit ns	never
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
cramfs	fixed	1970
efs	unsigned 32-bit seconds	2106
exofs	signed 32-bit seconds	2038
ext2	signed 32-bit seconds	2038
ext3	signed 32-bit seconds	2038
ext4 (good old inodes)	signed 32-bit seconds	2038
ext4 (new inodes	34 bit seconds / 30-bit ns (but broken)	2038
f2fs	64-bit seconds / 32-bit ns	never
fat	7-bit years since 1980, 2s resolution	2107
freevxfs	unsigned 32-bit seconds/u32 microseconds	2106
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	unsigned 32-bit seconds	2106
isofs	'char' year since 1900 (fixable)	2028
jffs2	unsigned 32-bit seconds	2106
jfs	unsigned 32-bit seconds/ns	2106
logfs	signed 64-bit ns	2262
minix	unsigned 32-bit seconds	2106
ncpfs	7-bit year since 1980	2107
nfsv2,v3	unsigned 32-bit seconds/ns	2106
nfsv4	u64 seconds/u32 ns	never
nfsd	unsigned 32-bit seconds/ns	2106
nilfs2	u64 seconds/u32 ns	never
ntfs	64-bit 100ns since 1601	30828
ocfs2	34-bit seconds/30-bit ns	2514
omfs	64-bit milliseconds	never
pstore	ascii seconds	2106
qnx4	unsigned 32-bit seconds	2106
qnx6	unsigned 32-bit seconds	2106
reiserfs	unsigned 32-bit seconds	2106
romfs	fixed	1970
squashfs	unsigned 32-bit seconds	2106
sysv	unsigned 32-bit seconds	2106
ubifs	u64 second/u32 ns	never
udf	u16 year	2038
ufs1	unsigned 32-bit seconds	2106
ufs2	signed 64-bit seconds/u32 ns	never
xfs	signed 32-bit seconds/ns	2106

-  ⇤ ← Revision 8 as of 2014-05-31 06:22:13 → 
  Size: 5216
  Editor: MichaelKerrisk
  Comment:
+   ← Revision 9 as of 2014-06-02 14:19:10 → ⇥
  Size: 5644
  Editor: arnd
  Comment: some file systems are actually 'signed' when I first marked them as 'unsigned'
-Deletions are marked like this.
+Additions are marked like this.
 Line 58:
-|| 9p || u32 seconds || 2106 ||
+|| 9p (9P2000) || unsigned 32-bit seconds || 2106 ||
|| 9p (9P2000.L) || signed 64-bit seconds, ns || never ||
-Line 61:
+Line 62:
-|| afs ||  s32 seconds (bug) ||2038 ||
|| befs ||  48-bit seconds ||  never ||
|| bfs ||  u32 seconds ||  2106 ||
|| btrfs || 64-bit seconds, 32-bit ns || never ||
+|| afs ||  signed 32-bit seconds  ||2038 ||
|| befs || unsigned 48-bit seconds ||  never ||
|| bfs ||  unsigned 32-bit seconds ||  2106 ||
|| btrfs || signed 64-bit seconds, 32-bit ns || never ||
-Line 70:
+Line 71:
-|| efs || u32 seconds || 2106 ||
|| exofs || s32 seconds || 2038 ||
|| ext2 || u32 seconds || 2106 ||
|| ext3 || u32 seconds || 2106 ||
|| ext4 || 34 bit seconds / 30-bit ns || 2514 ||
+|| efs || unsigned 32-bit seconds || 2106 ||
|| exofs || signed 32-bit seconds || 2038 ||
|| ext2 || signed 32-bit seconds || 2038 ||
|| ext3 || signed 32-bit seconds || 2038 ||
|| ext4 (good old inodes) || signed 32-bit seconds || 2038 ||
|| ext4 (new inodes || 34 bit seconds / 30-bit ns (but broken) || 2038 ||
-Line 77:
+Line 79:
-|| freevxfs ||  u32 seconds/u32 microseconds ||  2106 ||
+|| freevxfs ||  unsigned 32-bit seconds/u32 microseconds ||  2106 ||
-Line 83:
+Line 85:
-|| hpfs ||  u32 seconds ||  2106 ||
+|| hpfs ||  unsigned 32-bit seconds ||  2106 ||
-Line 85:
+Line 87:
-|| jffs2 || u32 seconds || 2106 ||
|| jfs ||  u32 seconds/ns ||  2106 ||
|| logfs || u64 ns || 2554 ||
|| minix ||  u32 seconds ||  2106 ||
+|| jffs2 || unsigned 32-bit seconds || 2106 ||
|| jfs ||  unsigned 32-bit seconds/ns ||  2106 ||
|| logfs || signed 64-bit ns || 2262 ||
|| minix ||  unsigned 32-bit seconds ||  2106 ||
-Line 90:
+Line 92:
-|| nfsv2,v3 || u32 seconds/ns || 2106 ||
+|| nfsv2,v3 || unsigned 32-bit seconds/ns || 2106 ||
-Line 92:
+Line 94:
-|| nfsd || u32 seconds/ns || 2106 ||
+|| nfsd || unsigned 32-bit seconds/ns || 2106 ||
-Line 98:
+Line 100:
-|| qnx4 ||  u32 seconds ||  2106 ||
|| qnx6 ||  u32 seconds ||  2106 ||
|| reiserfs || u32 seconds || 2106 ||
+|| qnx4 ||  unsigned 32-bit seconds ||  2106 ||
|| qnx6 ||  unsigned 32-bit seconds ||  2106 ||
|| reiserfs || unsigned 32-bit seconds || 2106 ||
-Line 102:
+Line 104:
-|| squashfs || u32 seconds || 2106 ||
|| sysv ||  u32 seconds ||  2106 ||
+|| squashfs || unsigned 32-bit seconds || 2106 ||
|| sysv ||  unsigned 32-bit seconds ||  2106 ||
-Line 106:
+Line 108:
-|| ufs1 || u32 seconds || 2106 ||
|| ufs2 || u64 seconds/u32 ns ||never ||
|| xfs || u32 seconds/ns || 2106 ||
+|| ufs1 || unsigned 32-bit seconds || 2106 ||
|| ufs2 || signed 64-bit seconds/u32 ns ||never ||
|| xfs || signed 32-bit seconds/ns || 2106 ||