Everything about the timesyncd service and binary seemed to be correct, but it was hitting a restart limit, and I was seeing the following error in the journal:

systemd-timesyncd.service failed to run 'start' task: No such file or directory


That usually indicates that there's something wrong with the ExecStart program, but it existed, and worked fine.

Shawn Landden (scientes) suggested I strace PID 1 to see what systemd thought was going on.

It turns out that because systemd fails to create a directory under /var/tmp, it thinks that timesyncd itself is broken. If I create /var/tmp, timesyncd is A-OK!

I don't know if the absence of /var/tmp is a problem with my science experiment of a distro (it's OpenWrt + systemd), but systemd should definitely not blame timesyncd for the failure to create a directory under /var/tmp, and... is it necessary anyway?

I create a symlink from /var/tmp to /tmp, and now that timesyncd is running, here's the contents of /var/tmp:

# find /var/tmp/
/var/tmp/
/var/tmp/systemd-private-496647d450244423afd7faaa2ac6420c-systemd-timesyncd.service-YJDsMU
/var/tmp/systemd-private-496647d450244423afd7faaa2ac6420c-systemd-timesyncd.service-YJDsMU/tmp
/var/tmp/systemd-private-496647d450244423afd7faaa2ac6420c-systemd-timesyncd.service-2vQ4v0
/var/tmp/systemd-private-496647d450244423afd7faaa2ac6420c-systemd-timesyncd.service-2vQ4v0/tmp


And here's the relevant PID 1 strace snippet showing the /var/tmp failure:

mkdir("/var/tmp/systemd-private-496647d450244423afd7faaa2ac6420c-systemd-timesyncd.service-OUeCeA", 0700) = -1 ENOENT (No such file or directory)
umask(0)                                = 077
rmdir("/tmp/systemd-private-496647d450244423afd7faaa2ac6420c-systemd-timesyncd.service-G5yGdf/tmp") = 0
rmdir("/tmp/systemd-private-496647d450244423afd7faaa2ac6420c-systemd-timesyncd.service-G5yGdf") = 0
sendmsg(28, {msg_name(0)=NULL, msg_iov(4)=[{"PRIORITY=4\nSYSLOG_FACILITY=3\nCOD"..., 172}, {"MESSAGE=", 8}, {"systemd-timesyncd.service failed"..., 79}, {"\n", 1}], msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL) = 260