DAEMON REPARENTED / INIT --USER
来源:互联网 发布:用户行为数据建模 编辑:程序博客网 时间:2024/05/21 10:55
from: https://www.osso.nl/blog/daemon-reparented-init-user/
While I was battling an obscure Ubuntu shutdown issue — more about that later — I noticed that daemonized jobs started from my X session were not reparented to PID 1 init
, but to a custom init --user
, owned by me.
What? I cannot start daemon that outlive my X session?
That's right, I cannot. Check this out:
$ sh -c 'sleep 61 &'$ ps faxu | egrep 'init|sleep 61'root 1 ... /sbin/initwalter 2198 ... \_ init --userwalter 6673 ... | | \_ egrep --color=auto init|sleep 61walter 6671 ... \_ sleep 61
Okay then. What is this black magic?
It's apparently caused by PR_SET_CHILD_SUBREAPER
; available through prctl
since Linux kernel 3.4. Ubuntu added that in Raring (13.04), according to Raring Upstart User Sessions, PID tracking.
Can I work around that?
Short answer: no, the PR_SET_CHILD_SUBREAPER
interface allows a single process to enable or disable the feature, but not for someone else to disable it.
Long answer: yes, but only if we alter the subreaper state of the User Session init; like this:
$ sudo gdb `which init` `pgrep -xf 'init --user'` \ -batch -ex 'call prctl(36,0,0,0,0)'Password: [Thread debugging using libthread_db enabled]Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".0x00007f3b7a6848c3 in __select_nocancel () at ../sysdeps/unix/syscall-template.S:8181 ../sysdeps/unix/syscall-template.S: No such file or directory.$1 = 0$ sh -c 'sleep 61 &'$ ps faxu | egrep 'init|sleep 61'root 1 ... /sbin/initwalter 2198 ... \_ init --userwalter 6986 ... | | \_ egrep --color=auto init|sleep 61walter 6957 ... | \_ man 5 initwalter 6984 ... sleep 61
Hah! sleep 61
is now owned by PID 1 directly. By the way, reverting that hack is as easy as changing the second argument to prctl from 0 to 1.
So, apparently I really am barred from creating PID 1 owned daemons unless I hack init --user.
That does raise the question how initctl daemons are spawned, but that's done by asking /sbin/init
to do that for us:
# netstat -lnAunix | grep '/com/ubuntu/upstart$'unix 2 [ ACC ] STREAM LISTENING 8049 @/com/ubuntu/upstart# strace start cups...connect(3, {sa_family=AF_LOCAL, sun_path=@"/com/ubuntu/upstart"}, 22) = 0...cups start/running, process 6883Yuck! Did I mention I'm glad we're moving to
systemd
?- daemon reparented / init --user
- DAEMON REPARENTED / INIT --USER
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