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Commit ebec18a6d3aa1e7d84aab16225e87fd25170ec2b

Authored by Lennart Poettering
Committed by Linus Torvalds
1 parent 953326cb60
Exists in smarc-l5.0.0_1.0.0-ga and in 5 other branches imx_3.10.17_1.0.1_ga, imx_3.10.53_1.1.0_ga, imx_3.14.28_1.0.0_ga, smarc-imx_3.10.53_1.1.0_ga, smarc-imx_3.14.28_1.0.0_ga

prctl: add PR_{SET,GET}_CHILD_SUBREAPER to allow simple process supervision 

Userspace service managers/supervisors need to track their started
services.  Many services daemonize by double-forking and get implicitly
re-parented to PID 1.  The service manager will no longer be able to
receive the SIGCHLD signals for them, and is no longer in charge of
reaping the children with wait().  All information about the children is
lost at the moment PID 1 cleans up the re-parented processes.

With this prctl, a service manager process can mark itself as a sort of
'sub-init', able to stay as the parent for all orphaned processes
created by the started services.  All SIGCHLD signals will be delivered
to the service manager.

Receiving SIGCHLD and doing wait() is in cases of a service-manager much
preferred over any possible asynchronous notification about specific
PIDs, because the service manager has full access to the child process
data in /proc and the PID can not be re-used until the wait(), the
service-manager itself is in charge of, has happened.

As a side effect, the relevant parent PID information does not get lost
by a double-fork, which results in a more elaborate process tree and
'ps' output:

before:
  # ps afx
  253 ?        Ss     0:00 /bin/dbus-daemon --system --nofork
  294 ?        Sl     0:00 /usr/libexec/polkit-1/polkitd
  328 ?        S      0:00 /usr/sbin/modem-manager
  608 ?        Sl     0:00 /usr/libexec/colord
  658 ?        Sl     0:00 /usr/libexec/upowerd
  819 ?        Sl     0:00 /usr/libexec/imsettings-daemon
  916 ?        Sl     0:00 /usr/libexec/udisks-daemon
  917 ?        S      0:00  \_ udisks-daemon: not polling any devices

after:
  # ps afx
  294 ?        Ss     0:00 /bin/dbus-daemon --system --nofork
  426 ?        Sl     0:00  \_ /usr/libexec/polkit-1/polkitd
  449 ?        S      0:00  \_ /usr/sbin/modem-manager
  635 ?        Sl     0:00  \_ /usr/libexec/colord
  705 ?        Sl     0:00  \_ /usr/libexec/upowerd
  959 ?        Sl     0:00  \_ /usr/libexec/udisks-daemon
  960 ?        S      0:00  |   \_ udisks-daemon: not polling any devices
  977 ?        Sl     0:00  \_ /usr/libexec/packagekitd

This prctl is orthogonal to PID namespaces.  PID namespaces are isolated
from each other, while a service management process usually requires the
services to live in the same namespace, to be able to talk to each
other.

Users of this will be the systemd per-user instance, which provides
init-like functionality for the user's login session and D-Bus, which
activates bus services on-demand.  Both need init-like capabilities to
be able to properly keep track of the services they start.

Many thanks to Oleg for several rounds of review and insights.

[akpm@linux-foundation.org: fix comment layout and spelling]
[akpm@linux-foundation.org: add lengthy code comment from Oleg]
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Acked-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Showing 5 changed files with 54 additions and 5 deletions Side-by-side Diff

include/linux/prctl.h
Diff comments   View file @ ebec18a
... ... @@ -121,5 +121,8 @@
121 121 #define PR_SET_PTRACER 0x59616d61
122 122 # define PR_SET_PTRACER_ANY ((unsigned long)-1)
123 123  
  124 +#define PR_SET_CHILD_SUBREAPER 36
  125 +#define PR_GET_CHILD_SUBREAPER 37
  126 +
124 127 #endif /* _LINUX_PRCTL_H */
include/linux/sched.h
Diff comments   View file @ ebec18a
... ... @@ -553,6 +553,18 @@
553 553 int group_stop_count;
554 554 unsigned int flags; /* see SIGNAL_* flags below */
555 555  
  556 + /*
  557 + * PR_SET_CHILD_SUBREAPER marks a process, like a service
  558 + * manager, to re-parent orphan (double-forking) child processes
  559 + * to this process instead of 'init'. The service manager is
  560 + * able to receive SIGCHLD signals and is able to investigate
  561 + * the process until it calls wait(). All children of this
  562 + * process will inherit a flag if they should look for a
  563 + * child_subreaper process at exit.
  564 + */
  565 + unsigned int is_child_subreaper:1;
  566 + unsigned int has_child_subreaper:1;
  567 +
556 568 /* POSIX.1b Interval Timers */
557 569 struct list_head posix_timers;
558 570  
... ... @@ -687,11 +687,11 @@
687 687 }
688 688  
689 689 /*
690   - * When we die, we re-parent all our children.
691   - * Try to give them to another thread in our thread
692   - * group, and if no such member exists, give it to
693   - * the child reaper process (ie "init") in our pid
694   - * space.
  690 + * When we die, we re-parent all our children, and try to:
  691 + * 1. give them to another thread in our thread group, if such a member exists
  692 + * 2. give it to the first ancestor process which prctl'd itself as a
  693 + * child_subreaper for its children (like a service manager)
  694 + * 3. give it to the init process (PID 1) in our pid namespace
695 695 */
696 696 static struct task_struct *find_new_reaper(struct task_struct *father)
697 697 __releases(&tasklist_lock)
... ... @@ -722,6 +722,29 @@
722 722 * forget_original_parent() must move them somewhere.
723 723 */
724 724 pid_ns->child_reaper = init_pid_ns.child_reaper;
  725 + } else if (father->signal->has_child_subreaper) {
  726 + struct task_struct *reaper;
  727 +
  728 + /*
  729 + * Find the first ancestor marked as child_subreaper.
  730 + * Note that the code below checks same_thread_group(reaper,
  731 + * pid_ns->child_reaper). This is what we need to DTRT in a
  732 + * PID namespace. However we still need the check above, see
  733 + * http://marc.info/?l=linux-kernel&m=131385460420380
  734 + */
  735 + for (reaper = father->real_parent;
  736 + reaper != &init_task;
  737 + reaper = reaper->real_parent) {
  738 + if (same_thread_group(reaper, pid_ns->child_reaper))
  739 + break;
  740 + if (!reaper->signal->is_child_subreaper)
  741 + continue;
  742 + thread = reaper;
  743 + do {
  744 + if (!(thread->flags & PF_EXITING))
  745 + return reaper;
  746 + } while_each_thread(reaper, thread);
  747 + }
725 748 }
726 749  
727 750 return pid_ns->child_reaper;
... ... @@ -1051,6 +1051,9 @@
1051 1051 sig->oom_score_adj = current->signal->oom_score_adj;
1052 1052 sig->oom_score_adj_min = current->signal->oom_score_adj_min;
1053 1053  
  1054 + sig->has_child_subreaper = current->signal->has_child_subreaper ||
  1055 + current->signal->is_child_subreaper;
  1056 +
1054 1057 mutex_init(&sig->cred_guard_mutex);
1055 1058  
1056 1059 return 0;
... ... @@ -1962,6 +1962,14 @@
1962 1962 case PR_SET_MM:
1963 1963 error = prctl_set_mm(arg2, arg3, arg4, arg5);
1964 1964 break;
  1965 + case PR_SET_CHILD_SUBREAPER:
  1966 + me->signal->is_child_subreaper = !!arg2;
  1967 + error = 0;
  1968 + break;
  1969 + case PR_GET_CHILD_SUBREAPER:
  1970 + error = put_user(me->signal->is_child_subreaper,
  1971 + (int __user *) arg2);
  1972 + break;
1965 1973 default:
1966 1974 error = -EINVAL;
1967 1975 break;