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>

2 files changed, 15 insertions, 0 deletions

diff --git a/include/linux/prctl.h b/include/linux/prctl.h
index a0413ac3abe8..e0cfec2490aa 100644
--- a/include/linux/prctl.h
+++ b/include/linux/prctl.h
@@ -121,4 +121,7 @@
 #define PR_SET_PTRACER 0x59616d61
 # define PR_SET_PTRACER_ANY ((unsigned long)-1)
 
+#define PR_SET_CHILD_SUBREAPER 36
+#define PR_GET_CHILD_SUBREAPER 37
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 0c147a4260a5..0c3854b0d4b1 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -553,6 +553,18 @@ struct signal_struct {
         int                     group_stop_count;
         unsigned int            flags; /* see SIGNAL_* flags below */
 
+        /*
+         * PR_SET_CHILD_SUBREAPER marks a process, like a service
+         * manager, to re-parent orphan (double-forking) child processes
+         * to this process instead of 'init'. The service manager is
+         * able to receive SIGCHLD signals and is able to investigate
+         * the process until it calls wait(). All children of this
+         * process will inherit a flag if they should look for a
+         * child_subreaper process at exit.
+         */
+        unsigned int            is_child_subreaper:1;
+        unsigned int            has_child_subreaper:1;
+
         /* POSIX.1b Interval Timers */
         struct list_head posix_timers;