connector: add comm change event report to proc connector

Add an event to monitor comm value changes of tasks.  Such an event
becomes vital, if someone desires to control threads of a process in
different manner.

A natural characteristic of threads is its comm value, and helpfully
application developers have an opportunity to change it in runtime.
Reporting about such events via proc connector allows to fine-grain
monitoring and control potentials, for instance a process control daemon
listening to proc connector and following comm value policies can place
specific threads to assigned cgroup partitions.

It might be possible to achieve a pale partial one-shot likeness without
this update, if an application changes comm value of a thread generator
task beforehand, then a new thread is cloned, and after that proc
connector listener gets the fork event and reads new thread's comm value
from procfs stat file, but this change visibly simplifies and extends the
matter.

Signed-off-by: Vladimir Zapolskiy <vzapolskiy@gmail.com>
Acked-by: Evgeniy Polyakov <zbr@ioremap.net>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/sys.c

@@ -1759,6 +1759,7 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 					      sizeof(me->comm) - 1) < 0)
 				return -EFAULT;
 			set_task_comm(me, comm);
+			proc_comm_connector(me);
 			return 0;
 		case PR_GET_NAME:
 			get_task_comm(comm, me);