On 08/16, Andrew Morton wrote:
>
> From: Lennart Poettering <lennart@xxxxxxxxxxxxxx>
>
> Userspace service managers/supervisors need to track their started
> services. Many services daemonize by double-forking and get implicitely
> re-parented to PID 1. The process manager will no longer be able to
> receive the SIGCHLD signals for them.
>
> With this prctl, a service manager can mark itself as a sort of 'sub-init'
> process, able to stay as the parent process for all processes created by
> the started services. All SIGCHLD signals will be delivered to the
> service manager.
I try to never argue with the new features. But to be honest, this
doesn't look very good to me.
OK, a service manager M does prctl(PR_SET_CHILD_REAPER), then it forks
a service X which forks another child C and exits. Then C exits and
notifies M.
But. How can M know that the service X should be restarted? It only
knows the pid. What if wait(WEXITED) succeeds because C in turn does
fork + exit? What M has 2 or more services?
Anyway, the implementation is certainly buggy.
> @@ -1296,6 +1296,8 @@ struct task_struct {
> * execve */
> unsigned in_iowait:1;
>
> + /* Reparent child processes to this process instead of pid 1. */
> + unsigned child_reaper:1;
First of all - this is already very wrong imho. This should be
per-process, not per-thread.
> + /* find the first ancestor which is marked as child_reaper */
> + for (reaper = father->parent;
> + reaper != &init_task && reaper != pid_ns->child_reaper;
> + reaper = reaper->parent)
This loop can never reach init_task/child_reaper and crash the kernel.
For example, father->parent can point to init_task's sub-thread.
OTOH you shouldn't use init_task at all.
Also. You shouldn't do this if the sub-namespace init exits, this is
wrong.
> + if (reaper->child_reaper)
> + return reaper;
No, we can't blindly return this task, it can be dead/exiting. More
precisely, we must not do this if it has already passed
forget_original_parent(). That is why the code above checks PF_EXITING.
Oleg.
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
