Messages in this thread | | | Date | Thu, 4 Mar 2010 15:08:22 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH] exit: PR_SET_ANCHOR for marking processes as reapers for child processes |
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On 02/02, Lennart Poettering wrote: > > This patch adds a simple flag for each process that marks it as an > "anchor" process for all its children and grandchildren. If a child of > such an anchor dies all its children will not be reparented to init, but > instead to this anchor, escaping this anchor process is not possible. A > task with this flag set hence acts is little "sub-init".
Lennart, this patch adds a noticeable linux-only feature. I see your point, but imho your idea needs the "strong" acks. I cc'ed some heavyweights, if someone dislikes your idea he can nack it right now.
Security. This is beyond my understanding, hopefully the cc'ed experts can help.
Should we clear ->child_anchor flags when the "sub-init" execs? Or, at least, when the task changes its credentials? Probably not, but dunno.
The more problematic case is when the descendant of the "sub-init" execs the setuid application. Should we allow the reparenting to !/sbin/init task in this case?
Should we clear ->pdeath_signal after reparenting to sub-init ?
Do we need the new security_operations->task_reparent() method ? Or, perhaps we can reuse ->task_wait() if we add the "parent" argument?
Something else we should think about?
As for the patch itself,
> static struct task_struct *find_new_reaper(struct task_struct *father) > { > struct pid_namespace *pid_ns = task_active_pid_ns(father); > - struct task_struct *thread; > + struct task_struct *thread, *anchor; > > thread = father; > while_each_thread(father, thread) { > @@ -715,6 +715,11 @@ static struct task_struct *find_new_reaper(struct task_struct *father) > return thread; > } > > + /* find the first ancestor which is marked child_anchor */ > + for (anchor = father->parent; anchor != &init_task; anchor = anchor->parent) > + if (anchor->child_anchor) > + return anchor; > + > if (unlikely(pid_ns->child_reaper == father)) { > write_unlock_irq(&tasklist_lock); > if (unlikely(pid_ns == &init_pid_ns))
This is not exactly right:
- We can race with the exiting anchor. IOW, we must not reparent to anchor if it has already passed exit_notify(). You can check PF_EXITING flag like while_each_thread() above does.
- "anchor != &init_task" is not correct, the task must not escape its container. We should stop checking the ->parent list when we hit ->child_reaper, not init_task
- if a sub-namespace init dies, we shouldn't skip zap_pid_ns_processes() logic, move the "for" loop below. This also closes another possible race, the anchor can be already dead when we take tasklist again.
> @@ -1578,6 +1578,13 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, > else > error = PR_MCE_KILL_DEFAULT; > break; > + case PR_SET_ANCHOR: > + me->child_anchor = !!arg2; > + error = 0; > + break;
It is a bit strange that PR_SET_ANCHOR acts per-thread, not per process.
Suppose that a task A does prtcl(PR_SET_ANCHOR) and marks itself as a local child reaper. Then its sub-thread B forks() the process C which also forks the child X. When C dies, X will be re-parented to init. Is this what we really want?
To me, it looks more natural if PR_SET_ANCHOR marks the whole process as a local reaper, not only the thread which called PR_SET_ANCHOR.
Oleg.
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