linux-kernel - Re: free_pid() && PIDNS_HASH

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Date:	Sun, 08 Sep 2013 14:25:50 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: free_pid() && PIDNS_HASH_ADDING

Oleg Nesterov <oleg@...hat.com> writes:

> On 09/08, Oleg Nesterov wrote:
>>
>> Off topic. What if the first alloc_pid() succeeds and then later
>> copy_process() fails. In this case free_pid() is called but
>> PIDNS_HASH_ADDING was not cleared, we miss kern_unmount(), no?
>
> Perhaps something like below?

I am thinking more:

diff --git a/kernel/pid.c b/kernel/pid.c
index ab75add..ef59516 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -273,6 +273,10 @@ void free_pid(struct pid *pid)
                         */
                        wake_up_process(ns->child_reaper);
                        break;
+               case PIDNS_HASH_ADDING:
+                       /* Handle a fork failure of the first process */
+                       ns->nr_hashed = 0;
+                       /* fall through */
                case 0:
                        schedule_work(&ns->proc_work);
                        break;

At which point I ask myself what of the pathlogocical case where the
first fork fails but because we created the pid namespace with unshare
there is a concurrent fork from another process into the pid namespace
that succeeds.  Resulting in one pid in the pid namespace that is not
the reaper.

So we also need something like this.

@@ -324,6 +328,8 @@ struct pid *alloc_pid(struct pid_namespace *ns)
        spin_lock_irq(&pidmap_lock);
        if (!(ns->nr_hashed & PIDNS_HASH_ADDING))
                goto out_unlock;
+       if (!is_child_reaper(pid) && !ns->child_reaper)
+               goto out_unlock;
        for ( ; upid >= pid->numbers; --upid) {
                hlist_add_head_rcu(&upid->pid_chain,
                                &pid_hash[pid_hashfn(upid->nr, upid->ns)]);

but I think my locking is wrong to safely test ns->child_reaper.

Perhaps I should prevent setns if there is no reaper?

Ideas?

Eric
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