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

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Date:	Mon, 9 Sep 2013 17:15:30 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.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

On 09/08, Eric W. Biederman wrote:
>
> 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;

Agreed, it also makes sense to clear ->nr_hashed. But I still think
that WARN_ON(ns->child_reaper) makes sense too.

> 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.

But how can setns() work before the first fork() succeeds and makes the
->child_reaper visible in /proc ?

Probably I missed something obvious, I didn't sleep today...

Oleg.

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