On 04/27, Kirill Tkhai wrote:
>
> On 27.04.2017 19:12, Oleg Nesterov wrote:
> > On 04/26, Kirill Tkhai wrote:
> >>
> >> On 26.04.2017 18:53, Oleg Nesterov wrote:
> >>>
> >>>> +static long set_last_pid_vec(struct pid_namespace *pid_ns,
> >>>> + struct pidns_ioc_req *req)
> >>>> +{
> >>>> + char *str, *p;
> >>>> + int ret = 0;
> >>>> + pid_t pid;
> >>>> +
> >>>> + read_lock(&tasklist_lock);
> >>>> + if (!pid_ns->child_reaper)
> >>>> + ret = -EINVAL;
> >>>> + read_unlock(&tasklist_lock);
> >>>> + if (ret)
> >>>> + return ret;
> >>>
> >>> why do you need to check ->child_reaper under tasklist_lock? this looks
> >>> pointless.
> >>>
> >>> In fact I do not understand how it is possible to hit
> >>> pid_ns->child_reaper == NULL,
> >>> there must be at least one task in this namespace, otherwise you can't
> >>> open a file
> >>> which has f_op == ns_file_operations, no?
> >>
> >> Sure, it's impossible to pick a pid_ns, if there is no the pid_ns's tasks.
> >> I added
> >> it under impression of
> >> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=dfda351c729733a401981e8738ce497eaffcaa00
> >> but here it's completely wrong. It will be removed in v2.
> >
> > Hmm. But if I read this commit correctly then we really need to check
> > pid_ns->child_reaper != NULL ?
> >
> > Currently we can't pick an "empty" pid_ns. But after the commit above a task
> > can do sys_unshare(CLONE_NEWPID), another (or the same) task can open its
> > /proc/$pid/ns/pid_for_children and call ns_ioctl() before the 1st
> > alloc_pid() ?
>
> Another task can't open /proc/$pid/ns/pid_for_children before the 1st
> alloc_pid(),
> because pid_for_children is available to open only after the 1st alloc_pid().
> So, it's impossible to call ioctl() on it.
Ah, OK, I didn't notice the ns->child_reaper check in pidns_for_children_get().
But note that it doesn't need tasklist_lock too.
Oleg.
