Re: [PATCH 1/2v6] procfs: show hierarchy of pid namespace

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Richard Weinberger (richard@nod.at):
> Am 05.11.2014 um 11:41 schrieb Chen Hanxiao:
> > We lack of pid hierarchy information, and this will lead to:
> > a) we don't know pids' relationship, who is whose child:
> >    /proc/PID/ns/pid only tell us whether two pids live in different ns
> > b) bring trouble to nested lxc container check/restore/migration
> > c) bring trouble to pid translation between containers;
> > 
> > This patch will show the hierarchy of pid namespace
> > by pidns_hierarchy like:
> > 
> > [root@localhost ~]#cat /proc/pidns_hierarchy
> > 18060 18102 1534
> > 18060 18102 1600
> > 1550
> 
> Hmm, what about printing the pid hierarchy in the same way as /proc/self/mountinfo
> does with mount namespaces?
> Your current approach is not bad but we should really try to be consistent with existing
> sources of information.

Good point.  How would you structure it to make it look mor elike mountinfo?
Adding the pidns inode number (in place of a mount sequence number) might be
useful, but it sounds like you have a more concrete idea?


> > +config PROC_PID_HIERARCHY
> > +	bool "Enable /proc/pidns_hierarchy support" if EXPERT
> > +	depends on PROC_FS
> > +	help
> > +	  Show pid namespace hierarchy information
> 
> Why does this depend on EXPERT?
> Every Linux distro will enable this option.

Agreed here.


> > +static int proc_pidns_list_refresh(struct pid_namespace *curr_ns,
> > +		struct list_head *pidns_pid_list,
> > +		struct list_head *pidns_pid_tree)
> > +{
> > +	struct pid *pid;
> > +	int new_nr, nr = 0;
> > +	int rc;
> > +
> > +	/* collect pids in current namespace */
> > +	while (nr < PID_MAX_LIMIT) {
> > +		rcu_read_lock();
> > +		pid = find_ge_pid(nr, curr_ns);
> > +		if (pid) {
> > +			new_nr = pid_vnr(pid);
> > +			if (!is_child_reaper(pid)) {
> > +				nr = new_nr + 1;
> > +				rcu_read_unlock();
> > +				continue;
> > +			}
> > +			get_pid(pid);
> > +			rcu_read_unlock();
> > +			rc = pidns_list_add(pid, pidns_pid_list);
> 
> This function allocates memory per PID. If we have lots of PIDs, how does this scale?
> I'd go so far and say this can be a DoS'able issue if the pidns_hierarchy file is opened multiple times...

It's not per pid, but per init-pid.  For non-reaper pids he bails and continue
through the loop a few lines above.  This still may be DOS-able if users don't
have kmem restrictions to prevent a ton of pid namespaces, but then the
namespaces themselves will take a lot more memory than the representation here.

-serge