linux-kernel - Re: [PATCH v7 1/2] fork: extend clone3() to support setting a PID

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Date:   Mon, 11 Nov 2019 16:40:28 +0100
From:   Adrian Reber <areber@...hat.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        linux-kernel@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>
Subject: Re: [PATCH v7 1/2] fork: extend clone3() to support setting a PID

On Mon, Nov 11, 2019 at 04:25:15PM +0100, Oleg Nesterov wrote:
> On 11/11, Adrian Reber wrote:
> >
> > v7:
> >  - changed set_tid to be an array to set the PID of a process
> >    in multiple nested PID namespaces at the same time as discussed
> >    at LPC 2019 (container MC)
> 
> cough... iirc you convinced me this is not needed when we discussed
> the previous version ;) Nevermind, probably my memory fools me.

You are right. You suggested the same thing and we didn't listen ;)

> So far I only have some cosmetic nits,

Thanks for the quick review. I will try to apply your suggestions.

> > @@ -175,6 +187,18 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> >
> >  	for (i = ns->level; i >= 0; i--) {
> >  		int pid_min = 1;
> > +		int t_pos = 0;
>                     ^^^^^
> 
> I won't insist, but I'd suggest to cache set_tid[t_pos] instead to make
> the code a bit more simple.
> 
> > @@ -186,12 +210,24 @@ struct pid *alloc_pid(struct pid_namespace *ns)
> >  		if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
> >  			pid_min = RESERVED_PIDS;
> 
> You can probably move this code into the "else" branch below.
> 
> IOW, something like
> 
> 
> 	for (i = ns->level; i >= 0; i--) {
> 		int xxx = 0;
> 
> 		if (set_tid_size) {
> 			int pos = ns->level - i;
> 
> 			xxx = set_tid[pos];
> 			if (xxx < 1 || xxx >= pid_max)
> 				return ERR_PTR(-EINVAL);
> 			/* Also fail if a PID != 1 is requested and no PID 1 exists */
> 			if (xxx != 1 && !tmp->child_reaper)
> 				return ERR_PTR(-EINVAL);
> 			if (!ns_capable(tmp->user_ns, CAP_SYS_ADMIN))
> 				return ERR_PTR(-EPERM);
> 			set_tid_size--;
> 		}
> 
> 		idr_preload(GFP_KERNEL);
> 		spin_lock_irq(&pidmap_lock);
> 
> 		if (xxx) {
> 			nr = idr_alloc(&tmp->idr, NULL, xxx, xxx + 1,
> 					GFP_ATOMIC);
> 			/*
> 			 * If ENOSPC is returned it means that the PID is
> 			 * alreay in use. Return EEXIST in that case.
> 			 */
> 			if (nr == -ENOSPC)
> 				nr = -EEXIST;
> 		} else {
> 			int pid_min = 1;
> 			/*
> 			 * init really needs pid 1, but after reaching the
> 			 * maximum wrap back to RESERVED_PIDS
> 			 */
> 			if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
> 				pid_min = RESERVED_PIDS;
> 			/*
> 			 * Store a null pointer so find_pid_ns does not find
> 			 * a partially initialized PID (see below).
> 			 */
> 			nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> 					      pid_max, GFP_ATOMIC);
> 		}
> 
> 		...
> 
> This way only the "if (set_tid_size)" block has to play with set_tid_size/set_tid.
> 
> note also that this way we can easily allow set_tid[some_level] == 0, we can
> simply do
> 
> 	-	if (xxx < 1 || xxx >= pid_max)
> 	+	if (xxx < 0 || xxx >= pid_max)
> 
> although I don't think this is really useful.

Yes. I explicitly didn't allow 0 as a PID as I didn't thought it would
be useful (or maybe even valid).

		Adrian