linux-kernel - Re: [PATCH v1 1/1] waitid: Add support for waiting for the current process group

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Date:   Wed, 14 Aug 2019 14:45:51 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-kernel@...r.kernel.org, libc-alpha@...rceware.org,
        alistair23@...il.com, ebiederm@...ssion.com, arnd@...db.de,
        dalias@...c.org, torvalds@...ux-foundation.org,
        adhemerval.zanella@...aro.org, fweimer@...hat.com,
        palmer@...ive.com, macro@....com, zongbox@...il.com,
        akpm@...ux-foundation.org, viro@...iv.linux.org.uk, hpa@...or.com
Subject: Re: [PATCH v1 1/1] waitid: Add support for waiting for the current
 process group

On Wed, Aug 14, 2019 at 02:29:10PM +0200, Oleg Nesterov wrote:
> On 08/14, christian.brauner@...ntu.com wrote:
> >
> >  	case P_PGID:
> >  		type = PIDTYPE_PGID;
> > -		if (upid <= 0)
> > +		if (upid < 0)
> >  			return -EINVAL;
> > +
> > +		if (upid == 0)
> > +			pid = get_pid(task_pgrp(current));
> 
> this needs rcu lock or tasklist_lock, this can race with another thread
> doing sys_setpgid/setsid (see change_pid(PIDTYPE_PGID)).

Oh, I naively assumed task_pgrp() would take an rcu lock...

kernel/sys.c takes both, i.e.

rcu_read_lock();
write_lock_irq(&tasklist_lock);

though I think we should be fine with just rcu_read_lock(). setpgid()
indicates that it wants to check real_parent and needs the
write_lock_irq() because it might change behind its back which we don't
care about since we're not changing the pgrp.

Christian