linux-kernel - Re: [PATCH RFC] pid: make setpgid() system call use RCU read-side critical section

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Date:	Fri, 10 Sep 2010 00:15:55 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jiri Slaby <jslaby@...e.cz>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, jmorris@...ei.org
Subject: Re: [PATCH RFC] pid: make setpgid() system call use RCU read-side
	critical section

On 08/30, Jiri Slaby wrote:
>
> Ccing Oleg.

Sorry for delay...

> > --- a/kernel/sys.c
> > +++ b/kernel/sys.c
> > @@ -938,6 +938,7 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
> >  	write_lock_irq(&tasklist_lock);
> >
> >  	err = -ESRCH;
> > +	rcu_read_lock();
> >  	p = find_task_by_vpid(pid);
>
> AFAICT the missing lock doesn't harm due to the write_lock of tasklist
> above. But is probably a good thing to do anyway.

The problem is, find_task_by_vpid() is not safe without RCU. It is not
that the returned task_struct can't go away, find_pid_ns() itself is
not safe. This is because the failing copy_process() calls free_pid()
without tasklist_lock and modifies pid_hash[] list.

Oleg.

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