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

[Oleg Nesterov <oleg@redhat.com>]

On 09/16, Jiri Slaby wrote:
>
> On 09/10/2010 12:15 AM, Oleg Nesterov wrote:
> > On 08/30, Jiri Slaby wrote:
> >>> --- 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.
>
> That said, it (950eaaca681c4) should probably go into stable. (Apply to
> all 32-35 whichever are maintained currently.)

Perhaps, but the race is mostly theoretical.

To be honest, I think 950eaaca681c4 needs a comment to explain what
rcu_read_lock() protects, or perhaps we can make it more explicit.

Oleg.

--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -931,7 +931,6 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid
 		pgid = pid;
 	if (pgid < 0)
 		return -EINVAL;
-	rcu_read_lock();
 
 	/* From this point forward we keep holding onto the tasklist lock
 	 * so that our parent does not change from under us. -DaveM
@@ -939,7 +938,9 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid
 	write_lock_irq(&tasklist_lock);
 
 	err = -ESRCH;
+	rcu_read_lock();
 	p = find_task_by_vpid(pid);
+	rcu_read_unlock();
 	if (!p)
 		goto out;
 
@@ -968,7 +969,9 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid
 	if (pgid != pid) {
 		struct task_struct *g;
 
+		rcu_read_lock();
 		pgrp = find_vpid(pgid);
+		rcu_read_unlock();
 		g = pid_task(pgrp, PIDTYPE_PGID);
 		if (!g || task_session(g) != task_session(group_leader))
 			goto out;
@@ -985,7 +988,6 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid
 out:
 	/* All paths lead to here, thus we are safe. -DaveM */
 	write_unlock_irq(&tasklist_lock);
-	rcu_read_unlock();
 	return err;
 }