pids: de_thread: don't clear session/pgrp pids for the old leader

Based on Eric W. Biederman's idea.

Unless task == current, without tasklist_lock held task_session()/task_pgrp()
can return NULL if the caller races with de_thread() which switches the group
leader.

Change transfer_pid() to not clear old->pids[type].pid for the old leader.
This means that its .pid can point to "nowhere", but this is already true for
sub-threads, and the old leader is not group_leader() any longer.  IOW, with
or without this change we can't trust task's special pids unless it is the
group leader.

With this change the following code

	rcu_read_lock();
	task = find_task_by_xxx();
	do_something(task_pgrp(task), task_session(task));
	rcu_read_unlock();

can't race with exec and hit the NULL pid.

Signed-off-by: Oleg Nesterov <[email protected]>
Cc:  "Eric W. Biederman" <[email protected]>
Cc: Pavel Emelyanov <[email protected]>
Cc: Roland McGrath <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions