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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 <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
65450cebc6