Messages in this thread Patch in this message | | | Date | Thu, 20 Mar 2008 18:28:17 +0300 | From | Oleg Nesterov <> | Subject | [PATCH 1/4] pids: de_thread: don't clear session/pgrp pids for the old leader |
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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>
--- 25/kernel/pid.c~1_PID_EXEC 2008-03-20 17:00:56.000000000 +0300 +++ 25/kernel/pid.c 2008-03-20 17:22:19.000000000 +0300 @@ -354,7 +354,6 @@ void transfer_pid(struct task_struct *ol { new->pids[type].pid = old->pids[type].pid; hlist_replace_rcu(&old->pids[type].node, &new->pids[type].node); - old->pids[type].pid = NULL; } struct task_struct *pid_task(struct pid *pid, enum pid_type type)
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