From: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Subject: [RFC][PATCH 09/14] Save leaders struct pid before detach_pid()

Save the struct pid of the thread group leader before detaching
our pid. See comments in the code below for more details.

Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
---
 fs/exec.c |   17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

Index: lx26-21-rc3-mm2/fs/exec.c
===================================================================
--- lx26-21-rc3-mm2.orig/fs/exec.c	2007-03-20 15:55:52.000000000 -0700
+++ lx26-21-rc3-mm2/fs/exec.c	2007-03-20 17:17:38.000000000 -0700
@@ -578,6 +578,7 @@ static int de_thread(struct task_struct 
 	struct sighand_struct *newsighand, *oldsighand = tsk->sighand;
 	spinlock_t *lock = &oldsighand->siglock;
 	struct task_struct *leader = NULL;
+	struct pid *leader_pid;
 	int count;
 
 	/*
@@ -698,10 +699,24 @@ static int de_thread(struct task_struct 
 		 * The old leader becomes a thread of the this thread group.
 		 * Note: The old leader also uses this pid until release_task
 		 *       is called.  Odd but simple and correct.
+		 * Note: Support for multiple pid namespaces implies that there
+		 * 	 can be multiple processes with a given pid_t value,
+		 * 	 and so, find_pid() needs/uses a notion of "current"
+		 * 	 pid namespace to select the appropriate process to
+		 * 	 return.
+		 *
+		 * 	 Since the current pid namespace of a process is stored
+		 * 	 in its pid->pid_nrs list, if we detach/free our struct
+		 * 	 pid, we would have no way of finding our current pid
+		 * 	 namespace and so no way of finding the leader pid.
+		 *
+		 * 	 To work around, find the leader pid, before detaching
+		 * 	 our pid from the task.
 		 */
+		leader_pid = find_pid(leader->pid);
 		detach_pid(tsk, PIDTYPE_PID);
 		tsk->pid = leader->pid;
-		attach_pid(tsk, PIDTYPE_PID,  find_pid(tsk->pid));
+		attach_pid(tsk, PIDTYPE_PID,  leader_pid);
 		transfer_pid(leader, tsk, PIDTYPE_PGID);
 		transfer_pid(leader, tsk, PIDTYPE_SID);
 		list_replace_rcu(&leader->tasks, &tsk->tasks);
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