pids: Move the pgrp and session pid pointers from task_struct to signal_struct

To access these fields the code always has to go to group leader so
going to signal struct is no loss and is actually a fundamental simplification.

This saves a little bit of memory by only allocating the pid pointer array
once instead of once for every thread, and even better this removes a
few potential races caused by the fact that group_leader can be changed
by de_thread, while signal_struct can not.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 23 insertions, 3 deletions

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index b95a272c1ab5..2dcded16eb1e 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -146,7 +146,9 @@ struct signal_struct {
 
 #endif
 
+	/* PID/PID hash table linkage. */
 	struct pid *leader_pid;
+	struct pid *pids[PIDTYPE_MAX];
 
 #ifdef CONFIG_NO_HZ_FULL
 	atomic_t tick_dep_mask;
@@ -559,9 +561,12 @@ void walk_process_tree(struct task_struct *top, proc_visitor, void *);
 static inline
 struct pid *task_pid_type(struct task_struct *task, enum pid_type type)
 {
-	if (type != PIDTYPE_PID)
-		task = task->group_leader;
-	return task->pids[type].pid;
+	struct pid *pid;
+	if (type == PIDTYPE_PID)
+		pid = task_pid(task);
+	else
+		pid = task->signal->pids[type];
+	return pid;
 }
 
 static inline struct pid *task_tgid(struct task_struct *task)
@@ -569,6 +574,21 @@ static inline struct pid *task_tgid(struct task_struct *task)
 	return task->signal->leader_pid;
 }
 
+/*
+ * Without tasklist or RCU lock it is not safe to dereference
+ * the result of task_pgrp/task_session even if task == current,
+ * we can race with another thread doing sys_setsid/sys_setpgid.
+ */
+static inline struct pid *task_pgrp(struct task_struct *task)
+{
+	return task->signal->pids[PIDTYPE_PGID];
+}
+
+static inline struct pid *task_session(struct task_struct *task)
+{
+	return task->signal->pids[PIDTYPE_SID];
+}
+
 static inline int get_nr_threads(struct task_struct *tsk)
 {
 	return tsk->signal->nr_threads;