[PATCH] kernel/pid.c: check pid whether be NULL in __change_pid()

[Chen Gang]

Within __change_pid(), 'new' may be NULL if it comes from detach_pid(),
and 'link->pid' also may be NULL ("link->pid = new"), so theoretically,\
the original 'link->pid' may be NULL, too.

In real world, at least now, all callers which will call detach_pid()
or change_pid() will not cause issue, but still recommend to check it
in __change_pid() to let itself consistency.

After the modification, it passed a simpe test: "build -> boot up ->
[s/g]et_[p/s/g]id() test by LTP tools".


Signed-off-by: Chen Gang <gang.chen@xxxxxxxxxxx>
---
 kernel/pid.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index 9b9a266..15b1b3d 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -399,6 +399,9 @@ static void __change_pid(struct task_struct *task, enum pid_type type,
 	hlist_del_rcu(&link->node);
 	link->pid = new;
 
+	if (!pid)
+		return;
+
 	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 		if (!hlist_empty(&pid->tasks[tmp]))
 			return;
-- 
1.7.7.6
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