[PATCH] Don't crash if we are self-checkpointing with a child

[Dan Smith]

If we are doing a self-checkpoint, we will not be frozen and the
get_freezer_task() will return NULL.  If we have children, then
may_checkpoint_task() will attempt to make sure that they are in the
same cgroup as the freezer task, which will attempt to lock a NULL
task pointer (and thus go boom).

This patch just inserts a check before in_same_cgroup_freezer() call
to make sure we have a freezer task.  The existing check is aimed to
determine if a task is not frozen alongside the root, so this addition
catches the case where it is impossible.

Signed-off-by: Dan Smith <danms at us.ibm.com>
---
 kernel/checkpoint/checkpoint.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/checkpoint/checkpoint.c b/kernel/checkpoint/checkpoint.c
index 0f46acf..33f53e9 100644
--- a/kernel/checkpoint/checkpoint.c
+++ b/kernel/checkpoint/checkpoint.c
@@ -262,8 +262,12 @@ static int may_checkpoint_task(struct ckpt_ctx *ctx, struct task_struct *t)
 	if (t->exit_state)
 		return 0;
 
-	/* verify that all tasks belongs to same freezer cgroup */
-	if (t != current && !in_same_cgroup_freezer(t, ctx->root_freezer)) {
+	/*
+	 * verify that we have a freezer cgroup and that all tasks
+	 * belong to the same one
+	 */
+	if (t != current && (!ctx->root_freezer ||
+			     !in_same_cgroup_freezer(t, ctx->root_freezer))) {
 		_ckpt_err(ctx, -EBUSY, "%(T)Not frozen or wrong cgroup\n");
 		return -EBUSY;
 	}
-- 
1.7.2.2