[PATCH] cgroups: fix a serious bug in cgroupstats

[Li Zefan <lizf@cn.fujitsu.com>]

Try this, and you'll get oops immediately:
 # cd Documentation/accounting/
 # gcc -o getdelays getdelays.c
 # mount -t cgroup -o debug xxx /mnt
 # ./getdelays -C /mnt/tasks

Because a normal file's dentry->d_fsdata is a pointer to struct cftype,
not struct cgroup.

After the patch, it returns EINVAL if we try to get cgroupstats
from a normal file.

CC: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
---
 kernel/cgroup.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 358e775..f6f5219 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2039,10 +2039,13 @@ int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry)
 	struct cgroup *cgrp;
 	struct cgroup_iter it;
 	struct task_struct *tsk;
+
 	/*
-	 * Validate dentry by checking the superblock operations
+	 * Validate dentry by checking the superblock operations,
+	 * and make sure it's a directory.
 	 */
-	if (dentry->d_sb->s_op != &cgroup_ops)
+	if (dentry->d_sb->s_op != &cgroup_ops ||
+	    !S_ISDIR(dentry->d_inode->i_mode))
 		 goto err;
 
 	ret = 0;
-- 
1.5.4.rc3