cgroup: fix cgroup_path() vs rename() race
rename() will change dentry->d_name. The result of this race can
be worse than seeing partially rewritten name, but we might access
a stale pointer because rename() will re-allocate memory to hold
a longer name.
As accessing dentry->name must be protected by dentry->d_lock or
parent inode's i_mutex, while on the other hand cgroup-path() can
be called with some irq-safe spinlocks held, we can't generate
cgroup path using dentry->d_name.
Alternatively we make a copy of dentry->d_name and save it in
cgrp->name when a cgroup is created, and update cgrp->name at
rename().
v5: use flexible array instead of zero-size array.
v4: - allocate root_cgroup_name and all root_cgroup->name points to it.
- add cgroup_name() wrapper.
v3: use kfree_rcu() instead of synchronize_rcu() in user-visible path.
v2: make cgrp->name RCU safe.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 900af59..75c6ec1 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -150,6 +150,11 @@
CGRP_CPUSET_CLONE_CHILDREN,
};
+struct cgroup_name {
+ struct rcu_head rcu_head;
+ char name[];
+};
+
struct cgroup {
unsigned long flags; /* "unsigned long" so bitops work */
@@ -172,6 +177,19 @@
struct cgroup *parent; /* my parent */
struct dentry *dentry; /* cgroup fs entry, RCU protected */
+ /*
+ * This is a copy of dentry->d_name, and it's needed because
+ * we can't use dentry->d_name in cgroup_path().
+ *
+ * You must acquire rcu_read_lock() to access cgrp->name, and
+ * the only place that can change it is rename(), which is
+ * protected by parent dir's i_mutex.
+ *
+ * Normally you should use cgroup_name() wrapper rather than
+ * access it directly.
+ */
+ struct cgroup_name __rcu *name;
+
/* Private pointers for each registered subsystem */
struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
@@ -404,6 +422,12 @@
void *data;
};
+/* Caller should hold rcu_read_lock() */
+static inline const char *cgroup_name(const struct cgroup *cgrp)
+{
+ return rcu_dereference(cgrp->name)->name;
+}
+
int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);
int cgroup_rm_cftypes(struct cgroup_subsys *ss, struct cftype *cfts);