netlink: use call_rcu for netlink_change_ngroups
For the network namespace work in generic netlink I need
to be able to call this function under rcu_read_lock(),
otherwise the locking becomes a nightmare and more locks
would be needed. Instead, just embed a struct rcu_head
(actually a struct listeners_rcu_head that also carries
the pointer to the memory block) into the listeners
memory so we can use call_rcu() instead of synchronising
and then freeing. No rcu_barrier() is needed since this
code cannot be modular.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index d7d1b82..d46da6c 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -83,6 +83,11 @@
struct module *module;
};
+struct listeners_rcu_head {
+ struct rcu_head rcu_head;
+ void *ptr;
+};
+
#define NETLINK_KERNEL_SOCKET 0x1
#define NETLINK_RECV_PKTINFO 0x2
#define NETLINK_BROADCAST_SEND_ERROR 0x4
@@ -1453,7 +1458,8 @@
if (groups < 32)
groups = 32;
- listeners = kzalloc(NLGRPSZ(groups), GFP_KERNEL);
+ listeners = kzalloc(NLGRPSZ(groups) + sizeof(struct listeners_rcu_head),
+ GFP_KERNEL);
if (!listeners)
goto out_sock_release;
@@ -1501,6 +1507,14 @@
EXPORT_SYMBOL(netlink_kernel_release);
+static void netlink_free_old_listeners(struct rcu_head *rcu_head)
+{
+ struct listeners_rcu_head *lrh;
+
+ lrh = container_of(rcu_head, struct listeners_rcu_head, rcu_head);
+ kfree(lrh->ptr);
+}
+
/**
* netlink_change_ngroups - change number of multicast groups
*
@@ -1516,6 +1530,7 @@
int netlink_change_ngroups(struct sock *sk, unsigned int groups)
{
unsigned long *listeners, *old = NULL;
+ struct listeners_rcu_head *old_rcu_head;
struct netlink_table *tbl = &nl_table[sk->sk_protocol];
int err = 0;
@@ -1524,7 +1539,9 @@
netlink_table_grab();
if (NLGRPSZ(tbl->groups) < NLGRPSZ(groups)) {
- listeners = kzalloc(NLGRPSZ(groups), GFP_ATOMIC);
+ listeners = kzalloc(NLGRPSZ(groups) +
+ sizeof(struct listeners_rcu_head),
+ GFP_ATOMIC);
if (!listeners) {
err = -ENOMEM;
goto out_ungrab;
@@ -1532,13 +1549,22 @@
old = tbl->listeners;
memcpy(listeners, old, NLGRPSZ(tbl->groups));
rcu_assign_pointer(tbl->listeners, listeners);
+ /*
+ * Free the old memory after an RCU grace period so we
+ * don't leak it. We use call_rcu() here in order to be
+ * able to call this function from atomic contexts. The
+ * allocation of this memory will have reserved enough
+ * space for struct listeners_rcu_head at the end.
+ */
+ old_rcu_head = (void *)(tbl->listeners +
+ NLGRPLONGS(tbl->groups));
+ old_rcu_head->ptr = old;
+ call_rcu(&old_rcu_head->rcu_head, netlink_free_old_listeners);
}
tbl->groups = groups;
out_ungrab:
netlink_table_ungrab();
- synchronize_rcu();
- kfree(old);
return err;
}