Subject: [PATCH net-next 2/4] rtnetlink: get reference on module before invoking handlers

From: Florian Westphal <[email-masked]>
Date: Sat, 2 Dec 2017 21:44:06 +0100

Add yet another rtnl_register function.  It will be used by modules
that can be removed.

The passed module struct is used to prevent module unload while
a netlink dump is in progress or when a DOIT_UNLOCKED doit callback
is called.

Cc: Peter Zijlstra <[email-masked]>
Signed-off-by: Florian Westphal <[email-masked]>
---
 include/net/rtnetlink.h |   2 +
 net/core/rtnetlink.c    | 113 +++++++++++++++++++++++++++++++++---------------
 2 files changed, 80 insertions(+), 35 deletions(-)

diff --git a/include/net/rtnetlink.h b/include/net/rtnetlink.h
index ead018744ff5..e326b3f9eb5f 100644
--- a/include/net/rtnetlink.h
+++ b/include/net/rtnetlink.h
@@ -17,6 +17,8 @@ int __rtnl_register(int protocol, int msgtype,
 		    rtnl_doit_func, rtnl_dumpit_func, unsigned int flags);
 void rtnl_register(int protocol, int msgtype,
 		   rtnl_doit_func, rtnl_dumpit_func, unsigned int flags);
+int rtnl_register_module(struct module *owner, int protocol, int msgtype,
+			 rtnl_doit_func, rtnl_dumpit_func, unsigned int flags);
 int rtnl_unregister(int protocol, int msgtype);
 void rtnl_unregister_all(int protocol);
 
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index ff292d3f2c41..de6390365c90 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -62,6 +62,7 @@
 struct rtnl_link {
 	rtnl_doit_func		doit;
 	rtnl_dumpit_func	dumpit;
+	struct module		*owner;
 	unsigned int		flags;
 	struct rcu_head		rcu;
 };
@@ -129,7 +130,6 @@ EXPORT_SYMBOL(lockdep_rtnl_is_held);
 #endif /* #ifdef CONFIG_PROVE_LOCKING */
 
 static struct rtnl_link __rcu **rtnl_msg_handlers[RTNL_FAMILY_MAX + 1];
-static refcount_t rtnl_msg_handlers_ref[RTNL_FAMILY_MAX + 1];
 
 static inline int rtm_msgindex(int msgtype)
 {
@@ -159,27 +159,10 @@ static struct rtnl_link *rtnl_get_link(int protocol, int msgtype)
 	return tab[msgtype];
 }
 
-/**
- * __rtnl_register - Register a rtnetlink message type
- * @protocol: Protocol family or PF_UNSPEC
- * @msgtype: rtnetlink message type
- * @doit: Function pointer called for each request message
- * @dumpit: Function pointer called for each dump request (NLM_F_DUMP) message
- * @flags: rtnl_link_flags to modifiy behaviour of doit/dumpit functions
- *
- * Registers the specified function pointers (at least one of them has
- * to be non-NULL) to be called whenever a request message for the
- * specified protocol family and message type is received.
- *
- * The special protocol family PF_UNSPEC may be used to define fallback
- * function pointers for the case when no entry for the specific protocol
- * family exists.
- *
- * Returns 0 on success or a negative error code.
- */
-int __rtnl_register(int protocol, int msgtype,
-		    rtnl_doit_func doit, rtnl_dumpit_func dumpit,
-		    unsigned int flags)
+static int rtnl_register_internal(struct module *owner,
+				  int protocol, int msgtype,
+				  rtnl_doit_func doit, rtnl_dumpit_func dumpit,
+				  unsigned int flags)
 {
 	struct rtnl_link **tab, *link, *old;
 	int msgindex;
@@ -210,6 +193,9 @@ int __rtnl_register(int protocol, int msgtype,
 			goto unlock;
 	}
 
+	WARN_ON(link->owner && link->owner != owner);
+	link->owner = owner;
+
 	WARN_ON(doit && link->doit && link->doit != doit);
 	if (doit)
 		link->doit = doit;
@@ -228,6 +214,54 @@ int __rtnl_register(int protocol, int msgtype,
 	rtnl_unlock();
 	return ret;
 }
+
+/**
+ * rtnl_register_module - Register a rtnetlink message type
+ *
+ * @owner: module registering the hook (THIS_MODULE)
+ * @protocol: Protocol family or PF_UNSPEC
+ * @msgtype: rtnetlink message type
+ * @doit: Function pointer called for each request message
+ * @dumpit: Function pointer called for each dump request (NLM_F_DUMP) message
+ * @flags: rtnl_link_flags to modifiy behaviour of doit/dumpit functions
+ *
+ * Like rtnl_register, but for use by removable modules.
+ */
+int rtnl_register_module(struct module *owner,
+			 int protocol, int msgtype,
+			 rtnl_doit_func doit, rtnl_dumpit_func dumpit,
+			 unsigned int flags)
+{
+	return rtnl_register_internal(owner, protocol, msgtype,
+				      doit, dumpit, flags);
+}
+EXPORT_SYMBOL_GPL(rtnl_register_module);
+
+/**
+ * __rtnl_register - Register a rtnetlink message type
+ * @protocol: Protocol family or PF_UNSPEC
+ * @msgtype: rtnetlink message type
+ * @doit: Function pointer called for each request message
+ * @dumpit: Function pointer called for each dump request (NLM_F_DUMP) message
+ * @flags: rtnl_link_flags to modifiy behaviour of doit/dumpit functions
+ *
+ * Registers the specified function pointers (at least one of them has
+ * to be non-NULL) to be called whenever a request message for the
+ * specified protocol family and message type is received.
+ *
+ * The special protocol family PF_UNSPEC may be used to define fallback
+ * function pointers for the case when no entry for the specific protocol
+ * family exists.
+ *
+ * Returns 0 on success or a negative error code.
+ */
+int __rtnl_register(int protocol, int msgtype,
+		    rtnl_doit_func doit, rtnl_dumpit_func dumpit,
+		    unsigned int flags)
+{
+	return rtnl_register_internal(NULL, protocol, msgtype,
+				      doit, dumpit, flags);
+}
 EXPORT_SYMBOL_GPL(__rtnl_register);
 
 /**
@@ -311,8 +345,6 @@ void rtnl_unregister_all(int protocol)
 
 	synchronize_net();
 
-	while (refcount_read(&rtnl_msg_handlers_ref[protocol]) > 1)
-		schedule();
 	kfree(tab);
 }
 EXPORT_SYMBOL_GPL(rtnl_unregister_all);
@@ -4372,6 +4404,7 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 {
 	struct net *net = sock_net(skb->sk);
 	struct rtnl_link *link;
+	struct module *owner;
 	int err = -EOPNOTSUPP;
 	rtnl_doit_func doit;
 	unsigned int flags;
@@ -4408,24 +4441,32 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 			if (!link || !link->dumpit)
 				goto err_unlock;
 		}
+		owner = link->owner;
 		dumpit = link->dumpit;
 
-		refcount_inc(&rtnl_msg_handlers_ref[family]);
-
 		if (type == RTM_GETLINK - RTM_BASE)
 			min_dump_alloc = rtnl_calcit(skb, nlh);
 
+		err = 0;
+		/* need to do this before rcu_read_unlock() */
+		if (!try_module_get(owner))
+			err = -EPROTONOSUPPORT;
+
 		rcu_read_unlock();
 
 		rtnl = net->rtnl;
-		{
+		if (err == 0) {
 			struct netlink_dump_control c = {
 				.dump		= dumpit,
 				.min_dump_alloc	= min_dump_alloc,
+				.module		= owner,
 			};
 			err = netlink_dump_start(rtnl, skb, nlh, &c);
+			/* netlink_dump_start() will keep a reference on
+			 * module if dump is still in progress.
+			 */
+			module_put(owner);
 		}
-		refcount_dec(&rtnl_msg_handlers_ref[family]);
 		return err;
 	}
 
@@ -4437,14 +4478,19 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 			goto out_unlock;
 	}
 
+	owner = link->owner;
+	if (!try_module_get(owner)) {
+		err = -EPROTONOSUPPORT;
+		goto out_unlock;
+	}
+
 	flags = link->flags;
 	if (flags & RTNL_FLAG_DOIT_UNLOCKED) {
-		refcount_inc(&rtnl_msg_handlers_ref[family]);
 		doit = link->doit;
 		rcu_read_unlock();
 		if (doit)
 			err = doit(skb, nlh, extack);
-		refcount_dec(&rtnl_msg_handlers_ref[family]);
+		module_put(owner);
 		return err;
 	}
 	rcu_read_unlock();
@@ -4455,6 +4501,8 @@ static int rtnetlink_rcv_msg(struct sk_buff *skb, struct nlmsghdr *nlh,
 		err = link->doit(skb, nlh, extack);
 	rtnl_unlock();
 
+	module_put(owner);
+
 	return err;
 
 out_unlock:
@@ -4546,11 +4594,6 @@ static struct pernet_operations rtnetlink_net_ops = {
 
 void __init rtnetlink_init(void)
 {
-	int i;
-
-	for (i = 0; i < ARRAY_SIZE(rtnl_msg_handlers_ref); i++)
-		refcount_set(&rtnl_msg_handlers_ref[i], 1);
-
 	if (register_pernet_subsys(&rtnetlink_net_ops))
 		panic("rtnetlink_init: cannot initialize rtnetlink\n");
 
-- 
2.13.6