[PATCH v2 net-next 07/14] rtnetlink: Protect struct rtnl_link_ops with SRCU.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

Once RTNL is replaced with rtnl_net_lock(), we need a mechanism to
guarantee that rtnl_link_ops is alive during inflight RTM_NEWLINK
even when its module is being unloaded.

Let's use SRCU to protect ops.

rtnl_link_ops_get() now iterates link_ops under RCU and returns
SRCU-protected ops pointer.  The caller must call rtnl_link_ops_put()
to release the pointer after the use.

Also, __rtnl_link_unregister() unlinks the ops first and calls
synchronize_srcu() to wait for inflight RTM_NEWLINK requests to
complete.

Note that link_ops needs to be protected by its dedicated lock
when RTNL is removed.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
---
v2:
  * Handle error of init_srcu_struct().
  * Call cleanup_srcu_struct() after synchronize_srcu().
---
 include/net/rtnetlink.h |  5 ++-
 net/core/rtnetlink.c    | 83 ++++++++++++++++++++++++++++++-----------
 2 files changed, 65 insertions(+), 23 deletions(-)

diff --git a/include/net/rtnetlink.h b/include/net/rtnetlink.h
index bb49c5708ce7..1a6aa5ca74f3 100644
--- a/include/net/rtnetlink.h
+++ b/include/net/rtnetlink.h
@@ -3,6 +3,7 @@
 #define __NET_RTNETLINK_H
 
 #include <linux/rtnetlink.h>
+#include <linux/srcu.h>
 #include <net/netlink.h>
 
 typedef int (*rtnl_doit_func)(struct sk_buff *, struct nlmsghdr *,
@@ -69,7 +70,8 @@ static inline int rtnl_msg_family(const struct nlmsghdr *nlh)
 /**
  *	struct rtnl_link_ops - rtnetlink link operations
  *
- *	@list: Used internally
+ *	@list: Used internally, protected by RTNL and SRCU
+ *	@srcu: Used internally
  *	@kind: Identifier
  *	@netns_refund: Physical device, move to init_net on netns exit
  *	@maxtype: Highest device specific netlink attribute number
@@ -100,6 +102,7 @@ static inline int rtnl_msg_family(const struct nlmsghdr *nlh)
  */
 struct rtnl_link_ops {
 	struct list_head	list;
+	struct srcu_struct	srcu;
 
 	const char		*kind;
 
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 9c9290a6c271..31b105b3a834 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -457,15 +457,29 @@ EXPORT_SYMBOL_GPL(__rtnl_unregister_many);
 
 static LIST_HEAD(link_ops);
 
-static const struct rtnl_link_ops *rtnl_link_ops_get(const char *kind)
+static struct rtnl_link_ops *rtnl_link_ops_get(const char *kind, int *srcu_index)
 {
-	const struct rtnl_link_ops *ops;
+	struct rtnl_link_ops *ops;
 
-	list_for_each_entry(ops, &link_ops, list) {
-		if (!strcmp(ops->kind, kind))
-			return ops;
+	rcu_read_lock();
+
+	list_for_each_entry_rcu(ops, &link_ops, list) {
+		if (!strcmp(ops->kind, kind)) {
+			*srcu_index = srcu_read_lock(&ops->srcu);
+			goto unlock;
+		}
 	}
-	return NULL;
+
+	ops = NULL;
+unlock:
+	rcu_read_unlock();
+
+	return ops;
+}
+
+static void rtnl_link_ops_put(struct rtnl_link_ops *ops, int srcu_index)
+{
+	srcu_read_unlock(&ops->srcu, srcu_index);
 }
 
 /**
@@ -480,8 +494,16 @@ static const struct rtnl_link_ops *rtnl_link_ops_get(const char *kind)
  */
 int __rtnl_link_register(struct rtnl_link_ops *ops)
 {
-	if (rtnl_link_ops_get(ops->kind))
-		return -EEXIST;
+	struct rtnl_link_ops *tmp;
+	int err;
+
+	/* When RTNL is removed, add lock for link_ops. */
+	ASSERT_RTNL();
+
+	list_for_each_entry(tmp, &link_ops, list) {
+		if (!strcmp(ops->kind, tmp->kind))
+			return -EEXIST;
+	}
 
 	/* The check for alloc/setup is here because if ops
 	 * does not have that filled up, it is not possible
@@ -491,7 +513,12 @@ int __rtnl_link_register(struct rtnl_link_ops *ops)
 	if ((ops->alloc || ops->setup) && !ops->dellink)
 		ops->dellink = unregister_netdevice_queue;
 
-	list_add_tail(&ops->list, &link_ops);
+	err = init_srcu_struct(&ops->srcu);
+	if (err)
+		return err;
+
+	list_add_tail_rcu(&ops->list, &link_ops);
+
 	return 0;
 }
 EXPORT_SYMBOL_GPL(__rtnl_link_register);
@@ -542,10 +569,12 @@ void __rtnl_link_unregister(struct rtnl_link_ops *ops)
 {
 	struct net *net;
 
-	for_each_net(net) {
+	list_del_rcu(&ops->list);
+	synchronize_srcu(&ops->srcu);
+	cleanup_srcu_struct(&ops->srcu);
+
+	for_each_net(net)
 		__rtnl_kill_links(net, ops);
-	}
-	list_del(&ops->list);
 }
 EXPORT_SYMBOL_GPL(__rtnl_link_unregister);
 
@@ -2158,10 +2187,11 @@ static const struct nla_policy ifla_xdp_policy[IFLA_XDP_MAX + 1] = {
 	[IFLA_XDP_PROG_ID]	= { .type = NLA_U32 },
 };
 
-static const struct rtnl_link_ops *linkinfo_to_kind_ops(const struct nlattr *nla)
+static struct rtnl_link_ops *linkinfo_to_kind_ops(const struct nlattr *nla,
+						  int *ops_srcu_index)
 {
-	const struct rtnl_link_ops *ops = NULL;
 	struct nlattr *linfo[IFLA_INFO_MAX + 1];
+	struct rtnl_link_ops *ops = NULL;
 
 	if (nla_parse_nested_deprecated(linfo, IFLA_INFO_MAX, nla, ifla_info_policy, NULL) < 0)
 		return NULL;
@@ -2170,7 +2200,7 @@ static const struct rtnl_link_ops *linkinfo_to_kind_ops(const struct nlattr *nla
 		char kind[MODULE_NAME_LEN];
 
 		nla_strscpy(kind, linfo[IFLA_INFO_KIND], sizeof(kind));
-		ops = rtnl_link_ops_get(kind);
+		ops = rtnl_link_ops_get(kind, ops_srcu_index);
 	}
 
 	return ops;
@@ -2290,8 +2320,8 @@ static int rtnl_valid_dump_ifinfo_req(const struct nlmsghdr *nlh,
 
 static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 {
-	const struct rtnl_link_ops *kind_ops = NULL;
 	struct netlink_ext_ack *extack = cb->extack;
+	struct rtnl_link_ops *kind_ops = NULL;
 	const struct nlmsghdr *nlh = cb->nlh;
 	struct net *net = sock_net(skb->sk);
 	unsigned int flags = NLM_F_MULTI;
@@ -2302,6 +2332,7 @@ static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 	struct net *tgt_net = net;
 	u32 ext_filter_mask = 0;
 	struct net_device *dev;
+	int ops_srcu_index;
 	int master_idx = 0;
 	int netnsid = -1;
 	int err, i;
@@ -2335,7 +2366,7 @@ static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 			master_idx = nla_get_u32(tb[i]);
 			break;
 		case IFLA_LINKINFO:
-			kind_ops = linkinfo_to_kind_ops(tb[i]);
+			kind_ops = linkinfo_to_kind_ops(tb[i], &ops_srcu_index);
 			break;
 		default:
 			if (cb->strict_check) {
@@ -2361,6 +2392,10 @@ static int rtnl_dump_ifinfo(struct sk_buff *skb, struct netlink_callback *cb)
 		if (err < 0)
 			break;
 	}
+
+	if (kind_ops)
+		rtnl_link_ops_put(kind_ops, ops_srcu_index);
+
 	cb->seq = tgt_net->dev_base_seq;
 	nl_dump_check_consistent(cb, nlmsg_hdr(skb));
 	if (netnsid >= 0)
@@ -3747,8 +3782,9 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 			struct netlink_ext_ack *extack)
 {
 	struct nlattr **tb, **linkinfo, **data = NULL;
-	const struct rtnl_link_ops *ops = NULL;
+	struct rtnl_link_ops *ops = NULL;
 	struct rtnl_newlink_tbs *tbs;
+	int ops_srcu_index;
 	int ret;
 
 	tbs = kmalloc(sizeof(*tbs), GFP_KERNEL);
@@ -3780,13 +3816,13 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 		char kind[MODULE_NAME_LEN];
 
 		nla_strscpy(kind, linkinfo[IFLA_INFO_KIND], sizeof(kind));
-		ops = rtnl_link_ops_get(kind);
+		ops = rtnl_link_ops_get(kind, &ops_srcu_index);
 #ifdef CONFIG_MODULES
 		if (!ops) {
 			__rtnl_unlock();
 			request_module("rtnl-link-%s", kind);
 			rtnl_lock();
-			ops = rtnl_link_ops_get(kind);
+			ops = rtnl_link_ops_get(kind, &ops_srcu_index);
 		}
 #endif
 	}
@@ -3800,7 +3836,7 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 							  linkinfo[IFLA_INFO_DATA],
 							  ops->policy, extack);
 			if (ret < 0)
-				goto free;
+				goto put_ops;
 
 			data = tbs->attr;
 		}
@@ -3808,12 +3844,15 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 		if (ops->validate) {
 			ret = ops->validate(tb, data, extack);
 			if (ret < 0)
-				goto free;
+				goto put_ops;
 		}
 	}
 
 	ret = __rtnl_newlink(skb, nlh, ops, tbs, data, extack);
 
+put_ops:
+	if (ops)
+		rtnl_link_ops_put(ops, ops_srcu_index);
 free:
 	kfree(tbs);
 	return ret;
-- 
2.39.5 (Apple Git-154)