bpf, netns: Keep a list of attached bpf_link's

To support multi-prog link-based attachments for new netns attach types, we
need to keep track of more than one bpf_link per attach type. Hence,
convert net->bpf.links into a list, that currently can be either empty or
have just one item.

Instead of reusing bpf_prog_list from bpf-cgroup, we link together
bpf_netns_link's themselves. This makes list management simpler as we don't
have to allocate, initialize, and later release list elements. We can do
this because multi-prog attachment will be available only for bpf_link, and
we don't need to build a list of programs attached directly and indirectly
via links.

No functional changes intended.
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200625141357.910330-4-jakub@cloudflare.com

include/net/netns/bpf.h

@@ -15,7 +15,7 @@ struct netns_bpf {
 	/* Array of programs to run compiled from progs or links */
 	struct bpf_prog_array __rcu *run_array[MAX_NETNS_BPF_ATTACH_TYPE];
 	struct bpf_prog *progs[MAX_NETNS_BPF_ATTACH_TYPE];
-	struct bpf_link *links[MAX_NETNS_BPF_ATTACH_TYPE];
+	struct list_head links[MAX_NETNS_BPF_ATTACH_TYPE];
 };
 
 #endif /* __NETNS_BPF_H__ */

kernel/bpf/net_namespace.c

@@ -19,20 +19,12 @@ struct bpf_netns_link {
 	 * with netns_bpf_mutex held.
 	 */
 	struct net *net;
+	struct list_head node; /* node in list of links attached to net */
 };
 
 /* Protects updates to netns_bpf */
 DEFINE_MUTEX(netns_bpf_mutex);
 
-/* Must be called with netns_bpf_mutex held. */
-static void __net_exit bpf_netns_link_auto_detach(struct bpf_link *link)
-{
-	struct bpf_netns_link *net_link =
-		container_of(link, struct bpf_netns_link, link);
-
-	net_link->net = NULL;
-}
-
 /* Must be called with netns_bpf_mutex held. */
 static void netns_bpf_run_array_detach(struct net *net,
 				       enum netns_bpf_attach_type type)
@@ -66,7 +58,7 @@ static void bpf_netns_link_release(struct bpf_link *link)
 		goto out_unlock;
 
 	netns_bpf_run_array_detach(net, type);
-	net->bpf.links[type] = NULL;
+	list_del(&net_link->node);
 
 out_unlock:
 	mutex_unlock(&netns_bpf_mutex);
@@ -225,7 +217,7 @@ int netns_bpf_prog_attach(const union bpf_attr *attr, struct bpf_prog *prog)
 	mutex_lock(&netns_bpf_mutex);
 
 	/* Attaching prog directly is not compatible with links */
-	if (net->bpf.links[type]) {
+	if (!list_empty(&net->bpf.links[type])) {
 		ret = -EEXIST;
 		goto out_unlock;
 	}
@@ -279,7 +271,7 @@ static int __netns_bpf_prog_detach(struct net *net,
 	struct bpf_prog *attached;
 
 	/* Progs attached via links cannot be detached */
-	if (net->bpf.links[type])
+	if (!list_empty(&net->bpf.links[type]))
 		return -EINVAL;
 
 	attached = net->bpf.progs[type];
@@ -310,13 +302,15 @@ int netns_bpf_prog_detach(const union bpf_attr *attr)
 static int netns_bpf_link_attach(struct net *net, struct bpf_link *link,
 				 enum netns_bpf_attach_type type)
 {
+	struct bpf_netns_link *net_link =
+		container_of(link, struct bpf_netns_link, link);
 	struct bpf_prog_array *run_array;
 	int err;
 
 	mutex_lock(&netns_bpf_mutex);
 
 	/* Allow attaching only one prog or link for now */
-	if (net->bpf.links[type]) {
+	if (!list_empty(&net->bpf.links[type])) {
 		err = -E2BIG;
 		goto out_unlock;
 	}
@@ -345,7 +339,7 @@ static int netns_bpf_link_attach(struct net *net, struct bpf_link *link,
 	run_array->items[0].prog = link->prog;
 	rcu_assign_pointer(net->bpf.run_array[type], run_array);
 
-	net->bpf.links[type] = link;
+	list_add_tail(&net_link->node, &net->bpf.links[type]);
 
 out_unlock:
 	mutex_unlock(&netns_bpf_mutex);
@@ -404,24 +398,34 @@ int netns_bpf_link_create(const union bpf_attr *attr, struct bpf_prog *prog)
 	return err;
 }
 
+static int __net_init netns_bpf_pernet_init(struct net *net)
+{
+	int type;
+
+	for (type = 0; type < MAX_NETNS_BPF_ATTACH_TYPE; type++)
+		INIT_LIST_HEAD(&net->bpf.links[type]);
+
+	return 0;
+}
+
 static void __net_exit netns_bpf_pernet_pre_exit(struct net *net)
 {
 	enum netns_bpf_attach_type type;
-	struct bpf_link *link;
+	struct bpf_netns_link *net_link;
 
 	mutex_lock(&netns_bpf_mutex);
 	for (type = 0; type < MAX_NETNS_BPF_ATTACH_TYPE; type++) {
 		netns_bpf_run_array_detach(net, type);
-		link = net->bpf.links[type];
-		if (link)
-			bpf_netns_link_auto_detach(link);
-		else if (net->bpf.progs[type])
+		list_for_each_entry(net_link, &net->bpf.links[type], node)
+			net_link->net = NULL; /* auto-detach link */
+		if (net->bpf.progs[type])
 			bpf_prog_put(net->bpf.progs[type]);
 	}
 	mutex_unlock(&netns_bpf_mutex);
 }
 
 static struct pernet_operations netns_bpf_pernet_ops __net_initdata = {
+	.init = netns_bpf_pernet_init,
 	.pre_exit = netns_bpf_pernet_pre_exit,
 };