packet: add classic BPF fanout mode

Add fanout mode PACKET_FANOUT_CBPF that accepts a classic BPF program
to select a socket.

This avoids having to keep adding special case fanout modes. One
example use case is application layer load balancing. The QUIC
protocol, for instance, encodes a connection ID in UDP payload.

Also add socket option SOL_PACKET/PACKET_FANOUT_DATA that updates data
associated with the socket group. Fanout mode PACKET_FANOUT_CBPF is the
only user so far.

Signed-off-by: Willem de Bruijn <[e-mail]>
Acked-by: Alexei Starovoitov <[e-mail]>
Acked-by: Daniel Borkmann <[e-mail]>
Acked-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/uapi/linux/if_packet.h

···
 #define PACKET_TX_HAS_OFF		19
 #define PACKET_QDISC_BYPASS		20
 #define PACKET_ROLLOVER_STATS		21
+#define PACKET_FANOUT_DATA		22
 
 #define PACKET_FANOUT_HASH		0
 #define PACKET_FANOUT_LB		1
···
 #define PACKET_FANOUT_ROLLOVER		3
 #define PACKET_FANOUT_RND		4
 #define PACKET_FANOUT_QM		5
+#define PACKET_FANOUT_CBPF		6
 #define PACKET_FANOUT_FLAG_ROLLOVER	0x1000
 #define PACKET_FANOUT_FLAG_DEFRAG	0x8000
 

net/packet/af_packet.c

···
 #ifdef CONFIG_INET
 #include <net/inet_common.h>
 #endif
+#include <linux/bpf.h>
 
 #include "internal.h"
 
···
 	return skb_get_queue_mapping(skb) % num;
 }
 
+static unsigned int fanout_demux_bpf(struct packet_fanout *f,
+				     struct sk_buff *skb,
+				     unsigned int num)
+{
+	struct bpf_prog *prog;
+	unsigned int ret = 0;
+
+	rcu_read_lock();
+	prog = rcu_dereference(f->bpf_prog);
+	if (prog)
+		ret = BPF_PROG_RUN(prog, skb) % num;
+	rcu_read_unlock();
+
+	return ret;
+}
+
 static bool fanout_has_flag(struct packet_fanout *f, u16 flag)
 {
 	return f->flags & (flag >> 8);
···
 		break;
 	case PACKET_FANOUT_ROLLOVER:
 		idx = fanout_demux_rollover(f, skb, 0, false, num);
+		break;
+	case PACKET_FANOUT_CBPF:
+		idx = fanout_demux_bpf(f, skb, num);
 		break;
 	}
 
···
 	return false;
 }
 
+static void fanout_init_data(struct packet_fanout *f)
+{
+	switch (f->type) {
+	case PACKET_FANOUT_LB:
+		atomic_set(&f->rr_cur, 0);
+		break;
+	case PACKET_FANOUT_CBPF:
+		RCU_INIT_POINTER(f->bpf_prog, NULL);
+		break;
+	}
+}
+
+static void __fanout_set_data_bpf(struct packet_fanout *f, struct bpf_prog *new)
+{
+	struct bpf_prog *old;
+
+	spin_lock(&f->lock);
+	old = rcu_dereference_protected(f->bpf_prog, lockdep_is_held(&f->lock));
+	rcu_assign_pointer(f->bpf_prog, new);
+	spin_unlock(&f->lock);
+
+	if (old) {
+		synchronize_net();
+		bpf_prog_destroy(old);
+	}
+}
+
+static int fanout_set_data_cbpf(struct packet_sock *po, char __user *data,
+				unsigned int len)
+{
+	struct bpf_prog *new;
+	struct sock_fprog fprog;
+	int ret;
+
+	if (sock_flag(&po->sk, SOCK_FILTER_LOCKED))
+		return -EPERM;
+	if (len != sizeof(fprog))
+		return -EINVAL;
+	if (copy_from_user(&fprog, data, len))
+		return -EFAULT;
+
+	ret = bpf_prog_create_from_user(&new, &fprog, NULL);
+	if (ret)
+		return ret;
+
+	__fanout_set_data_bpf(po->fanout, new);
+	return 0;
+}
+
+static int fanout_set_data(struct packet_sock *po, char __user *data,
+			   unsigned int len)
+{
+	switch (po->fanout->type) {
+	case PACKET_FANOUT_CBPF:
+		return fanout_set_data_cbpf(po, data, len);
+	default:
+		return -EINVAL;
+	};
+}
+
+static void fanout_release_data(struct packet_fanout *f)
+{
+	switch (f->type) {
+	case PACKET_FANOUT_CBPF:
+		__fanout_set_data_bpf(f, NULL);
+	};
+}
+
 static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 {
 	struct packet_sock *po = pkt_sk(sk);
···
 	case PACKET_FANOUT_CPU:
 	case PACKET_FANOUT_RND:
 	case PACKET_FANOUT_QM:
+	case PACKET_FANOUT_CBPF:
 		break;
 	default:
 		return -EINVAL;
···
 		match->id = id;
 		match->type = type;
 		match->flags = flags;
-		atomic_set(&match->rr_cur, 0);
 		INIT_LIST_HEAD(&match->list);
 		spin_lock_init(&match->lock);
 		atomic_set(&match->sk_ref, 0);
+		fanout_init_data(match);
 		match->prot_hook.type = po->prot_hook.type;
 		match->prot_hook.dev = po->prot_hook.dev;
 		match->prot_hook.func = packet_rcv_fanout;
···
 	if (atomic_dec_and_test(&f->sk_ref)) {
 		list_del(&f->list);
 		dev_remove_pack(&f->prot_hook);
+		fanout_release_data(f);
 		kfree(f);
 	}
 	mutex_unlock(&fanout_mutex);
···
 			return -EFAULT;
 
 		return fanout_add(sk, val & 0xffff, val >> 16);
+	}
+	case PACKET_FANOUT_DATA:
+	{
+		if (!po->fanout)
+			return -EINVAL;
+
+		return fanout_set_data(po, optval, optlen);
 	}
 	case PACKET_TX_HAS_OFF:
 	{

net/packet/internal.h

···
 	u16			id;
 	u8			type;
 	u8			flags;
-	atomic_t		rr_cur;
+	union {
+		atomic_t		rr_cur;
+		struct bpf_prog __rcu	*bpf_prog;
+	};
 	struct list_head	list;
 	struct sock		*arr[PACKET_FANOUT_MAX];
 	spinlock_t		lock;