packet: add PACKET_RESERVE sockopt

Add new sockopt to reserve some headroom in the mmaped ring frames in
front of the packet payload. This can be used f.i. when the VLAN header
needs to be (re)constructed to avoid moving the entire payload.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/if_packet.h

@@ -45,6 +45,7 @@ struct sockaddr_ll
 #define PACKET_ORIGDEV			9
 #define PACKET_VERSION			10
 #define PACKET_HDRLEN			11
+#define PACKET_RESERVE			12
 
 struct tpacket_stats
 {

net/packet/af_packet.c

@@ -188,6 +188,7 @@ struct packet_sock {
 	unsigned int		pg_vec_len;
 	enum tpacket_versions	tp_version;
 	unsigned int		tp_hdrlen;
+	unsigned int		tp_reserve;
 #endif
 };
 
@@ -635,11 +636,13 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev, struct packe
 		snaplen = res;
 
 	if (sk->sk_type == SOCK_DGRAM) {
-		macoff = netoff = TPACKET_ALIGN(po->tp_hdrlen) + 16;
+		macoff = netoff = TPACKET_ALIGN(po->tp_hdrlen) + 16 +
+				  po->tp_reserve;
 	} else {
 		unsigned maclen = skb_network_offset(skb);
 		netoff = TPACKET_ALIGN(po->tp_hdrlen +
-				       (maclen < 16 ? 16 : maclen));
+				       (maclen < 16 ? 16 : maclen)) +
+			po->tp_reserve;
 		macoff = netoff - maclen;
 	}
 
@@ -1448,6 +1451,19 @@ packet_setsockopt(struct socket *sock, int level, int optname, char __user *optv
 			return -EINVAL;
 		}
 	}
+	case PACKET_RESERVE:
+	{
+		unsigned int val;
+
+		if (optlen != sizeof(val))
+			return -EINVAL;
+		if (po->pg_vec)
+			return -EBUSY;
+		if (copy_from_user(&val, optval, sizeof(val)))
+			return -EFAULT;
+		po->tp_reserve = val;
+		return 0;
+	}
 #endif
 	case PACKET_AUXDATA:
 	{
@@ -1547,6 +1563,12 @@ static int packet_getsockopt(struct socket *sock, int level, int optname,
 		}
 		data = &val;
 		break;
+	case PACKET_RESERVE:
+		if (len > sizeof(unsigned int))
+			len = sizeof(unsigned int);
+		val = po->tp_reserve;
+		data = &val;
+		break;
 #endif
 	default:
 		return -ENOPROTOOPT;
@@ -1790,7 +1812,8 @@ static int packet_set_ring(struct sock *sk, struct tpacket_req *req, int closing
 			return -EINVAL;
 		if (unlikely(req->tp_block_size & (PAGE_SIZE - 1)))
 			return -EINVAL;
-		if (unlikely(req->tp_frame_size < po->tp_hdrlen))
+		if (unlikely(req->tp_frame_size < po->tp_hdrlen +
+						  po->tp_reserve))
 			return -EINVAL;
 		if (unlikely(req->tp_frame_size & (TPACKET_ALIGNMENT - 1)))
 			return -EINVAL;