[PATCH net-next v2] packet: tx_ring: allow the user to choose tx data offset

@@ -163,6 +163,19 @@ As capture, each frame contains two parts:
 
  A complete tutorial is available at: http://wiki.gnu-log.net/
 
+By default, the user should put data at :
+ frame base + TPACKET_HDRLEN - sizeof(struct sockaddr_ll)
+
+So, whatever you choose for the socket mode (SOCK_DGRAM or SOCK_RAW),
+the beginning of the user data will be at :
+ frame base + TPACKET_ALIGN(sizeof(struct tpacket_hdr))
+
+If you wish to put user data at a custom offset from the beginning of
+the frame (for payload alignment with SOCK_RAW mode for instance) you
+can set tp_net (with SOCK_DGRAM) or tp_mac (with SOCK_RAW). In order
+to make this work it must be enabled previously with setsockopt()
+and the PACKET_TX_HAS_OFF option.
+
 --------------------------------------------------------------------------------
 + PACKET_MMAP settings
 --------------------------------------------------------------------------------

@@ -50,6 +50,7 @@ struct sockaddr_ll {
 #define PACKET_TX_TIMESTAMP		16
 #define PACKET_TIMESTAMP		17
 #define PACKET_FANOUT			18
+#define PACKET_TX_HAS_OFF		19
 
 #define PACKET_FANOUT_HASH		0
 #define PACKET_FANOUT_LB		1

@@ -1881,7 +1881,38 @@ static int tpacket_fill_skb(struct packet_sock *po, struct sk_buff *skb,
 	skb_reserve(skb, hlen);
 	skb_reset_network_header(skb);
 
-	data = ph.raw + po->tp_hdrlen - sizeof(struct sockaddr_ll);
+	if (po->tp_tx_has_off) {
+		int off_min, off_max, off;
+		off_min = po->tp_hdrlen - sizeof(struct sockaddr_ll);
+		off_max = po->tx_ring.frame_size - tp_len;
+		if (sock->type == SOCK_DGRAM) {
+			switch (po->tp_version) {
+			case TPACKET_V2:
+				off = ph.h2->tp_net;
+				break;
+			default:
+				off = ph.h1->tp_net;
+				break;
+			}
+		} else {
+			switch (po->tp_version) {
+			case TPACKET_V2:
+				off = ph.h2->tp_mac;
+				break;
+			default:
+				off = ph.h1->tp_mac;
+				break;
+			}
+		}
+		if (unlikely((off < off_min) || (off_max < off))) {
+			pr_err("payload offset (%d) out of range [%d;%d]\n",
+				off, off_min, off_max);
+			return -EINVAL;
+		}
+		data = ph.raw + off;
+	} else {
+		data = ph.raw + po->tp_hdrlen - sizeof(struct sockaddr_ll);
+	}
 	to_write = tp_len;
 
 	if (sock->type == SOCK_DGRAM) {

@@ -3111,6 +3142,19 @@ packet_setsockopt(struct socket *sock, int level, int optname, char __user *optv
 
 		return fanout_add(sk, val & 0xffff, val >> 16);
 	}
+	case PACKET_TX_HAS_OFF:
+	{
+		unsigned int val;
+
+		if (optlen != sizeof(val))
+			return -EINVAL;
+		if (po->rx_ring.pg_vec || po->tx_ring.pg_vec)
+			return -EBUSY;
+		if (copy_from_user(&val, optval, sizeof(val)))
+			return -EFAULT;
+		po->tp_tx_has_off = !!val;
+		return 0;
+	}
 	default:
 		return -ENOPROTOOPT;
 	}

@@ -3202,6 +3246,9 @@ static int packet_getsockopt(struct socket *sock, int level, int optname,
 			((u32)po->fanout->type << 16)) :
 		       0);
 		break;
+	case PACKET_TX_HAS_OFF:
+		val = po->tp_tx_has_off;
+		break;
 	default:
 		return -ENOPROTOOPT;
 	}

@@ -110,6 +110,7 @@ struct packet_sock {
 	unsigned int		tp_reserve;
 	unsigned int		tp_loss:1;
 	unsigned int		tp_tstamp;
+	unsigned int		tp_tx_has_off:1;
 	struct packet_type	prot_hook ____cacheline_aligned_in_smp;
 };