net: packet: use sockaddr_ll fields as storage for skb original length in recvmsg path

As part of an effort to move skb->dropcount to skb->cb[], 4 bytes
of additional room are needed in skb->cb[] in packet sockets.

Store the skb original length in the first two fields of sockaddr_ll
(sll_family and sll_protocol) as they can be derived from the skb when
needed.

Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 24 insertions, 6 deletions

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 9c28cec1a083..7eea30b9c8e5 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -216,10 +216,16 @@ static void prb_fill_vlan_info(struct tpacket_kbdq_core *,
 static void packet_flush_mclist(struct sock *sk);
 
 struct packet_skb_cb {
-        unsigned int origlen;
         union {
                 struct sockaddr_pkt pkt;
-                struct sockaddr_ll ll;
+                union {
+                        /* Trick: alias skb original length with
+                         * ll.sll_family and ll.protocol in order
+                         * to save room.
+                         */
+                        unsigned int origlen;
+                        struct sockaddr_ll ll;
+                };
         } sa;
 };
 
@@ -1814,9 +1820,7 @@ static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
                      sizeof(skb->cb));
 
         sll = &PACKET_SKB_CB(skb)->sa.ll;
-        sll->sll_family = AF_PACKET;
         sll->sll_hatype = dev->type;
-        sll->sll_protocol = skb->protocol;
         sll->sll_pkttype = skb->pkt_type;
         if (unlikely(po->origdev))
                 sll->sll_ifindex = orig_dev->ifindex;
@@ -1825,7 +1829,10 @@ static int packet_rcv(struct sk_buff *skb, struct net_device *dev,
 
         sll->sll_halen = dev_parse_header(skb, sll->sll_addr);
 
-        PACKET_SKB_CB(skb)->origlen = skb->len;
+        /* sll->sll_family and sll->sll_protocol are set in packet_recvmsg().
+         * Use their space for storing the original skb length.
+         */
+        PACKET_SKB_CB(skb)->sa.origlen = skb->len;
 
         if (pskb_trim(skb, snaplen))
                 goto drop_n_acct;
@@ -2883,6 +2890,7 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
         struct sk_buff *skb;
         int copied, err;
         int vnet_hdr_len = 0;
+        unsigned int origlen = 0;
 
         err = -EINVAL;
         if (flags & ~(MSG_PEEK|MSG_DONTWAIT|MSG_TRUNC|MSG_CMSG_COMPAT|MSG_ERRQUEUE))
@@ -2982,6 +2990,15 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
         if (err)
                 goto out_free;
 
+        if (sock->type != SOCK_PACKET) {
+                struct sockaddr_ll *sll = &PACKET_SKB_CB(skb)->sa.ll;
+
+                /* Original length was stored in sockaddr_ll fields */
+                origlen = PACKET_SKB_CB(skb)->sa.origlen;
+                sll->sll_family = AF_PACKET;
+                sll->sll_protocol = skb->protocol;
+        }
+
         sock_recv_ts_and_drops(msg, sk, skb);
 
         if (msg->msg_name) {
@@ -2993,6 +3010,7 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
                         msg->msg_namelen = sizeof(struct sockaddr_pkt);
                 } else {
                         struct sockaddr_ll *sll = &PACKET_SKB_CB(skb)->sa.ll;
+
                         msg->msg_namelen = sll->sll_halen +
                                 offsetof(struct sockaddr_ll, sll_addr);
                 }
@@ -3006,7 +3024,7 @@ static int packet_recvmsg(struct kiocb *iocb, struct socket *sock,
                 aux.tp_status = TP_STATUS_USER;
                 if (skb->ip_summed == CHECKSUM_PARTIAL)
                         aux.tp_status |= TP_STATUS_CSUMNOTREADY;
-                aux.tp_len = PACKET_SKB_CB(skb)->origlen;
+                aux.tp_len = origlen;
                 aux.tp_snaplen = skb->len;
                 aux.tp_mac = 0;
                 aux.tp_net = skb_network_offset(skb);