net-timestamp: no-payload option

Add timestamping option SOF_TIMESTAMPING_OPT_TSONLY. For transmit
timestamps, this loops timestamps on top of empty packets.

Doing so reduces the pressure on SO_RCVBUF. Payload inspection and
cmsg reception (aside from timestamps) are no longer possible. This
works together with a follow on patch that allows administrators to
only allow tx timestamping if it does not loop payload or metadata.

Signed-off-by: Willem de Bruijn <willemb@google.com>

----

Changes (rfc -> v1)
  - add documentation
  - remove unnecessary skb->len test (thanks to Richard Cochran)
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 3 deletions

diff --git a/net/ipv6/datagram.c b/net/ipv6/datagram.c
index 49f5e73db122..c215be70cac0 100644
--- a/net/ipv6/datagram.c
+++ b/net/ipv6/datagram.c
@@ -369,7 +369,7 @@ int ipv6_recv_error(struct sock *sk, struct msghdr *msg, int len, int *addr_len)
 
         serr = SKB_EXT_ERR(skb);
 
-        if (sin) {
+        if (sin && skb->len) {
                 const unsigned char *nh = skb_network_header(skb);
                 sin->sin6_family = AF_INET6;
                 sin->sin6_flowinfo = 0;
@@ -394,8 +394,7 @@ int ipv6_recv_error(struct sock *sk, struct msghdr *msg, int len, int *addr_len)
         memcpy(&errhdr.ee, &serr->ee, sizeof(struct sock_extended_err));
         sin = &errhdr.offender;
         memset(sin, 0, sizeof(*sin));
-
-        if (serr->ee.ee_origin != SO_EE_ORIGIN_LOCAL) {
+        if (serr->ee.ee_origin != SO_EE_ORIGIN_LOCAL && skb->len) {
                 sin->sin6_family = AF_INET6;
                 if (np->rxopt.all) {
                         if (serr->ee.ee_origin != SO_EE_ORIGIN_ICMP &&