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, 1 deletions

diff --git a/include/uapi/linux/net_tstamp.h b/include/uapi/linux/net_tstamp.h
index edbc888ceb51..6d1abea9746e 100644
--- a/include/uapi/linux/net_tstamp.h
+++ b/include/uapi/linux/net_tstamp.h
@@ -24,8 +24,9 @@ enum {
         SOF_TIMESTAMPING_TX_SCHED = (1<<8),
         SOF_TIMESTAMPING_TX_ACK = (1<<9),
         SOF_TIMESTAMPING_OPT_CMSG = (1<<10),
+        SOF_TIMESTAMPING_OPT_TSONLY = (1<<11),
 
-        SOF_TIMESTAMPING_LAST = SOF_TIMESTAMPING_OPT_CMSG,
+        SOF_TIMESTAMPING_LAST = SOF_TIMESTAMPING_OPT_TSONLY,
         SOF_TIMESTAMPING_MASK = (SOF_TIMESTAMPING_LAST - 1) |
                                  SOF_TIMESTAMPING_LAST
 };