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, 14 insertions, 5 deletions

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 56db472e9b86..65a3798f43e6 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3710,19 +3710,28 @@ void __skb_tstamp_tx(struct sk_buff *orig_skb,
                      struct sock *sk, int tstype)
 {
         struct sk_buff *skb;
+        bool tsonly = sk->sk_tsflags & SOF_TIMESTAMPING_OPT_TSONLY;
 
         if (!sk)
                 return;
 
-        if (hwtstamps)
-                *skb_hwtstamps(orig_skb) = *hwtstamps;
+        if (tsonly)
+                skb = alloc_skb(0, GFP_ATOMIC);
         else
-                orig_skb->tstamp = ktime_get_real();
-
-        skb = skb_clone(orig_skb, GFP_ATOMIC);
+                skb = skb_clone(orig_skb, GFP_ATOMIC);
         if (!skb)
                 return;
 
+        if (tsonly) {
+                skb_shinfo(skb)->tx_flags = skb_shinfo(orig_skb)->tx_flags;
+                skb_shinfo(skb)->tskey = skb_shinfo(orig_skb)->tskey;
+        }
+
+        if (hwtstamps)
+                *skb_hwtstamps(skb) = *hwtstamps;
+        else
+                skb->tstamp = ktime_get_real();
+
         __skb_complete_tx_timestamp(skb, sk, tstype);
 }
 EXPORT_SYMBOL_GPL(__skb_tstamp_tx);