sock: enable timestamping using control messages

Currently, SOL_TIMESTAMPING can only be enabled using setsockopt.
This is very costly when users want to sample writes to gather
tx timestamps.

Add support for enabling SO_TIMESTAMPING via control messages by
using tsflags added in `struct sockcm_cookie` (added in the previous
patches in this series) to set the tx_flags of the last skb created in
a sendmsg. With this patch, the timestamp recording bits in tx_flags
of the skbuff is overridden if SO_TIMESTAMPING is passed in a cmsg.

Please note that this is only effective for overriding the recording
timestamps flags. Users should enable timestamp reporting (e.g.,
SOF_TIMESTAMPING_SOFTWARE | SOF_TIMESTAMPING_OPT_ID) using
socket options and then should ask for SOF_TIMESTAMPING_TX_*
using control messages per sendmsg to sample timestamps for each
write.

Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 3 deletions

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index bccb4e11047a..45ff590661f4 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1027,12 +1027,11 @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
                  */
                 connected = 1;
         }
-        ipc.addr = inet->inet_saddr;
 
+        ipc.sockc.tsflags = sk->sk_tsflags;
+        ipc.addr = inet->inet_saddr;
         ipc.oif = sk->sk_bound_dev_if;
 
-        sock_tx_timestamp(sk, &ipc.tx_flags);
-
         if (msg->msg_controllen) {
                 err = ip_cmsg_send(sk, msg, &ipc, sk->sk_family == AF_INET6);
                 if (unlikely(err)) {
@@ -1059,6 +1058,8 @@ int udp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
         saddr = ipc.addr;
         ipc.addr = faddr = daddr;
 
+        sock_tx_timestamp(sk, ipc.sockc.tsflags, &ipc.tx_flags);
+
         if (ipc.opt && ipc.opt->opt.srr) {
                 if (!daddr)
                         return -EINVAL;