[PATCH v3 net] tcp: preserve timestamps across receive queue collapse

[Jason Xing <kerneljasonxing@gmail.com>]

When tcp_collapse() rebuilds skbs under memory pressure, the copy
process doesn't include the right tstamp and hwtstamp from the
old skb. And memcpy(nskb->cb, skb->cb, ...) copies has_rxtstamp,
but nskb->tstamp and hwtstamps are left at zero, so
tcp_recv_timestamp() ends up emitting no cmsg at all.

In net timestamping case, if such an skb happens to be the last
one consumed in a recvmsg() call, the application receives no RX
timestamp for that call.

Fix this by copying both tstamp and hwtstamp of the last skb to
the new skb, matching tcp_try_coalesce()/tcp_add_backlog().

Note that the has_rxtstamp flag can still be inherited through
the cb memcpy from an skb that contributes no bytes (fully covered
skb left in the ofo tree by the tcp_ooo_try_coalesce() ->
coalesce_done path), so set TCP_SKB_CB(nskb)->has_rxtstamp to false
which makes the new block the only place setting it.

Fixes: 98aaa913b4ed ("tcp: Extend SOF_TIMESTAMPING_RX_SOFTWARE to TCP recvmsg")
Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
---
V3
Link: https://lore.kernel.org/all/20260917132742.87117-1-kerneljasonxing@gmail.com/
1. revise the commit message with more accurate description (Eric)
2. fix a corner case (where a skb can contribute no bytes if OOO happens)
   spotted by AI and Eric

V2
Link: https://lore.kernel.org/all/20260916124153.43457-1-kerneljasonxing@gmail.com/
1. add one sentence in commit log from Eric's reply
2. use the same semantic as tcp_add_backlog which is to sync the
   last skb (Eric)
---
 net/ipv4/tcp_input.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 92bc60716f33..f58954ddecee 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5834,6 +5834,7 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
 			break;
 
 		memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
+		TCP_SKB_CB(nskb)->has_rxtstamp = false;
 		skb_copy_decrypted(nskb, skb);
 		TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(nskb)->end_seq = start;
 		if (list)
@@ -5854,6 +5855,12 @@ tcp_collapse(struct sock *sk, struct sk_buff_head *list, struct rb_root *root,
 				if (skb_copy_bits(skb, offset, skb_put(nskb, size), size))
 					BUG();
 				TCP_SKB_CB(nskb)->end_seq += size;
+				if (TCP_SKB_CB(skb)->has_rxtstamp) {
+					TCP_SKB_CB(nskb)->has_rxtstamp = true;
+					nskb->tstamp = skb->tstamp;
+					skb_hwtstamps(nskb)->hwtstamp =
+						skb_hwtstamps(skb)->hwtstamp;
+				}
 				copy -= size;
 				start += size;
 			}
-- 
2.43.7