[PATCH RFC net-next 6/9] bpf-timestamping: restore skb hwtstamp if it is used by start time

[Jason Xing <kerneljasonxing@gmail.com>]

When BPF Timestamping V1 (SKBTX_BPF) and V2 are both active,
skb_tstamp_tx_report_bpf_timestamping() writes the hardware
timestamp into orig_skb:

    *skb_hwtstamps(skb) = *hwtstamps;

Since orig_skb shares skb_shared_info with the retransmit-queue
original via skb_clone(), this clobbers V2's start_time on the
retransmit-queue skb.

Save start_time before the write and restore it after the V1 callback
returns, so both timestamping generations can coexist.

Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
---
 net/core/skbuff.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 44aeea713da7..98cd813006b8 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -5686,6 +5686,7 @@ static void skb_tstamp_tx_report_bpf_timestamping(struct sk_buff *skb,
 						  struct sock *sk,
 						  int tstype)
 {
+	ktime_t start = 0;
 	int op;
 
 	switch (tstype) {
@@ -5695,6 +5696,8 @@ static void skb_tstamp_tx_report_bpf_timestamping(struct sk_buff *skb,
 	case SCM_TSTAMP_SND:
 		if (hwtstamps) {
 			op = BPF_SOCK_OPS_TSTAMP_SND_HW_CB;
+			if (static_branch_unlikely(&bpfts_v2_needed_key))
+				start = skb_hwtstamps(skb)->start_time;
 			*skb_hwtstamps(skb) = *hwtstamps;
 		} else {
 			op = BPF_SOCK_OPS_TSTAMP_SND_SW_CB;
@@ -5708,6 +5711,8 @@ static void skb_tstamp_tx_report_bpf_timestamping(struct sk_buff *skb,
 	}
 
 	bpf_skops_tx_timestamping(sk, skb, op);
+	if (static_branch_unlikely(&bpfts_v2_needed_key) && start)
+		skb_hwtstamps(skb)->start_time = start;
 }
 
 void __skb_tstamp_tx(struct sk_buff *orig_skb,
-- 
2.43.7