bpf: Add BPF_SOCK_OPS_TSTAMP_SND_SW_CB callback

Support sw SCM_TSTAMP_SND case for bpf timestamping.

Add a new sock_ops callback, BPF_SOCK_OPS_TSTAMP_SND_SW_CB. This
callback will occur at the same timestamping point as the user
space's software SCM_TSTAMP_SND. The BPF program can use it to
get the same SCM_TSTAMP_SND timestamp without modifying the
user-space application.

Based on this patch, BPF program will get the software
timestamp when the driver is ready to send the skb. In the
sebsequent patch, the hardware timestamp will be supported.

Signed-off-by: Jason Xing <kerneljasonxing@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250220072940.99994-8-kerneljasonxing@gmail.com

include/linux/skbuff.h

@@ -4568,7 +4568,7 @@ void skb_tstamp_tx(struct sk_buff *orig_skb,
 static inline void skb_tx_timestamp(struct sk_buff *skb)
 {
 	skb_clone_tx_timestamp(skb);
-	if (skb_shinfo(skb)->tx_flags & SKBTX_SW_TSTAMP)
+	if (skb_shinfo(skb)->tx_flags & (SKBTX_SW_TSTAMP | SKBTX_BPF))
 		skb_tstamp_tx(skb, NULL);
 }
 

include/uapi/linux/bpf.h

@@ -7036,6 +7036,10 @@ enum {
 					 * SK_BPF_CB_TX_TIMESTAMPING
 					 * feature is on.
 					 */
+	BPF_SOCK_OPS_TSTAMP_SND_SW_CB,	/* Called when skb is about to send
+					 * to the nic when SK_BPF_CB_TX_TIMESTAMPING
+					 * feature is on.
+					 */
 };
 
 /* List of TCP states. There is a build check in net/ipv4/tcp.c to detect

net/core/skbuff.c

@@ -5557,6 +5557,7 @@ static bool skb_tstamp_tx_report_so_timestamping(struct sk_buff *skb,
 }
 
 static void skb_tstamp_tx_report_bpf_timestamping(struct sk_buff *skb,
+						  struct skb_shared_hwtstamps *hwtstamps,
 						  struct sock *sk,
 						  int tstype)
 {
@@ -5566,6 +5567,11 @@ static void skb_tstamp_tx_report_bpf_timestamping(struct sk_buff *skb,
 	case SCM_TSTAMP_SCHED:
 		op = BPF_SOCK_OPS_TSTAMP_SCHED_CB;
 		break;
+	case SCM_TSTAMP_SND:
+		if (hwtstamps)
+			return;
+		op = BPF_SOCK_OPS_TSTAMP_SND_SW_CB;
+		break;
 	default:
 		return;
 	}
@@ -5586,7 +5592,8 @@ void __skb_tstamp_tx(struct sk_buff *orig_skb,
 		return;
 
 	if (skb_shinfo(orig_skb)->tx_flags & SKBTX_BPF)
-		skb_tstamp_tx_report_bpf_timestamping(orig_skb, sk, tstype);
+		skb_tstamp_tx_report_bpf_timestamping(orig_skb, hwtstamps,
+						      sk, tstype);
 
 	if (!skb_tstamp_tx_report_so_timestamping(orig_skb, hwtstamps, tstype))
 		return;

tools/include/uapi/linux/bpf.h

@@ -7036,6 +7036,10 @@ enum {
 					 * SK_BPF_CB_TX_TIMESTAMPING
 					 * feature is on.
 					 */
+	BPF_SOCK_OPS_TSTAMP_SND_SW_CB,	/* Called when skb is about to send
+					 * to the nic when SK_BPF_CB_TX_TIMESTAMPING
+					 * feature is on.
+					 */
 };
 
 /* List of TCP states. There is a build check in net/ipv4/tcp.c to detect