bpf/bpf_get,set_sockopt: add option to set TCP-BPF sock ops flags

Currently the only opportunity to set sock ops flags dictating
which callbacks fire for a socket is from within a TCP-BPF sockops
program. This is problematic if the connection is already set up
as there is no further chance to specify callbacks for that socket.
Add TCP_BPF_SOCK_OPS_CB_FLAGS to bpf_setsockopt() and bpf_getsockopt()
to allow users to specify callbacks later, either via an iterator
over sockets or via a socket-specific program triggered by a
setsockopt() on the socket.

Previous discussion on this here [1].

[1] https://lore.kernel.org/bpf/[e-mail]/

Signed-off-by: Alan Maguire <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Martin KaFai Lau <[e-mail]>

include/uapi/linux/bpf.h

···
  * 		  **TCP_SYNCNT**, **TCP_USER_TIMEOUT**, **TCP_NOTSENT_LOWAT**,
  * 		  **TCP_NODELAY**, **TCP_MAXSEG**, **TCP_WINDOW_CLAMP**,
  * 		  **TCP_THIN_LINEAR_TIMEOUTS**, **TCP_BPF_DELACK_MAX**,
- * 		  **TCP_BPF_RTO_MIN**.
+ *		  **TCP_BPF_RTO_MIN**, **TCP_BPF_SOCK_OPS_CB_FLAGS**.
  * 		* **IPPROTO_IP**, which supports *optname* **IP_TOS**.
  * 		* **IPPROTO_IPV6**, which supports the following *optname*\ s:
  * 		  **IPV6_TCLASS**, **IPV6_AUTOFLOWLABEL**.
···
 	TCP_BPF_SYN		= 1005, /* Copy the TCP header */
 	TCP_BPF_SYN_IP		= 1006, /* Copy the IP[46] and TCP header */
 	TCP_BPF_SYN_MAC         = 1007, /* Copy the MAC, IP[46], and TCP header */
+	TCP_BPF_SOCK_OPS_CB_FLAGS = 1008, /* Get or Set TCP sock ops flags */
 };
 
 enum {

net/core/filter.c

···
 			return -EINVAL;
 		inet_csk(sk)->icsk_rto_min = timeout;
 		break;
+	case TCP_BPF_SOCK_OPS_CB_FLAGS:
+		if (val & ~(BPF_SOCK_OPS_ALL_CB_FLAGS))
+			return -EINVAL;
+		tp->bpf_sock_ops_cb_flags = val;
+		break;
 	default:
 		return -EINVAL;
 	}
···
 		if (*optlen < 1)
 			return -EINVAL;
 		break;
+	case TCP_BPF_SOCK_OPS_CB_FLAGS:
+		if (*optlen != sizeof(int))
+			return -EINVAL;
+		if (getopt) {
+			struct tcp_sock *tp = tcp_sk(sk);
+			int cb_flags = tp->bpf_sock_ops_cb_flags;
+
+			memcpy(optval, &cb_flags, *optlen);
+			return 0;
+		}
+		return bpf_sol_tcp_setsockopt(sk, optname, optval, *optlen);
 	default:
 		if (getopt)
 			return -EINVAL;

tools/include/uapi/linux/bpf.h

···
  * 		  **TCP_SYNCNT**, **TCP_USER_TIMEOUT**, **TCP_NOTSENT_LOWAT**,
  * 		  **TCP_NODELAY**, **TCP_MAXSEG**, **TCP_WINDOW_CLAMP**,
  * 		  **TCP_THIN_LINEAR_TIMEOUTS**, **TCP_BPF_DELACK_MAX**,
- * 		  **TCP_BPF_RTO_MIN**.
+ *		  **TCP_BPF_RTO_MIN**, **TCP_BPF_SOCK_OPS_CB_FLAGS**.
  * 		* **IPPROTO_IP**, which supports *optname* **IP_TOS**.
  * 		* **IPPROTO_IPV6**, which supports the following *optname*\ s:
  * 		  **IPV6_TCLASS**, **IPV6_AUTOFLOWLABEL**.
···
 	TCP_BPF_SYN		= 1005, /* Copy the TCP header */
 	TCP_BPF_SYN_IP		= 1006, /* Copy the IP[46] and TCP header */
 	TCP_BPF_SYN_MAC         = 1007, /* Copy the MAC, IP[46], and TCP header */
+	TCP_BPF_SOCK_OPS_CB_FLAGS = 1008, /* Get or Set TCP sock ops flags */
 };
 
 enum {