bpf: tcp: Support bpf_(get|set)sockopt in bpf tcp iter

This patch allows bpf tcp iter to call bpf_(get|set)sockopt.
To allow a specific bpf iter (tcp here) to call a set of helpers,
get_func_proto function pointer is added to bpf_iter_reg.
The bpf iter is a tracing prog which currently requires
CAP_PERFMON or CAP_SYS_ADMIN, so this patch does not
impose other capability checks for bpf_(get|set)sockopt.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210701200619.1036715-1-kafai@fb.com

net/ipv4/tcp_ipv4.c

@@ -3259,6 +3259,20 @@ static const struct bpf_iter_seq_info tcp_seq_info = {
 	.seq_priv_size		= sizeof(struct bpf_tcp_iter_state),
 };
 
+static const struct bpf_func_proto *
+bpf_iter_tcp_get_func_proto(enum bpf_func_id func_id,
+			    const struct bpf_prog *prog)
+{
+	switch (func_id) {
+	case BPF_FUNC_setsockopt:
+		return &bpf_sk_setsockopt_proto;
+	case BPF_FUNC_getsockopt:
+		return &bpf_sk_getsockopt_proto;
+	default:
+		return NULL;
+	}
+}
+
 static struct bpf_iter_reg tcp_reg_info = {
 	.target			= "tcp",
 	.ctx_arg_info_size	= 1,
@@ -3266,6 +3280,7 @@ static struct bpf_iter_reg tcp_reg_info = {
 		{ offsetof(struct bpf_iter__tcp, sk_common),
 		  PTR_TO_BTF_ID_OR_NULL },
 	},
+	.get_func_proto		= bpf_iter_tcp_get_func_proto,
 	.seq_info		= &tcp_seq_info,
 };