bpf: Require only one of cong_avoid() and cong_control() from a TCP CC

Remove the check for required and optional functions in a struct
tcp_congestion_ops from bpf_tcp_ca.c. Rely on
tcp_register_congestion_control() to reject a BPF CC that does not
implement all required functions, as it will do for a non-BPF CC.

When a CC implements tcp_congestion_ops.cong_control(), the alternate
cong_avoid() is not in use in the TCP stack. Previously, a BPF CC was
still forced to implement cong_avoid() as a no-op since it was
non-optional in bpf_tcp_ca.c.
Signed-off-by: Jörn-Thorben Hinz <jthinz@mailbox.tu-berlin.de>
Reviewed-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/r/20220622191227.898118-3-jthinz@mailbox.tu-berlin.deSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/bpf_struct_ops.c

@@ -503,10 +503,9 @@ static int bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
 		goto unlock;
 	}
 
-	/* Error during st_ops->reg().  It is very unlikely since
-	 * the above init_member() should have caught it earlier
-	 * before reg().  The only possibility is if there was a race
-	 * in registering the struct_ops (under the same name) to
+	/* Error during st_ops->reg(). Can happen if this struct_ops needs to be
+	 * verified as a whole, after all init_member() calls. Can also happen if
+	 * there was a race in registering the struct_ops (under the same name) to
 	 * a sub-system through different struct_ops's maps.
 	 */
 	set_memory_nx((long)st_map->image, 1);

net/ipv4/bpf_tcp_ca.c

@@ -14,18 +14,6 @@
 /* "extern" is to avoid sparse warning.  It is only used in bpf_struct_ops.c. */
 extern struct bpf_struct_ops bpf_tcp_congestion_ops;
 
-static u32 optional_ops[] = {
-	offsetof(struct tcp_congestion_ops, init),
-	offsetof(struct tcp_congestion_ops, release),
-	offsetof(struct tcp_congestion_ops, set_state),
-	offsetof(struct tcp_congestion_ops, cwnd_event),
-	offsetof(struct tcp_congestion_ops, in_ack_event),
-	offsetof(struct tcp_congestion_ops, pkts_acked),
-	offsetof(struct tcp_congestion_ops, min_tso_segs),
-	offsetof(struct tcp_congestion_ops, sndbuf_expand),
-	offsetof(struct tcp_congestion_ops, cong_control),
-};
-
 static u32 unsupported_ops[] = {
 	offsetof(struct tcp_congestion_ops, get_info),
 };
@@ -51,18 +39,6 @@ static int bpf_tcp_ca_init(struct btf *btf)
 	return 0;
 }
 
-static bool is_optional(u32 member_offset)
-{
-	unsigned int i;
-
-	for (i = 0; i < ARRAY_SIZE(optional_ops); i++) {
-		if (member_offset == optional_ops[i])
-			return true;
-	}
-
-	return false;
-}
-
 static bool is_unsupported(u32 member_offset)
 {
 	unsigned int i;
@@ -246,7 +222,6 @@ static int bpf_tcp_ca_init_member(const struct btf_type *t,
 {
 	const struct tcp_congestion_ops *utcp_ca;
 	struct tcp_congestion_ops *tcp_ca;
-	int prog_fd;
 	u32 moff;
 
 	utcp_ca = (const struct tcp_congestion_ops *)udata;
@@ -268,14 +243,6 @@ static int bpf_tcp_ca_init_member(const struct btf_type *t,
 		return 1;
 	}
 
-	if (!btf_type_resolve_func_ptr(btf_vmlinux, member->type, NULL))
-		return 0;
-
-	/* Ensure bpf_prog is provided for compulsory func ptr */
-	prog_fd = (int)(*(unsigned long *)(udata + moff));
-	if (!prog_fd && !is_optional(moff) && !is_unsupported(moff))
-		return -EINVAL;
-
 	return 0;
 }