bpf: Support all gso types in bpf_skb_change_proto()

Since we no longer modify gso_size, it is now theoretically
safe to not set SKB_GSO_DODGY and reset gso_segs to zero.

This also means the skb_is_gso_tcp() check should no longer
be necessary.

Unfortunately we cannot remove the skb_{decrease,increase}_gso_size()
helpers, as they are still used elsewhere:

  bpf_skb_net_grow() without BPF_F_ADJ_ROOM_FIXED_GSO
  bpf_skb_net_shrink() without BPF_F_ADJ_ROOM_FIXED_GSO
  net/core/lwt_bpf.c's handle_gso_type()
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Dongseok Yi <dseok.yi@samsung.com>
Cc: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/bpf/20210617000953.2787453-3-zenczykowski@gmail.com

net/core/filter.c

@@ -3241,9 +3241,6 @@ static int bpf_skb_proto_4_to_6(struct sk_buff *skb)
 	u32 off = skb_mac_header_len(skb);
 	int ret;
 
-	if (skb_is_gso(skb) && !skb_is_gso_tcp(skb))
-		return -ENOTSUPP;
-
 	ret = skb_cow(skb, len_diff);
 	if (unlikely(ret < 0))
 		return ret;
@@ -3255,17 +3252,11 @@ static int bpf_skb_proto_4_to_6(struct sk_buff *skb)
 	if (skb_is_gso(skb)) {
 		struct skb_shared_info *shinfo = skb_shinfo(skb);
 
-		/* SKB_GSO_TCPV4 needs to be changed into
-		 * SKB_GSO_TCPV6.
-		 */
+		/* SKB_GSO_TCPV4 needs to be changed into SKB_GSO_TCPV6. */
 		if (shinfo->gso_type & SKB_GSO_TCPV4) {
 			shinfo->gso_type &= ~SKB_GSO_TCPV4;
 			shinfo->gso_type |=  SKB_GSO_TCPV6;
 		}
-
-		/* Header must be checked, and gso_segs recomputed. */
-		shinfo->gso_type |= SKB_GSO_DODGY;
-		shinfo->gso_segs = 0;
 	}
 
 	skb->protocol = htons(ETH_P_IPV6);
@@ -3280,9 +3271,6 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
 	u32 off = skb_mac_header_len(skb);
 	int ret;
 
-	if (skb_is_gso(skb) && !skb_is_gso_tcp(skb))
-		return -ENOTSUPP;
-
 	ret = skb_unclone(skb, GFP_ATOMIC);
 	if (unlikely(ret < 0))
 		return ret;
@@ -3294,17 +3282,11 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
 	if (skb_is_gso(skb)) {
 		struct skb_shared_info *shinfo = skb_shinfo(skb);
 
-		/* SKB_GSO_TCPV6 needs to be changed into
-		 * SKB_GSO_TCPV4.
-		 */
+		/* SKB_GSO_TCPV6 needs to be changed into SKB_GSO_TCPV4. */
 		if (shinfo->gso_type & SKB_GSO_TCPV6) {
 			shinfo->gso_type &= ~SKB_GSO_TCPV6;
 			shinfo->gso_type |=  SKB_GSO_TCPV4;
 		}
-
-		/* Header must be checked, and gso_segs recomputed. */
-		shinfo->gso_type |= SKB_GSO_DODGY;
-		shinfo->gso_segs = 0;
 	}
 
 	skb->protocol = htons(ETH_P_IP);