Revert "bpf: Check for BPF_F_ADJ_ROOM_FIXED_GSO when bpf_skb_change_proto"

This reverts commit fa7b83bf.

See the followup commit for the reasoning why I believe the appropriate
approach is to simply make this change without a flag, but it can basically
be summarized as using this helper without the flag is bug-prone or outright
buggy, and thus the default should be this new behaviour.

As this commit has only made it into net-next/master, but not into
any real release, such a backwards incompatible change is still ok.
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/20210617000953.2787453-1-zenczykowski@gmail.com

net/core/filter.c

@@ -3235,7 +3235,7 @@ static int bpf_skb_net_hdr_pop(struct sk_buff *skb, u32 off, u32 len)
 	return ret;
 }
 
-static int bpf_skb_proto_4_to_6(struct sk_buff *skb, u64 flags)
+static int bpf_skb_proto_4_to_6(struct sk_buff *skb)
 {
 	const u32 len_diff = sizeof(struct ipv6hdr) - sizeof(struct iphdr);
 	u32 off = skb_mac_header_len(skb);
@@ -3264,9 +3264,7 @@ static int bpf_skb_proto_4_to_6(struct sk_buff *skb, u64 flags)
 		}
 
 		/* Due to IPv6 header, MSS needs to be downgraded. */
-		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
-			skb_decrease_gso_size(shinfo, len_diff);
-
+		skb_decrease_gso_size(shinfo, len_diff);
 		/* Header must be checked, and gso_segs recomputed. */
 		shinfo->gso_type |= SKB_GSO_DODGY;
 		shinfo->gso_segs = 0;
@@ -3278,7 +3276,7 @@ static int bpf_skb_proto_4_to_6(struct sk_buff *skb, u64 flags)
 	return 0;
 }
 
-static int bpf_skb_proto_6_to_4(struct sk_buff *skb, u64 flags)
+static int bpf_skb_proto_6_to_4(struct sk_buff *skb)
 {
 	const u32 len_diff = sizeof(struct ipv6hdr) - sizeof(struct iphdr);
 	u32 off = skb_mac_header_len(skb);
@@ -3307,9 +3305,7 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb, u64 flags)
 		}
 
 		/* Due to IPv4 header, MSS can be upgraded. */
-		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
-			skb_increase_gso_size(shinfo, len_diff);
-
+		skb_increase_gso_size(shinfo, len_diff);
 		/* Header must be checked, and gso_segs recomputed. */
 		shinfo->gso_type |= SKB_GSO_DODGY;
 		shinfo->gso_segs = 0;
@@ -3321,17 +3317,17 @@ static int bpf_skb_proto_6_to_4(struct sk_buff *skb, u64 flags)
 	return 0;
 }
 
-static int bpf_skb_proto_xlat(struct sk_buff *skb, __be16 to_proto, u64 flags)
+static int bpf_skb_proto_xlat(struct sk_buff *skb, __be16 to_proto)
 {
 	__be16 from_proto = skb->protocol;
 
 	if (from_proto == htons(ETH_P_IP) &&
 	      to_proto == htons(ETH_P_IPV6))
-		return bpf_skb_proto_4_to_6(skb, flags);
+		return bpf_skb_proto_4_to_6(skb);
 
 	if (from_proto == htons(ETH_P_IPV6) &&
 	      to_proto == htons(ETH_P_IP))
-		return bpf_skb_proto_6_to_4(skb, flags);
+		return bpf_skb_proto_6_to_4(skb);
 
 	return -ENOTSUPP;
 }
@@ -3341,7 +3337,7 @@ BPF_CALL_3(bpf_skb_change_proto, struct sk_buff *, skb, __be16, proto,
 {
 	int ret;
 
-	if (unlikely(flags & ~(BPF_F_ADJ_ROOM_FIXED_GSO)))
+	if (unlikely(flags))
 		return -EINVAL;
 
 	/* General idea is that this helper does the basic groundwork
@@ -3361,7 +3357,7 @@ BPF_CALL_3(bpf_skb_change_proto, struct sk_buff *, skb, __be16, proto,
 	 * that. For offloads, we mark packet as dodgy, so that headers
 	 * need to be verified first.
 	 */
-	ret = bpf_skb_proto_xlat(skb, proto, flags);
+	ret = bpf_skb_proto_xlat(skb, proto);
 	bpf_compute_data_pointers(skb);
 	return ret;
 }