bpf: clear decap state on skb_adjust_room shrink path

commit-author Nick Hudson <nhudson@akamai.com>
commit -
commit-source https://lore.kernel.org/netdev/20260504101759.3319427-6-nhudson@akamai.com/

On shrink in bpf_skb_adjust_room(), apply decapsulation state updates
according to BPF_F_ADJ_ROOM_DECAP_* flags.

For GSO skbs, clear only the tunnel gso_type bits that correspond to the
requested decap layer:
- DECAP_L4_UDP: SKB_GSO_UDP_TUNNEL{,_CSUM}
- DECAP_L4_GRE: SKB_GSO_GRE{,_CSUM}
- DECAP_IPXIP4: SKB_GSO_IPXIP4
- DECAP_IPXIP6: SKB_GSO_IPXIP6

Then clear skb->encapsulation only if no tunnel GSO bits remain, keeping
encapsulation set for cases such as ESP-in-UDP where tunnel state remains.

For non-GSO skbs, there are no tunnel GSO bits to consult, so clear
skb->encapsulation directly when DECAP_L4_* or DECAP_IPXIP_* flags are set.

This keeps decap state handling consistent between GSO and non-GSO packets.

	Co-developed-by: Max Tottenham <mtottenh@akamai.com>
	Signed-off-by: Max Tottenham <mtottenh@akamai.com>
	Co-developed-by: Anna Glasgall <aglasgal@akamai.com>
	Signed-off-by: Anna Glasgall <aglasgal@akamai.com>
	Signed-off-by: Nick Hudson <nhudson@akamai.com>
	Signed-off-by: Brett Mastbergen <bmastbergen@ciq.com>

Author: bmastbergen

net/core/filter.c

@@ -3596,9 +3596,48 @@ static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
 		if (!(flags & BPF_F_ADJ_ROOM_FIXED_GSO))
 			skb_increase_gso_size(shinfo, len_diff);
 
+		/* Selective GSO flag clearing based on decap type.
+		 * Only clear the flags for the tunnel layer being removed.
+		 */
+		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_UDP) &&
+		    (shinfo->gso_type & (SKB_GSO_UDP_TUNNEL |
+					 SKB_GSO_UDP_TUNNEL_CSUM)))
+			shinfo->gso_type &= ~(SKB_GSO_UDP_TUNNEL |
+					      SKB_GSO_UDP_TUNNEL_CSUM);
+		if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_GRE) &&
+		    (shinfo->gso_type & (SKB_GSO_GRE | SKB_GSO_GRE_CSUM)))
+			shinfo->gso_type &= ~(SKB_GSO_GRE |
+					      SKB_GSO_GRE_CSUM);
+		if ((flags & BPF_F_ADJ_ROOM_DECAP_IPXIP4) &&
+		    (shinfo->gso_type & SKB_GSO_IPXIP4))
+			shinfo->gso_type &= ~SKB_GSO_IPXIP4;
+		if ((flags & BPF_F_ADJ_ROOM_DECAP_IPXIP6) &&
+		    (shinfo->gso_type & SKB_GSO_IPXIP6))
+			shinfo->gso_type &= ~SKB_GSO_IPXIP6;
+
+		/* Clear encapsulation flag only when no tunnel GSO flags remain */
+		if (flags & (BPF_F_ADJ_ROOM_DECAP_L4_MASK |
+			     BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK)) {
+			if (!(shinfo->gso_type & (SKB_GSO_UDP_TUNNEL |
+						  SKB_GSO_UDP_TUNNEL_CSUM |
+						  SKB_GSO_GRE |
+						  SKB_GSO_GRE_CSUM |
+						  SKB_GSO_IPXIP4 |
+						  SKB_GSO_IPXIP6 |
+						  SKB_GSO_ESP)))
+				if (skb->encapsulation)
+					skb->encapsulation = 0;
+		}
+
 		/* Header must be checked, and gso_segs recomputed. */
 		shinfo->gso_type |= SKB_GSO_DODGY;
 		shinfo->gso_segs = 0;
+	} else {
+		/* For non-GSO packets, clear encapsulation if decap flags are set */
+		if ((flags & (BPF_F_ADJ_ROOM_DECAP_L4_MASK |
+			      BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK)) &&
+		    skb->encapsulation)
+			skb->encapsulation = 0;
 	}
 
 	return 0;