there are pointers in __sk_buff

[QiongwenXu]

struct __sk_buff {
	__u32 len;
	__u32 pkt_type;
	__u32 mark;
	__u32 queue_mapping;
	__u32 protocol;
	__u32 vlan_present;
	__u32 vlan_tci;
	__u32 vlan_proto;
	__u32 priority;
	__u32 ingress_ifindex;
	__u32 ifindex;
	__u32 tc_index;
	__u32 cb[5];
	__u32 hash;
	__u32 tc_classid;
	__u32 data;             // pkt start
	__u32 data_end;         // pkt end
...

bpf bytecode example:
(cilium, bpf_overlay.o, section from-overlay)

       8:	61 62 50 00 00 00 00 00	r2 = *(u32 *)(r6 + 80) // pkt end
       9:	61 61 4c 00 00 00 00 00	r1 = *(u32 *)(r6 + 76) // pkt end
      10:	bf 13 00 00 00 00 00 00	r3 = r1
      11:	07 03 00 00 0e 00 00 00	r3 += 14
      12:	2d 23 71 00 00 00 00 00	if r3 > r2 goto +113 <LBB6_16>
      13:	71 12 0c 00 00 00 00 00	r2 = *(u8 *)(r1 + 12)
      14:	71 18 0d 00 00 00 00 00	r8 = *(u8 *)(r1 + 13)  // load from pkt