bpf: allow narrow loads of some sk_reuseport_md fields with offset > 0

test_select_reuseport fails on s390 due to verifier rejecting
test_select_reuseport_kern.o with the following message:

	; data_check.eth_protocol = reuse_md->eth_protocol;
	18: (69) r1 = *(u16 *)(r6 +22)
	invalid bpf_context access off=22 size=2

This is because on big-endian machines casts from __u32 to __u16 are
generated by referencing the respective variable as __u16 with an offset
of 2 (as opposed to 0 on little-endian machines).

The verifier already has all the infrastructure in place to allow such
accesses, it's just that they are not explicitly enabled for
eth_protocol field. Enable them for eth_protocol field by using
bpf_ctx_range instead of offsetof.

Ditto for ip_protocol, bind_inany and len, since they already allow
narrowing, and the same problem can arise when working with them.

Fixes: 2dbb9b9e ("bpf: Introduce BPF_PROG_TYPE_SK_REUSEPORT")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/filter.c

@@ -8757,13 +8757,13 @@ sk_reuseport_is_valid_access(int off, int size,
 		return size == size_default;
 
 	/* Fields that allow narrowing */
-	case offsetof(struct sk_reuseport_md, eth_protocol):
+	case bpf_ctx_range(struct sk_reuseport_md, eth_protocol):
 		if (size < FIELD_SIZEOF(struct sk_buff, protocol))
 			return false;
 		/* fall through */
-	case offsetof(struct sk_reuseport_md, ip_protocol):
-	case offsetof(struct sk_reuseport_md, bind_inany):
-	case offsetof(struct sk_reuseport_md, len):
+	case bpf_ctx_range(struct sk_reuseport_md, ip_protocol):
+	case bpf_ctx_range(struct sk_reuseport_md, bind_inany):
+	case bpf_ctx_range(struct sk_reuseport_md, len):
 		bpf_ctx_record_field_size(info, size_default);
 		return bpf_ctx_narrow_access_ok(off, size, size_default);