bpf: make skb->tc_classid also readable

Currently, the tc_classid from eBPF skb context is write-only, but there's
no good reason for tc programs to limit it to write-only. For example,
it can be used to transfer its state via tail calls where the resulting
tc_classid gets filled gradually.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/filter.c

@@ -2069,9 +2069,6 @@ static bool sk_filter_is_valid_access(int off, int size,
 static bool tc_cls_act_is_valid_access(int off, int size,
 				       enum bpf_access_type type)
 {
-	if (off == offsetof(struct __sk_buff, tc_classid))
-		return type == BPF_WRITE ? true : false;
-
 	if (type == BPF_WRITE) {
 		switch (off) {
 		case offsetof(struct __sk_buff, mark):
@@ -2079,6 +2076,7 @@ static bool tc_cls_act_is_valid_access(int off, int size,
 		case offsetof(struct __sk_buff, priority):
 		case offsetof(struct __sk_buff, cb[0]) ...
 		     offsetof(struct __sk_buff, cb[4]):
+		case offsetof(struct __sk_buff, tc_classid):
 			break;
 		default:
 			return false;
@@ -2195,8 +2193,10 @@ static u32 bpf_net_convert_ctx_access(enum bpf_access_type type, int dst_reg,
 		ctx_off -= offsetof(struct __sk_buff, tc_classid);
 		ctx_off += offsetof(struct sk_buff, cb);
 		ctx_off += offsetof(struct qdisc_skb_cb, tc_classid);
-		WARN_ON(type != BPF_WRITE);
+		if (type == BPF_WRITE)
 			*insn++ = BPF_STX_MEM(BPF_H, dst_reg, src_reg, ctx_off);
+		else
+			*insn++ = BPF_LDX_MEM(BPF_H, dst_reg, src_reg, ctx_off);
 		break;
 
 	case offsetof(struct __sk_buff, tc_index):