bpf: add bpf_set_hash helper for tc progs

Allow for tc BPF programs to set a skb->hash, apart from clearing
and triggering a recalc that we have right now. It allows for BPF
to implement a custom hashing routine for skb_get_hash().

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/include/uapi/linux/bpf.h

@@ -513,6 +513,11 @@ union bpf_attr {
  *     Get the owner uid of the socket stored inside sk_buff.
  *     @skb: pointer to skb
  *     Return: uid of the socket owner on success or overflowuid if failed.
+ *
+ * u32 bpf_set_hash(skb, hash)
+ *     Set full skb->hash.
+ *     @skb: pointer to skb
+ *     @hash: hash to set
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -562,7 +567,8 @@ union bpf_attr {
 	FN(xdp_adjust_head),		\
 	FN(probe_read_str),		\
 	FN(get_socket_cookie),		\
-	FN(get_socket_uid),
+	FN(get_socket_uid),		\
+	FN(set_hash),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call