bpf: sync bpf.h to tools/

Sync updated documentation for bpf_redirect_map.

Sync the bpf_tcp_gen_syncookie helper function definition with the one
in tools/uapi.
Signed-off-by: Petar Penkov <ppenkov@google.com>
Reviewed-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/include/uapi/linux/bpf.h

@@ -1572,8 +1572,11 @@ union bpf_attr {
  * 		but this is only implemented for native XDP (with driver
  * 		support) as of this writing).
  *
- * 		All values for *flags* are reserved for future usage, and must
- * 		be left at zero.
+ * 		The lower two bits of *flags* are used as the return code if
+ * 		the map lookup fails. This is so that the return value can be
+ * 		one of the XDP program return codes up to XDP_TX, as chosen by
+ * 		the caller. Any higher bits in the *flags* argument must be
+ * 		unset.
  *
  * 		When used to redirect packets to net devices, this helper
  * 		provides a high performance increase over **bpf_redirect**\ ().
@@ -2711,6 +2714,33 @@ union bpf_attr {
  *		**-EPERM** if no permission to send the *sig*.
  *
  *		**-EAGAIN** if bpf program can try again.
+ *
+ * s64 bpf_tcp_gen_syncookie(struct bpf_sock *sk, void *iph, u32 iph_len, struct tcphdr *th, u32 th_len)
+ *	Description
+ *		Try to issue a SYN cookie for the packet with corresponding
+ *		IP/TCP headers, *iph* and *th*, on the listening socket in *sk*.
+ *
+ *		*iph* points to the start of the IPv4 or IPv6 header, while
+ *		*iph_len* contains **sizeof**\ (**struct iphdr**) or
+ *		**sizeof**\ (**struct ip6hdr**).
+ *
+ *		*th* points to the start of the TCP header, while *th_len*
+ *		contains the length of the TCP header.
+ *
+ *	Return
+ *		On success, lower 32 bits hold the generated SYN cookie in
+ *		followed by 16 bits which hold the MSS value for that cookie,
+ *		and the top 16 bits are unused.
+ *
+ *		On failure, the returned value is one of the following:
+ *
+ *		**-EINVAL** SYN cookie cannot be issued due to error
+ *
+ *		**-ENOENT** SYN cookie should not be issued (no SYN flood)
+ *
+ *		**-EOPNOTSUPP** kernel configuration does not enable SYN cookies
+ *
+ *		**-EPROTONOSUPPORT** IP packet version is not 4 or 6
  */
 #define __BPF_FUNC_MAPPER(FN)		\
 	FN(unspec),			\
@@ -2822,7 +2852,8 @@ union bpf_attr {
 	FN(strtoul),			\
 	FN(sk_storage_get),		\
 	FN(sk_storage_delete),		\
-	FN(send_signal),
+	FN(send_signal),		\
+	FN(tcp_gen_syncookie),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call