netlink: allow sending extended ACK with cookie on success

Now that we have extended error reporting and a new message format for
netlink ACK messages, also extend this to be able to return arbitrary
cookie data on success.

This will allow, for example, nl80211 to not send an extra message for
cookies identifying newly created objects, but return those directly
in the ACK message.

The cookie data size is currently limited to 20 bytes (since Jamal
talked about using SHA1 for identifiers.)

Thanks to Jamal Hadi Salim for bringing up this idea during the
discussions.

Signed-off-by: Johannes Berg <[e-mail]>
Reviewed-by: Jiri Pirko <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/netlink.h

···
 	return __netlink_kernel_create(net, unit, THIS_MODULE, cfg);
 }
 
+/* this can be increased when necessary - don't expose to userland */
+#define NETLINK_MAX_COOKIE_LEN	20
+
 /**
  * struct netlink_ext_ack - netlink extended ACK report struct
  * @_msg: message string to report - don't access directly, use
  *	%NL_SET_ERR_MSG
  * @bad_attr: attribute with error
+ * @cookie: cookie data to return to userspace (for success)
+ * @cookie_len: actual cookie data length
  */
 struct netlink_ext_ack {
 	const char *_msg;
 	const struct nlattr *bad_attr;
+	u8 cookie[NETLINK_MAX_COOKIE_LEN];
+	u8 cookie_len;
 };
 
 /* Always use this macro, this allows later putting the

include/uapi/linux/netlink.h

···
  * @NLMSGERR_ATTR_MSG: error message string (string)
  * @NLMSGERR_ATTR_OFFS: offset of the invalid attribute in the original
  *	 message, counting from the beginning of the header (u32)
+ * @NLMSGERR_ATTR_COOKIE: arbitrary subsystem specific cookie to
+ *	be used - in the success case - to identify a created
+ *	object or operation or similar (binary)
  * @__NLMSGERR_ATTR_MAX: number of attributes
  * @NLMSGERR_ATTR_MAX: highest attribute number
  */
···
 	NLMSGERR_ATTR_UNUSED,
 	NLMSGERR_ATTR_MSG,
 	NLMSGERR_ATTR_OFFS,
+	NLMSGERR_ATTR_COOKIE,
 
 	__NLMSGERR_ATTR_MAX,
 	NLMSGERR_ATTR_MAX = __NLMSGERR_ATTR_MAX - 1

net/netlink/af_netlink.c

···
 		}
 	} else {
 		flags |= NLM_F_CAPPED;
+
+		if (nlk->flags & NETLINK_F_EXT_ACK &&
+		    extack && extack->cookie_len)
+			tlvlen += nla_total_size(extack->cookie_len);
 	}
 
 	if (tlvlen)
···
 	errmsg->error = err;
 	memcpy(&errmsg->msg, nlh, payload > sizeof(*errmsg) ? nlh->nlmsg_len : sizeof(*nlh));
 
-	if (err && nlk->flags & NETLINK_F_EXT_ACK && extack) {
-		if (extack->_msg)
-			WARN_ON(nla_put_string(skb, NLMSGERR_ATTR_MSG,
-					       extack->_msg));
-		if (extack->bad_attr &&
-		    !WARN_ON((u8 *)extack->bad_attr < in_skb->data ||
-			     (u8 *)extack->bad_attr >= in_skb->data +
-						       in_skb->len))
-			WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_OFFS,
-					    (u8 *)extack->bad_attr -
-					    in_skb->data));
+	if (nlk->flags & NETLINK_F_EXT_ACK && extack) {
+		if (err) {
+			if (extack->_msg)
+				WARN_ON(nla_put_string(skb, NLMSGERR_ATTR_MSG,
+						       extack->_msg));
+			if (extack->bad_attr &&
+			    !WARN_ON((u8 *)extack->bad_attr < in_skb->data ||
+				     (u8 *)extack->bad_attr >= in_skb->data +
+							       in_skb->len))
+				WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_OFFS,
+						    (u8 *)extack->bad_attr -
+						    in_skb->data));
+		} else {
+			if (extack->cookie_len)
+				WARN_ON(nla_put(skb, NLMSGERR_ATTR_COOKIE,
+						extack->cookie_len,
+						extack->cookie));
+		}
 	}
 
 	nlmsg_end(skb, rep);