[PATCH net-next v4 3/5] netlink: allow sending extended ACK with cookie on success

[Johannes Berg <johannes@sipsolutions.net>]

From: Johannes Berg <johannes.berg@intel.com>

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 <johannes.berg@intel.com>
---
 include/linux/netlink.h      |  7 +++++++
 include/uapi/linux/netlink.h |  4 ++++
 net/netlink/af_netlink.c     | 33 ++++++++++++++++++++++-----------
 3 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 60e7137f840d..8d2a8924705c 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -62,15 +62,22 @@ netlink_kernel_create(struct net *net, int unit, struct netlink_kernel_cfg *cfg)
 	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
diff --git a/include/uapi/linux/netlink.h b/include/uapi/linux/netlink.h
index 96e613db68cb..67fcde069b9c 100644
--- a/include/uapi/linux/netlink.h
+++ b/include/uapi/linux/netlink.h
@@ -122,6 +122,9 @@ struct nlmsgerr {
  * @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)
  * @NUM_NLMSGERR_ATTRS: number of attributes
  * @NLMSGERR_ATTR_MAX: highest attribute number
  */
@@ -129,6 +132,7 @@ enum nlmsgerr_attrs {
 	NLMSGERR_ATTR_UNUSED,
 	NLMSGERR_ATTR_MSG,
 	NLMSGERR_ATTR_OFFS,
+	NLMSGERR_ATTR_COOKIE,
 
 	NUM_NLMSGERR_ATTRS,
 	NLMSGERR_ATTR_MAX = NUM_NLMSGERR_ATTRS - 1
diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index c1564768000e..ee841f00a6ec 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2311,6 +2311,10 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 		}
 	} 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)
@@ -2337,17 +2341,24 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 	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);
-- 
2.11.0