xfrm: Return detailed errors from xfrmi_newlink

Currently all failure modes of xfrm interface creation return EEXIST.
This change improves the granularity of errnos provided by also
returning ENODEV or EINVAL if failures happen in looking up the
underlying interface, or a required parameter is not provided.

This change has been tested against the Android Kernel Networking Tests,
with additional xfrmi_newlink tests here:

https://android-review.googlesource.com/c/kernel/tests/+/715755Signed-off-by: Benedict Wong <benedictwong@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_interface.c

@@ -149,14 +149,18 @@ static struct xfrm_if *xfrmi_create(struct net *net, struct xfrm_if_parms *p)
 	char name[IFNAMSIZ];
 	int err;
 
-	if (p->name[0])
+	if (p->name[0]) {
 		strlcpy(name, p->name, IFNAMSIZ);
-	else
+	} else {
+		err = -EINVAL;
 		goto failed;
+	}
 
 	dev = alloc_netdev(sizeof(*xi), name, NET_NAME_UNKNOWN, xfrmi_dev_setup);
-	if (!dev)
+	if (!dev) {
+		err = -EAGAIN;
 		goto failed;
+	}
 
 	dev_net_set(dev, net);
 
@@ -165,8 +169,10 @@ static struct xfrm_if *xfrmi_create(struct net *net, struct xfrm_if_parms *p)
 	xi->net = net;
 	xi->dev = dev;
 	xi->phydev = dev_get_by_index(net, p->link);
-	if (!xi->phydev)
+	if (!xi->phydev) {
+		err = -ENODEV;
 		goto failed_free;
+	}
 
 	err = xfrmi_create2(dev);
 	if (err < 0)
@@ -179,7 +185,7 @@ static struct xfrm_if *xfrmi_create(struct net *net, struct xfrm_if_parms *p)
 failed_free:
 	free_netdev(dev);
 failed:
-	return NULL;
+	return ERR_PTR(err);
 }
 
 static struct xfrm_if *xfrmi_locate(struct net *net, struct xfrm_if_parms *p,
@@ -194,13 +200,13 @@ static struct xfrm_if *xfrmi_locate(struct net *net, struct xfrm_if_parms *p,
 	     xip = &xi->next) {
 		if (xi->p.if_id == p->if_id) {
 			if (create)
-				return NULL;
+				return ERR_PTR(-EEXIST);
 
 			return xi;
 		}
 	}
 	if (!create)
-		return NULL;
+		return ERR_PTR(-ENODEV);
 	return xfrmi_create(net, p);
 }
 
@@ -682,8 +688,9 @@ static int xfrmi_newlink(struct net *src_net, struct net_device *dev,
 
 	nla_strlcpy(p->name, tb[IFLA_IFNAME], IFNAMSIZ);
 
-	if (!xfrmi_locate(net, p, 1))
-		return -EEXIST;
+	xi = xfrmi_locate(net, p, 1);
+	if (IS_ERR(xi))
+		return PTR_ERR(xi);
 
 	return 0;
 }
@@ -704,11 +711,12 @@ static int xfrmi_changelink(struct net_device *dev, struct nlattr *tb[],
 
 	xi = xfrmi_locate(net, &xi->p, 0);
 
-	if (xi) {
+	if (IS_ERR_OR_NULL(xi)) {
+		xi = netdev_priv(dev);
+	} else {
 		if (xi->dev != dev)
 			return -EEXIST;
-	} else
-		xi = netdev_priv(dev);
+	}
 
 	return xfrmi_update(xi, &xi->p);
 }