net: tun: rebuild error handling in tun_get_user

The error handling in tun_get_user is very scattered.
This patch unifies error handling, reduces duplication of code, and
makes the logic clearer.
Signed-off-by: Chuang Wang <nashuiliang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1746,7 +1746,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 	u32 rxhash = 0;
 	int skb_xdp = 1;
 	bool frags = tun_napi_frags_enabled(tfile);
-	enum skb_drop_reason drop_reason;
+	enum skb_drop_reason drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
 
 	if (!(tun->flags & IFF_NO_PI)) {
 		if (len < sizeof(pi))
@@ -1807,10 +1807,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		 * skb was created with generic XDP routine.
 		 */
 		skb = tun_build_skb(tun, tfile, from, &gso, len, &skb_xdp);
-		if (IS_ERR(skb)) {
-			dev_core_stats_rx_dropped_inc(tun->dev);
-			return PTR_ERR(skb);
-		}
+		err = PTR_ERR_OR_ZERO(skb);
+		if (err)
+			goto drop;
 		if (!skb)
 			return total_len;
 	} else {
@@ -1835,13 +1834,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 					    noblock);
 		}
 
-		if (IS_ERR(skb)) {
-			if (PTR_ERR(skb) != -EAGAIN)
-				dev_core_stats_rx_dropped_inc(tun->dev);
-			if (frags)
-				mutex_unlock(&tfile->napi_mutex);
-			return PTR_ERR(skb);
-		}
+		err = PTR_ERR_OR_ZERO(skb);
+		if (err)
+			goto drop;
 
 		if (zerocopy)
 			err = zerocopy_sg_from_iter(skb, from);
@@ -1851,27 +1846,14 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		if (err) {
 			err = -EFAULT;
 			drop_reason = SKB_DROP_REASON_SKB_UCOPY_FAULT;
-drop:
-			dev_core_stats_rx_dropped_inc(tun->dev);
-			kfree_skb_reason(skb, drop_reason);
-			if (frags) {
-				tfile->napi.skb = NULL;
-				mutex_unlock(&tfile->napi_mutex);
-			}
-
-			return err;
+			goto drop;
 		}
 	}
 
 	if (virtio_net_hdr_to_skb(skb, &gso, tun_is_little_endian(tun))) {
 		atomic_long_inc(&tun->rx_frame_errors);
-		kfree_skb(skb);
-		if (frags) {
-			tfile->napi.skb = NULL;
-			mutex_unlock(&tfile->napi_mutex);
-		}
-
-		return -EINVAL;
+		err = -EINVAL;
+		goto free_skb;
 	}
 
 	switch (tun->flags & TUN_TYPE_MASK) {
@@ -1887,9 +1869,8 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 				pi.proto = htons(ETH_P_IPV6);
 				break;
 			default:
-				dev_core_stats_rx_dropped_inc(tun->dev);
-				kfree_skb(skb);
-				return -EINVAL;
+				err = -EINVAL;
+				goto drop;
 			}
 		}
 
@@ -1931,11 +1912,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 			if (ret != XDP_PASS) {
 				rcu_read_unlock();
 				local_bh_enable();
-				if (frags) {
-					tfile->napi.skb = NULL;
-					mutex_unlock(&tfile->napi_mutex);
-				}
-				return total_len;
+				goto unlock_frags;
 			}
 		}
 		rcu_read_unlock();
@@ -2015,6 +1992,22 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		tun_flow_update(tun, rxhash, tfile);
 
 	return total_len;
+
+drop:
+	if (err != -EAGAIN)
+		dev_core_stats_rx_dropped_inc(tun->dev);
+
+free_skb:
+	if (!IS_ERR_OR_NULL(skb))
+		kfree_skb_reason(skb, drop_reason);
+
+unlock_frags:
+	if (frags) {
+		tfile->napi.skb = NULL;
+		mutex_unlock(&tfile->napi_mutex);
+	}
+
+	return err ?: total_len;
 }
 
 static ssize_t tun_chr_write_iter(struct kiocb *iocb, struct iov_iter *from)