linux-kernel - [PATCH net-next v2 3/9] tun: correct drop statistics in tun_put

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Message-ID: <20251125200041.1565663-4-jon@nutanix.com>
Date: Tue, 25 Nov 2025 13:00:30 -0700
From: Jon Kohler <jon@...anix.com>
To: netdev@...r.kernel.org, Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Jason Wang <jasowang@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        linux-kernel@...r.kernel.org (open list)
Cc: Jon Kohler <jon@...anix.com>
Subject: [PATCH net-next v2 3/9] tun: correct drop statistics in tun_put_user

Fold kfree_skb and consume_skb for tun_put_user into tun_put_user and
rework kfree_skb to take a drop reason. Add drop reason to all drop
sites and ensure that all failing paths properly increment drop
counter.

Signed-off-by: Jon Kohler <jon@...anix.com>
---
 drivers/net/tun.c | 51 +++++++++++++++++++++++++++++++----------------
 1 file changed, 34 insertions(+), 17 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 68ad46ab04a4..e0f5e1fe4bd0 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -2035,6 +2035,7 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 			    struct sk_buff *skb,
 			    struct iov_iter *iter)
 {
+	enum skb_drop_reason drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
 	struct tun_pi pi = { 0, skb->protocol };
 	ssize_t total;
 	int vlan_offset = 0;
@@ -2051,8 +2052,11 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 	total = skb->len + vlan_hlen + vnet_hdr_sz;
 
 	if (!(tun->flags & IFF_NO_PI)) {
-		if (iov_iter_count(iter) < sizeof(pi))
-			return -EINVAL;
+		if (iov_iter_count(iter) < sizeof(pi)) {
+			ret = -EINVAL;
+			drop_reason = SKB_DROP_REASON_PKT_TOO_SMALL;
+			goto drop;
+		}
 
 		total += sizeof(pi);
 		if (iov_iter_count(iter) < total) {
@@ -2060,8 +2064,11 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 			pi.flags |= TUN_PKT_STRIP;
 		}
 
-		if (copy_to_iter(&pi, sizeof(pi), iter) != sizeof(pi))
-			return -EFAULT;
+		if (copy_to_iter(&pi, sizeof(pi), iter) != sizeof(pi)) {
+			ret = -EFAULT;
+			drop_reason = SKB_DROP_REASON_SKB_UCOPY_FAULT;
+			goto drop;
+		}
 	}
 
 	if (vnet_hdr_sz) {
@@ -2070,8 +2077,10 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 
 		ret = tun_vnet_hdr_tnl_from_skb(tun->flags, tun->dev, skb,
 						&hdr);
-		if (ret)
-			return ret;
+		if (ret) {
+			drop_reason = SKB_DROP_REASON_DEV_HDR;
+			goto drop;
+		}
 
 		/*
 		 * Drop the packet if the configured header size is too small
@@ -2080,8 +2089,10 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 		gso = (struct virtio_net_hdr *)&hdr;
 		ret = __tun_vnet_hdr_put(vnet_hdr_sz, tun->dev->features,
 					 iter, gso);
-		if (ret)
-			return ret;
+		if (ret) {
+			drop_reason = SKB_DROP_REASON_DEV_HDR;
+			goto drop;
+		}
 	}
 
 	if (vlan_hlen) {
@@ -2094,23 +2105,33 @@ static ssize_t tun_put_user(struct tun_struct *tun,
 		vlan_offset = offsetof(struct vlan_ethhdr, h_vlan_proto);
 
 		ret = skb_copy_datagram_iter(skb, 0, iter, vlan_offset);
-		if (ret || !iov_iter_count(iter))
-			goto done;
+		if (ret || !iov_iter_count(iter)) {
+			drop_reason = SKB_DROP_REASON_DEV_HDR;
+			goto drop;
+		}
 
 		ret = copy_to_iter(&veth, sizeof(veth), iter);
-		if (ret != sizeof(veth) || !iov_iter_count(iter))
-			goto done;
+		if (ret != sizeof(veth) || !iov_iter_count(iter)) {
+			drop_reason = SKB_DROP_REASON_DEV_HDR;
+			goto drop;
+		}
 	}
 
 	skb_copy_datagram_iter(skb, vlan_offset, iter, skb->len - vlan_offset);
 
-done:
 	/* caller is in process context, */
 	preempt_disable();
 	dev_sw_netstats_tx_add(tun->dev, 1, skb->len + vlan_hlen);
 	preempt_enable();
 
+	consume_skb(skb);
+
 	return total;
+
+drop:
+	dev_core_stats_tx_dropped_inc(tun->dev);
+	kfree_skb_reason(skb, drop_reason);
+	return ret;
 }
 
 static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
@@ -2182,10 +2203,6 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
 		struct sk_buff *skb = ptr;
 
 		ret = tun_put_user(tun, tfile, skb, to);
-		if (unlikely(ret < 0))
-			kfree_skb(skb);
-		else
-			consume_skb(skb);
 	}
 
 	return ret;
-- 
2.43.0