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

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Message-ID: <F48BA9F9-7E15-49B3-896A-5AE367DAD060@nutanix.com>
Date: Tue, 2 Dec 2025 16:40:30 +0000
From: Jon Kohler <jon@...anix.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        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>,
        open list
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next v2 3/9] tun: correct drop statistics in
 tun_put_user



> On Nov 28, 2025, at 10:07 PM, Willem de Bruijn <willemdebruijn.kernel@...il.com> wrote:
> 
> Jon Kohler wrote:
>> 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;
> 
> PI counts as SKB_DROP_REASON_DEV_HDR?

Are you saying I should change this use case to DEV_HDR?

This one seemed like a pretty straight forward “It’s too small” case,
no? Or am I misreading into what you’re saying here?

Happy to take a suggestion if I’ve got the drop reason wired
wrong (or if we need to cook up a brand new drop reason for any of
these)

Jon

> 
>> + 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