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

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Message-ID: <willemdebruijn.kernel.1c90f25a9b9a9@gmail.com>
Date: Fri, 28 Nov 2025 22:07:30 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jon Kohler <jon@...anix.com>, 
 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: Re: [PATCH net-next v2 3/9] tun: correct drop statistics in
 tun_put_user

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?

> +			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
>