Subject: Re: [PATCH] virtio_net: Use NETDEV_TX_BUSY when has no buf to send
To:     Angus Chen <angus.chen@jaguarmicro.com>, <mst@redhat.com>,
        <jasowang@redhat.com>, <davem@davemloft.net>,
        <edumazet@google.com>, <kuba@kernel.org>, <pabeni@redhat.com>,
        <ast@kernel.org>, <daniel@iogearbox.net>, <hawk@kernel.org>,
        <john.fastabend@gmail.com>
CC:     <virtualization@lists.linux-foundation.org>,
        <netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <bpf@vger.kernel.org>, Xuan Zhuo <xuanzhuo@linux.alibaba.com>
References: <20230309074952.975-1-angus.chen@jaguarmicro.com>
From:   Yunsheng Lin <linyunsheng@huawei.com>
Message-ID: <866b9077-1600-4c93-3da8-4006cbf6abe7@huawei.com>
Date:   Thu, 9 Mar 2023 18:52:30 +0800
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On 2023/3/9 15:49, Angus Chen wrote:
> Don't consume skb if virtqueue_add return -ENOSPC.

Is this fixing the same out of space problem caused by
xdp xmit as Xuan Zhuo is fixing, or it is fixing a different
case?

https://lore.kernel.org/netdev/20230308071921-mutt-send-email-mst@kernel.org/T/#mc4c5766a59fb8be988bb6a4dfa48f49e58df3ea6

> 
> Signed-off-by: Angus Chen <angus.chen@jaguarmicro.com>
> ---
>  drivers/net/virtio_net.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index fb5e68ed3ec2..4096ea3d2eb6 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1980,7 +1980,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
>  				 qnum, err);
>  		dev->stats.tx_dropped++;
>  		dev_kfree_skb_any(skb);

Returning NETDEV_TX_BUSY will caused stack to requeue the skb
and send it again when space is available, but you have freed the skb here,
isn't this cause use-after-free problem?

> -		return NETDEV_TX_OK;
> +		return (err == -ENOSPC) ? NETDEV_TX_BUSY : NETDEV_TX_OK;
>  	}
>  
>  	/* Don't wait up for transmitted skbs to be freed. */
>