Subject: Re: [PATCH 1/2] virtio_net: fix error handling for mergeable buffers

From: Jason Wang <[email-masked]>
Date: Thu, 28 Nov 2013 11:14:45 +0800

On 11/28/2013 12:31 AM, Michael S. Tsirkin wrote:
> Eric Dumazet noticed that if we encounter an error
> when processing a mergeable buffer, we don't
> dequeue all of the buffers from this packet,
> the result is almost sure to be loss of networking.
>
> Jason Wang noticed that we also leak a page and that we don't decrement
> the rq buf count, so we won't repost buffers (a resource leak).
>
> Fix both issues, and also make the logic a bit more
> robust against device errors by not looping when e.g. because of a leak
> like the one we are fixing here the number of buffers is 0.
>
> Cc: Rusty Russell <[email-masked]>
> Cc: Michael Dalton <[email-masked]>
> Reported-by: Eric Dumazet <[email-masked]>
> Reported-by: Jason Wang <[email-masked]>
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> ---
>
> Note: this bugfix is needed on stable too, but backport
> might not be trivial.
> I'll send a backport for stable separately.

That will be fine.
>
>  drivers/net/virtio_net.c | 84 ++++++++++++++++++++++++++++++------------------
>  1 file changed, 52 insertions(+), 32 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 7bab4de..0e6ea69 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -299,41 +299,53 @@ static struct sk_buff *page_to_skb(struct receive_queue *rq,
>  	return skb;
>  }
>  
> -static int receive_mergeable(struct receive_queue *rq, struct sk_buff *head_skb)
> +static struct sk_buff *receive_mergeable(struct net_device *dev,
> +					 struct receive_queue *rq,
> +					 void *buf,
> +					 unsigned int len)
>  {
> -	struct skb_vnet_hdr *hdr = skb_vnet_hdr(head_skb);
> +	struct skb_vnet_hdr *hdr = buf;
> +	int num_buf = hdr->mhdr.num_buffers;
> +	struct page *page = virt_to_head_page(buf);
> +	int offset = buf - page_address(page);
> +	struct sk_buff *head_skb = page_to_skb(rq, page, offset, len,
> +					       MERGE_BUFFER_LEN);
>  	struct sk_buff *curr_skb = head_skb;
> -	char *buf;
> -	struct page *page;
> -	int num_buf, len, offset;
>  
> -	num_buf = hdr->mhdr.num_buffers;
> +	if (unlikely(!curr_skb))
> +		goto err_skb;
> +
>  	while (--num_buf) {
> -		int num_skb_frags = skb_shinfo(curr_skb)->nr_frags;
> +		int num_skb_frags;
> +
>  		buf = virtqueue_get_buf(rq->vq, &len);
>  		if (unlikely(!buf)) {
> -			pr_debug("%s: rx error: %d buffers missing\n",
> -				 head_skb->dev->name, hdr->mhdr.num_buffers);
> -			head_skb->dev->stats.rx_length_errors++;
> -			return -EINVAL;
> +			pr_debug("%s: rx error: %d buffers out of %d missing\n",
> +				 dev->name, num_buf, hdr->mhdr.num_buffers);
> +			dev->stats.rx_length_errors++;
> +			goto err_buf;

Not sure it's correct here. Since the we break immediately if buffer is
missed in err_buf and rx_length_error will be miss counted.

Maybe an ERR_PTR(-EINVAL) is better.
>  		}
>  		if (unlikely(len > MERGE_BUFFER_LEN)) {
>  			pr_debug("%s: rx error: merge buffer too long\n",
> -				 head_skb->dev->name);
> +				 dev->name);
>  			len = MERGE_BUFFER_LEN;
>  		}
> +
> +		page = virt_to_head_page(buf);
> +		--rq->num;
> +
> +		num_skb_frags = skb_shinfo(curr_skb)->nr_frags;
>  		if (unlikely(num_skb_frags == MAX_SKB_FRAGS)) {
>  			struct sk_buff *nskb = alloc_skb(0, GFP_ATOMIC);
> -			if (unlikely(!nskb)) {
> -				head_skb->dev->stats.rx_dropped++;
> -				return -ENOMEM;
> -			}
> +
> +			if (unlikely(!nskb))
> +				goto err_skb;
>  			if (curr_skb == head_skb)
>  				skb_shinfo(curr_skb)->frag_list = nskb;
>  			else
>  				curr_skb->next = nskb;
> -			curr_skb = nskb;
>  			head_skb->truesize += nskb->truesize;
> +			curr_skb = nskb;

This change seems unnecessary.

Other looks good.
>  			num_skb_frags = 0;
>  		}
>  		if (curr_skb != head_skb) {
> @@ -341,8 +353,7 @@ static int receive_mergeable(struct receive_queue *rq, struct sk_buff *head_skb)
>  			head_skb->len += len;
>  			head_skb->truesize += MERGE_BUFFER_LEN;
>  		}
> -		page = virt_to_head_page(buf);
> -		offset = buf - (char *)page_address(page);
> +		offset = buf - page_address(page);
>  		if (skb_can_coalesce(curr_skb, num_skb_frags, page, offset)) {
>  			put_page(page);
>  			skb_coalesce_rx_frag(curr_skb, num_skb_frags - 1,
> @@ -351,9 +362,28 @@ static int receive_mergeable(struct receive_queue *rq, struct sk_buff *head_skb)
>  			skb_add_rx_frag(curr_skb, num_skb_frags, page,
>  					offset, len, MERGE_BUFFER_LEN);
>  		}
> +	}
> +
> +	return head_skb;
> +
> +err_skb:
> +	put_page(page);
> +err_buf:
> +	dev->stats.rx_dropped++;
> +	dev_kfree_skb(head_skb);
> +	while (--num_buf) {
> +		buf = virtqueue_get_buf(rq->vq, &len);
> +		if (unlikely(!buf)) {
> +			pr_debug("%s: rx error: %d buffers missing\n",
> +				 dev->name, num_buf);
> +			dev->stats.rx_length_errors++;
> +			break;
> +		}
> +		page = virt_to_head_page(buf);
> +		put_page(page);
>  		--rq->num;
>  	}
> -	return 0;
> +	return NULL;
>  }
>  
>  static void receive_buf(struct receive_queue *rq, void *buf, unsigned int len)
> @@ -382,19 +412,9 @@ static void receive_buf(struct receive_queue *rq, void *buf, unsigned int len)
>  		len -= sizeof(struct virtio_net_hdr);
>  		skb_trim(skb, len);
>  	} else if (vi->mergeable_rx_bufs) {
> -		struct page *page = virt_to_head_page(buf);
> -		skb = page_to_skb(rq, page,
> -				  (char *)buf - (char *)page_address(page),
> -				  len, MERGE_BUFFER_LEN);
> -		if (unlikely(!skb)) {
> -			dev->stats.rx_dropped++;
> -			put_page(page);
> +		skb = receive_mergeable(dev, rq, buf, len);
> +		if (unlikely(!skb))
>  			return;
> -		}
> -		if (receive_mergeable(rq, skb)) {
> -			dev_kfree_skb(skb);
> -			return;
> -		}
>  	} else {
>  		page = buf;
>  		skb = page_to_skb(rq, page, 0, len, PAGE_SIZE);