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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 28 Nov 2013 13:14:47 +0200

On Thu, Nov 28, 2013 at 11:14:45AM +0800, Jason Wang wrote:
> 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.

Note that this never happens: it's an internal device error, we are
just trying to be defensive here. I would not worry about getting
error counters right in this case.

But I agree - since we know virtqueue_get_buf failed it's useless to try
and get more. So let's move the loop before the err_buf label.

> Maybe an ERR_PTR(-EINVAL) is better.

This will just make callers worry about checking IS_ERR, and
caller returns void so it has nothing to do with the return code anyway.

> >  		}
> >  		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.

Right, I'll drop it.

> 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);