Re: [PATCH net v1] virtio_net: Introduce skb_vnet_common_hdr to avoid typecasting

[Simon Horman]

On Mon, Aug 14, 2023 at 01:18:45PM -0400, Feng Liu wrote:

+ "David S. Miller" <davem@xxxxxxxxxxxxx>
  Eric Dumazet <edumazet@xxxxxxxxxx>
  Jakub Kicinski <kuba@xxxxxxxxxx>
  Paolo Abeni <pabeni@xxxxxxxxxx>

> The virtio_net driver currently deals with different versions and types
> of virtio net headers, such as virtio_net_hdr_mrg_rxbuf,
> virtio_net_hdr_v1_hash, etc. Due to these variations, the code relies
> on multiple type casts to convert memory between different structures,
> potentially leading to bugs when there are changes in these structures.
> 
> Introduces the "struct skb_vnet_common_hdr" as a unifying header
> structure using a union. With this approach, various virtio net header
> structures can be converted by accessing different members of this
> structure, thus eliminating the need for type casting and reducing the
> risk of potential bugs.
> 
> For example following code:
> static struct sk_buff *page_to_skb(struct virtnet_info *vi,
> 		struct receive_queue *rq,
> 		struct page *page, unsigned int offset,
> 		unsigned int len, unsigned int truesize,
> 		unsigned int headroom)
> {
> [...]
> 	struct virtio_net_hdr_mrg_rxbuf *hdr;
> [...]
> 	hdr_len = vi->hdr_len;
> [...]
> ok:
> 	hdr = skb_vnet_hdr(skb);
> 	memcpy(hdr, hdr_p, hdr_len);
> [...]
> }
> 
> When VIRTIO_NET_F_HASH_REPORT feature is enabled, hdr_len = 20
> But the sizeof(*hdr) is 12,
> memcpy(hdr, hdr_p, hdr_len); will copy 20 bytes to the hdr,
> which make a potential risk of bug. And this risk can be avoided by
> introducing struct virtio_net_hdr_mrg_rxbuf.
> 
> Signed-off-by: Feng Liu <feliu@xxxxxxxxxx>
> Reviewed-by: Jiri Pirko <jiri@xxxxxxxxxx>

I'm unsure if this is 'net' material.

> ---
>  drivers/net/virtio_net.c        | 29 ++++++++++++++++-------------
>  include/uapi/linux/virtio_net.h |  7 +++++++
>  2 files changed, 23 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 1270c8d23463..6ce0fbcabda9 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -344,9 +344,10 @@ static int rxq2vq(int rxq)
>  	return rxq * 2;
>  }
>  
> -static inline struct virtio_net_hdr_mrg_rxbuf *skb_vnet_hdr(struct sk_buff *skb)
> +static inline struct virtio_net_common_hdr *
> +skb_vnet_common_hdr(struct sk_buff *skb)
>  {
> -	return (struct virtio_net_hdr_mrg_rxbuf *)skb->cb;
> +	return (struct virtio_net_common_hdr *)skb->cb;
>  }
>  
>  /*
> @@ -469,7 +470,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  				   unsigned int headroom)
>  {
>  	struct sk_buff *skb;
> -	struct virtio_net_hdr_mrg_rxbuf *hdr;
> +	struct virtio_net_common_hdr *hdr;
>  	unsigned int copy, hdr_len, hdr_padded_len;
>  	struct page *page_to_free = NULL;
>  	int tailroom, shinfo_size;
> @@ -554,7 +555,7 @@ static struct sk_buff *page_to_skb(struct virtnet_info *vi,
>  		give_pages(rq, page);
>  
>  ok:
> -	hdr = skb_vnet_hdr(skb);
> +	hdr = skb_vnet_common_hdr(skb);
>  	memcpy(hdr, hdr_p, hdr_len);
>  	if (page_to_free)
>  		put_page(page_to_free);
> @@ -966,7 +967,7 @@ static struct sk_buff *receive_small_build_skb(struct virtnet_info *vi,
>  		return NULL;
>  
>  	buf += header_offset;
> -	memcpy(skb_vnet_hdr(skb), buf, vi->hdr_len);
> +	memcpy(skb_vnet_common_hdr(skb), buf, vi->hdr_len);
>  
>  	return skb;
>  }
> @@ -1577,7 +1578,8 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>  {
>  	struct net_device *dev = vi->dev;
>  	struct sk_buff *skb;
> -	struct virtio_net_hdr_mrg_rxbuf *hdr;
> +	struct virtio_net_common_hdr *common_hdr;
> +	struct virtio_net_hdr_mrg_rxbuf	*mrg_hdr;
>  
>  	if (unlikely(len < vi->hdr_len + ETH_HLEN)) {
>  		pr_debug("%s: short packet %i\n", dev->name, len);
> @@ -1597,18 +1599,19 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>  	if (unlikely(!skb))
>  		return;
>  
> -	hdr = skb_vnet_hdr(skb);
> +	common_hdr = skb_vnet_common_hdr(skb);
>  	if (dev->features & NETIF_F_RXHASH && vi->has_rss_hash_report)
> -		virtio_skb_set_hash((const struct virtio_net_hdr_v1_hash *)hdr, skb);
> +		virtio_skb_set_hash(&common_hdr->hash_v1_hdr, skb);
>  
> -	if (hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
> +	mrg_hdr = &common_hdr->mrg_hdr;
> +	if (mrg_hdr->hdr.flags & VIRTIO_NET_HDR_F_DATA_VALID)
>  		skb->ip_summed = CHECKSUM_UNNECESSARY;
>  
> -	if (virtio_net_hdr_to_skb(skb, &hdr->hdr,
> +	if (virtio_net_hdr_to_skb(skb, &mrg_hdr->hdr,
>  				  virtio_is_little_endian(vi->vdev))) {
>  		net_warn_ratelimited("%s: bad gso: type: %u, size: %u\n",
> -				     dev->name, hdr->hdr.gso_type,
> -				     hdr->hdr.gso_size);
> +				     dev->name, mrg_hdr->hdr.gso_type,
> +				     mrg_hdr->hdr.gso_size);
>  		goto frame_err;
>  	}
>  
> @@ -2105,7 +2108,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
>  	if (can_push)
>  		hdr = (struct virtio_net_hdr_mrg_rxbuf *)(skb->data - hdr_len);
>  	else
> -		hdr = skb_vnet_hdr(skb);
> +		hdr = &skb_vnet_common_hdr(skb)->mrg_hdr;
>  
>  	if (virtio_net_hdr_from_skb(skb, &hdr->hdr,
>  				    virtio_is_little_endian(vi->vdev), false,
> diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
> index 12c1c9699935..db40f93ae8b3 100644
> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -201,6 +201,13 @@ struct virtio_net_hdr_mrg_rxbuf {
>  	struct virtio_net_hdr hdr;
>  	__virtio16 num_buffers;	/* Number of merged rx buffers */
>  };
> +
> +struct virtio_net_common_hdr {
> +	union {
> +		struct virtio_net_hdr_mrg_rxbuf	mrg_hdr;
> +		struct virtio_net_hdr_v1_hash hash_v1_hdr;
> +	};
> +};

Does this belong in the UAPI?
I would have assumed it's a Kernel implementation detail.

>  #endif /* ...VIRTIO_NET_NO_LEGACY */
>  
>  /*
> -- 
> 2.37.1 (Apple Git-137.1)
> 
>