linux-kernel - Re: [PATCH v2 net 1/2] tuntap: correctly linearize skb when zerocopy is used

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Date:	Wed, 10 Jul 2013 09:15:31 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net 1/2] tuntap: correctly linearize skb when zerocopy
 is used

On Wed, Jul 10, 2013 at 01:43:27PM +0800, Jason Wang wrote:
> Userspace may produce vectors greater than MAX_SKB_FRAGS. When we try to
> linearize parts of the skb to let the rest of iov to be fit in
> the frags, we need count copylen into linear when calling tun_alloc_skb()
> instead of partly counting it into data_len. Since this breaks
> zerocopy_sg_from_iovec() since its inner counter assumes nr_frags should
> be zero at beginning. This cause nr_frags to be increased wrongly without
> setting the correct frags.
> 
> This bug were introduced from 0690899b4d4501b3505be069b9a687e68ccbe15b
> (tun: experimental zero copy tx support)
> 
> Cc: Michael S. Tsirkin <mst@...hat.com>
> Signed-off-by: Jason Wang <jasowang@...hat.com>


Acked-by: Michael S. Tsirkin <mst@...hat.com>

> ---
> - This patch is needed for stable.
> - Changes from v1: introduce a local variable to track linear size
> ---
>  drivers/net/tun.c |    9 ++++++---
>  1 files changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 9c61f87..c3cb60b 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1044,7 +1044,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>  {
>  	struct tun_pi pi = { 0, cpu_to_be16(ETH_P_IP) };
>  	struct sk_buff *skb;
> -	size_t len = total_len, align = NET_SKB_PAD;
> +	size_t len = total_len, align = NET_SKB_PAD, linear;
>  	struct virtio_net_hdr gso = { 0 };
>  	int offset = 0;
>  	int copylen;
> @@ -1108,10 +1108,13 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>  			copylen = gso.hdr_len;
>  		if (!copylen)
>  			copylen = GOODCOPY_LEN;
> -	} else
> +		linear = copylen;
> +	} else {
>  		copylen = len;
> +		linear = gso.hdr_len;
> +	}
>  
> -	skb = tun_alloc_skb(tfile, align, copylen, gso.hdr_len, noblock);
> +	skb = tun_alloc_skb(tfile, align, copylen, linear, noblock);
>  	if (IS_ERR(skb)) {
>  		if (PTR_ERR(skb) != -EAGAIN)
>  			tun->dev->stats.rx_dropped++;
> -- 
> 1.7.1
--
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