Subject: Re: [PATCH net 1/2] tun: unbreak truncated packet signalling

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 9 Dec 2013 12:56:38 +0200

On Mon, Dec 09, 2013 at 12:55:29PM +0200, Michael S. Tsirkin wrote:
> On Mon, Dec 09, 2013 at 06:25:16PM +0800, Jason Wang wrote:
> > Commit 6680ec68eff47d36f67b4351bc9836fd6cba9532
> > (tuntap: hardware vlan tx support) breaks the truncated packet signal by never
> > return a length greater than iov length in tun_put_user(). This patch fixes this
> > by always return the length of packet plus possible vlan header. Caller can
> > detect the truncated packet by comparing the return value and the size of iov
> > length.
> > 
> > Reported-by: Vlad Yasevich <[email-masked]>
> > Cc: Vlad Yasevich <[email-masked]>
> > Cc: Zhi Yong Wu <[email-masked]>
> > Signed-off-by: Jason Wang <[email-masked]>
> 
> So writer gets back a value greater than what was written?

Pls ignore this question - wrote it before I understood the
patch, and forgot to remove.
The rest of the comments and the proposed alternative patch
still stand.

> > ---
> > The patch is needed for stable.
> > ---
> >  drivers/net/tun.c | 23 ++++++++++++-----------
> >  1 file changed, 12 insertions(+), 11 deletions(-)
> > 
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index e26cbea..dd1bd7a 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1183,7 +1183,11 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> >  			    const struct iovec *iv, int len)
> >  {
> >  	struct tun_pi pi = { 0, skb->protocol };
> > -	ssize_t total = 0;
> > +	struct {
> > +		__be16 h_vlan_proto;
> > +		__be16 h_vlan_TCI;
> > +	} veth;
> > +	ssize_t total = 0, off = 0;
> 
> Why off = 0 here?
> We initialize it to total unconditionally, don't we?
> 
> >  	int vlan_offset = 0;
> >  
> >  	if (!(tun->flags & TUN_NO_PI)) {
> > @@ -1248,14 +1252,11 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> >  		total += tun->vnet_hdr_sz;
> >  	}
> >  
> > +	off = total;
> >  	if (!vlan_tx_tag_present(skb)) {
> >  		len = min_t(int, skb->len, len);
> >  	} else {
> >  		int copy, ret;
> > -		struct {
> > -			__be16 h_vlan_proto;
> > -			__be16 h_vlan_TCI;
> > -		} veth;
> >  
> >  		veth.h_vlan_proto = skb->vlan_proto;
> >  		veth.h_vlan_TCI = htons(vlan_tx_tag_get(skb));
> > @@ -1264,22 +1265,22 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> >  		len = min_t(int, skb->len + VLAN_HLEN, len);
> >  
> >  		copy = min_t(int, vlan_offset, len);
> > -		ret = skb_copy_datagram_const_iovec(skb, 0, iv, total, copy);
> > +		ret = skb_copy_datagram_const_iovec(skb, 0, iv, off, copy);
> >  		len -= copy;
> > -		total += copy;
> > +		off += copy;
> >  		if (ret || !len)
> >  			goto done;
> >  
> >  		copy = min_t(int, sizeof(veth), len);
> > -		ret = memcpy_toiovecend(iv, (void *)&veth, total, copy);
> > +		ret = memcpy_toiovecend(iv, (void *)&veth, off, copy);
> >  		len -= copy;
> > -		total += copy;
> > +		off += copy;
> >  		if (ret || !len)
> >  			goto done;
> 
> This seems wrong: if one of the branches above is taken, total is
> never incremented.
> 
> >  	}
> >  
> > -	skb_copy_datagram_const_iovec(skb, vlan_offset, iv, total, len);
> > -	total += len;
> > +	skb_copy_datagram_const_iovec(skb, vlan_offset, iv, off, len);
> > +	total += skb->len + (vlan_offset ? sizeof(veth) : 0);
> >  
> >  done:
> >  	tun->dev->stats.tx_packets++;
> 
> I also think it's inelegant that the veth struct is now in the
> outside scope, and the extra ? is also ugly.
> 
> Here's a smaller patch to fix all these problems - what do you think?
> 
> 
> 
> Signed-off-by: Michael S. Tsirkin <[email-masked]>
> 
> ---
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index 782e38b..3297e41 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1183,7 +1183,7 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>  			    const struct iovec *iv, int len)
>  {
>  	struct tun_pi pi = { 0, skb->protocol };
> -	ssize_t total = 0;
> +	ssize_t total = 0, offset;
>  	int vlan_offset = 0;
>  
>  	if (!(tun->flags & TUN_NO_PI)) {
> @@ -1248,6 +1248,8 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>  		total += tun->vnet_hdr_sz;
>  	}
>  
> +	offset = total;
> +	total += skb->len;
>  	if (!vlan_tx_tag_present(skb)) {
>  		len = min_t(int, skb->len, len);
>  	} else {
> @@ -1257,6 +1259,8 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>  			__be16 h_vlan_TCI;
>  		} veth;
>  
> +		total += sizeof(veth);
> +
>  		veth.h_vlan_proto = skb->vlan_proto;
>  		veth.h_vlan_TCI = htons(vlan_tx_tag_get(skb));
>  
> @@ -1279,7 +1283,6 @@ static ssize_t tun_put_user(struct tun_struct *tun,
>  	}
>  
>  	skb_copy_datagram_const_iovec(skb, vlan_offset, iv, total, len);
> -	total += len;
>  
>  done:
>  	tun->dev->stats.tx_packets++;
> 
> > -- 
> > 1.8.3.2