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

From: Vlad Yasevich <[email-masked]>
Date: Mon, 09 Dec 2013 10:31:10 -0500

On 12/09/2013 05:55 AM, 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?
>
>> ---
>> 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++;
>

You have to use 'offset' instead of 'total' when doing skb_copy and
adjust offset as you write the vlan header.

I think something like this will fix it:

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 782e38b..d71c393 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 {
@@ -1262,24 +1264,24 @@ static ssize_t tun_put_user(struct tun_struct *tun,

 		vlan_offset = offsetof(struct vlan_ethhdr, h_vlan_proto);
 		len = min_t(int, skb->len + VLAN_HLEN, len);
+		total += VLAN_HLEN;

 		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, offset, copy);
 		len -= copy;
-		total += copy;
+		offset += 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, offset, copy);
 		len -= copy;
-		total += copy;
+		offset += copy;
 		if (ret || !len)
 			goto done;
 	}

-	skb_copy_datagram_const_iovec(skb, vlan_offset, iv, total, len);
-	total += len;
+	skb_copy_datagram_const_iovec(skb, vlan_offset, iv, offset, len);

 done:
 	tun->dev->stats.tx_packets++;

-vlad