Xen devel - Nov 2013 - [PATCH net v2] xen-netback: fix fragment detection in checksum setup

The code to detect fragments in checksum_setup() was missing for IPv4 and
too eager for IPv6. (It transpires that Windows seems to send IPv6 packets
with a fragment header even if they are not a fragment - i.e. offset is zero,
and M bit is not set).

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: David Vrabel <david.vrabel@citrix.com>
---
v2

- Added comments noting what fragment/offset masks mean
  
 drivers/net/xen-netback/netback.c |   33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/drivers/net/xen-netback/netback.c
b/drivers/net/xen-netback/netback.c
index 919b650..c7464d8 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -1165,15 +1165,28 @@ static int checksum_setup_ip(struct xenvif *vif, struct
sk_buff *skb,
 	struct iphdr *iph = (void *)skb->data;
 	unsigned int header_size;
 	unsigned int off;
+	bool fragment;
 	int err = -EPROTO;
 
+	fragment = false;
+
 	off = sizeof(struct iphdr);
 
 	header_size = skb->network_header + off + MAX_IPOPTLEN;
 	maybe_pull_tail(skb, header_size);
 
+	/* 3fff -> fragment offset != 0 OR more fragments */
+	if (ntohs(iph->frag_off) & 0x3fff)
+		fragment = true;
+
 	off = iph->ihl * 4;
 
+	if (fragment) {
+		if (net_ratelimit())
+			netdev_err(vif->dev, "Packet is a fragment!\n");
+		goto out;
+	}
+
 	switch (iph->protocol) {
 	case IPPROTO_TCP:
 		if (!skb_partial_csum_set(skb, off,
@@ -1237,6 +1250,7 @@ static int checksum_setup_ipv6(struct xenvif *vif, struct
sk_buff *skb,
 	bool fragment;
 	bool done;
 
+	fragment = false;
 	done = false;
 
 	off = sizeof(struct ipv6hdr);
@@ -1275,9 +1289,22 @@ static int checksum_setup_ipv6(struct xenvif *vif, struct
sk_buff *skb,
 			off += (hp->hdrlen+2)<<2;
 			break;
 		}
-		case IPPROTO_FRAGMENT:
-			fragment = true;
-			/* fall through */
+		case IPPROTO_FRAGMENT: {
+			struct frag_hdr *hp = (void *)(skb->data + off);
+
+			header_size = skb->network_header +
+				off +
+				sizeof(struct frag_hdr);
+			maybe_pull_tail(skb, header_size);
+
+			/* fff9 -> fragment offset != 0 OR more fragments */
+			if (ntohs(hp->frag_off) & 0xfff9)
+				fragment = true;
+
+			nexthdr = hp->nexthdr;
+			off += sizeof(struct frag_hdr);
+			break;
+		}
 		default:
 			done = true;
 			break;
-- 
1.7.10.4

On Thu, 2013-11-28 at 13:23 +0000, Paul Durrant wrote:
> The code to detect fragments in checksum_setup() was missing for IPv4 and
> too eager for IPv6. (It transpires that Windows seems to send IPv6 packets
> with a fragment header even if they are not a fragment - i.e. offset is
zero,
> and M bit is not set).
> +	/* 3fff -> fragment offset != 0 OR more fragments */
> +	if (ntohs(iph->frag_off) & 0x3fff)
> +		fragment = true;
> +
What about the more self documented and faster :

if (iph->frag_off & htons(IP_OFFSET | IP_MF))
	fragment = true;

On 2013/11/28 21:23, Paul Durrant wrote:
> The code to detect fragments in checksum_setup() was missing for IPv4 and
> too eager for IPv6. (It transpires that Windows seems to send IPv6 packets
> with a fragment header even if they are not a fragment - i.e. offset is
zero,
> and M bit is not set).
>
> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> Cc: Wei Liu <wei.liu2@citrix.com>
> Cc: Ian Campbell <ian.campbell@citrix.com>
> Cc: David Vrabel <david.vrabel@citrix.com>
> ---
> v2
>
> - Added comments noting what fragment/offset masks mean
>    
>   drivers/net/xen-netback/netback.c |   33
++++++++++++++++++++++++++++++---
>   1 file changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/net/xen-netback/netback.c
b/drivers/net/xen-netback/netback.c
> index 919b650..c7464d8 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -1165,15 +1165,28 @@ static int checksum_setup_ip(struct xenvif *vif,
struct sk_buff *skb,
>   	struct iphdr *iph = (void *)skb->data;
>   	unsigned int header_size;
>   	unsigned int off;
> +	bool fragment;
>   	int err = -EPROTO;
>   
> +	fragment = false;
Is it better to initialize fragment directly as following?
bool fragment = false;
> +
>   	off = sizeof(struct iphdr);
>   
>   	header_size = skb->network_header + off + MAX_IPOPTLEN;
>   	maybe_pull_tail(skb, header_size);
>   
> +	/* 3fff -> fragment offset != 0 OR more fragments */
> +	if (ntohs(iph->frag_off) & 0x3fff)
> +		fragment = true;
> +
>   	off = iph->ihl * 4;
>   
> +	if (fragment) {
> +		if (net_ratelimit())
> +			netdev_err(vif->dev, "Packet is a fragment!\n");
> +		goto out;
> +	}
> +
>   	switch (iph->protocol) {
>   	case IPPROTO_TCP:
>   		if (!skb_partial_csum_set(skb, off,
> @@ -1237,6 +1250,7 @@ static int checksum_setup_ipv6(struct xenvif *vif,
struct sk_buff *skb,
>   	bool fragment;
>   	bool done;
>   
> +	fragment = false;
>   	done = false;
Same as above for "done" and "fragment"...

Thanks
Annie

> -----Original Message-----
> From: annie li [mailto:annie.li@oracle.com]
> Sent: 29 November 2013 05:36
> To: Paul Durrant
> Cc: xen-devel@lists.xen.org; netdev@vger.kernel.org; Wei Liu; Ian Campbell;
> David Vrabel
> Subject: Re: [PATCH net v2] xen-netback: fix fragment detection in checksum
> setup
> 
> 
> On 2013/11/28 21:23, Paul Durrant wrote:
> > The code to detect fragments in checksum_setup() was missing for IPv4
> and
> > too eager for IPv6. (It transpires that Windows seems to send IPv6
packets
> > with a fragment header even if they are not a fragment - i.e. offset
is zero,
> > and M bit is not set).
> >
> > Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
> > Cc: Wei Liu <wei.liu2@citrix.com>
> > Cc: Ian Campbell <ian.campbell@citrix.com>
> > Cc: David Vrabel <david.vrabel@citrix.com>
> > ---
> > v2
> >
> > - Added comments noting what fragment/offset masks mean
> >
> >   drivers/net/xen-netback/netback.c |   33
> ++++++++++++++++++++++++++++++---
> >   1 file changed, 30 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
> netback/netback.c
> > index 919b650..c7464d8 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -1165,15 +1165,28 @@ static int checksum_setup_ip(struct xenvif
*vif,
> struct sk_buff *skb,
> >   	struct iphdr *iph = (void *)skb->data;
> >   	unsigned int header_size;
> >   	unsigned int off;
> > +	bool fragment;
> >   	int err = -EPROTO;
> >
> > +	fragment = false;
> 
> Is it better to initialize fragment directly as following?
> bool fragment = false;
> 
I think that''s a matter of personal taste. I tend to favour this style.

  Paul
> > +
> >   	off = sizeof(struct iphdr);
> >
> >   	header_size = skb->network_header + off + MAX_IPOPTLEN;
> >   	maybe_pull_tail(skb, header_size);
> >
> > +	/* 3fff -> fragment offset != 0 OR more fragments */
> > +	if (ntohs(iph->frag_off) & 0x3fff)
> > +		fragment = true;
> > +
> >   	off = iph->ihl * 4;
> >
> > +	if (fragment) {
> > +		if (net_ratelimit())
> > +			netdev_err(vif->dev, "Packet is a fragment!\n");
> > +		goto out;
> > +	}
> > +
> >   	switch (iph->protocol) {
> >   	case IPPROTO_TCP:
> >   		if (!skb_partial_csum_set(skb, off,
> > @@ -1237,6 +1250,7 @@ static int checksum_setup_ipv6(struct xenvif
*vif,
> struct sk_buff *skb,
> >   	bool fragment;
> >   	bool done;
> >
> > +	fragment = false;
> >   	done = false;
> 
> Same as above for "done" and "fragment"...
> 
> Thanks
> Annie

On 2013/11/29 17:10, Paul Durrant wrote:
>> -----Original Message-----
>> From: annie li [mailto:annie.li@oracle.com]
>> Sent: 29 November 2013 05:36
>> To: Paul Durrant
>> Cc: xen-devel@lists.xen.org; netdev@vger.kernel.org; Wei Liu; Ian
Campbell;
>> David Vrabel
>> Subject: Re: [PATCH net v2] xen-netback: fix fragment detection in
checksum
>> setup
>>
>>
>> On 2013/11/28 21:23, Paul Durrant wrote:
>>> The code to detect fragments in checksum_setup() was missing for
IPv4
>> and
>>> too eager for IPv6. (It transpires that Windows seems to send IPv6
packets
>>> with a fragment header even if they are not a fragment - i.e.
offset is zero,
>>> and M bit is not set).
>>>
>>> Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
>>> Cc: Wei Liu <wei.liu2@citrix.com>
>>> Cc: Ian Campbell <ian.campbell@citrix.com>
>>> Cc: David Vrabel <david.vrabel@citrix.com>
>>> ---
>>> v2
>>>
>>> - Added comments noting what fragment/offset masks mean
>>>
>>>    drivers/net/xen-netback/netback.c |   33
>> ++++++++++++++++++++++++++++++---
>>>    1 file changed, 30 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
>> netback/netback.c
>>> index 919b650..c7464d8 100644
>>> --- a/drivers/net/xen-netback/netback.c
>>> +++ b/drivers/net/xen-netback/netback.c
>>> @@ -1165,15 +1165,28 @@ static int checksum_setup_ip(struct xenvif
*vif,
>> struct sk_buff *skb,
>>>    	struct iphdr *iph = (void *)skb->data;
>>>    	unsigned int header_size;
>>>    	unsigned int off;
>>> +	bool fragment;
>>>    	int err = -EPROTO;
>>>
>>> +	fragment = false;
>> Is it better to initialize fragment directly as following?
>> bool fragment = false;
>>
> I think that''s a matter of personal taste. I tend to favour this
style.
>
That is OK, I point this out because it is inconsistent with other 
variable initialization in netback.c.

Thanks
Annie
>
>>> +
>>>    	off = sizeof(struct iphdr);
>>>
>>>    	header_size = skb->network_header + off + MAX_IPOPTLEN;
>>>    	maybe_pull_tail(skb, header_size);
>>>
>>> +	/* 3fff -> fragment offset != 0 OR more fragments */
>>> +	if (ntohs(iph->frag_off) & 0x3fff)
>>> +		fragment = true;
>>> +
>>>    	off = iph->ihl * 4;
>>>
>>> +	if (fragment) {
>>> +		if (net_ratelimit())
>>> +			netdev_err(vif->dev, "Packet is a fragment!\n");
>>> +		goto out;
>>> +	}
>>> +
>>>    	switch (iph->protocol) {
>>>    	case IPPROTO_TCP:
>>>    		if (!skb_partial_csum_set(skb, off,
>>> @@ -1237,6 +1250,7 @@ static int checksum_setup_ipv6(struct xenvif
*vif,
>> struct sk_buff *skb,
>>>    	bool fragment;
>>>    	bool done;
>>>
>>> +	fragment = false;
>>>    	done = false;
>> Same as above for "done" and "fragment"...
>>
>> Thanks
>> Annie
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

> -----Original Message-----
> From: Eric Dumazet [mailto:eric.dumazet@gmail.com]
> Sent: 28 November 2013 17:29
> To: Paul Durrant
> Cc: xen-devel@lists.xen.org; netdev@vger.kernel.org; Wei Liu; Ian Campbell;
> David Vrabel
> Subject: Re: [PATCH net v2] xen-netback: fix fragment detection in checksum
> setup
> 
> On Thu, 2013-11-28 at 13:23 +0000, Paul Durrant wrote:
> > The code to detect fragments in checksum_setup() was missing for IPv4
> and
> > too eager for IPv6. (It transpires that Windows seems to send IPv6
packets
> > with a fragment header even if they are not a fragment - i.e. offset
is zero,
> > and M bit is not set).
> 
> > +	/* 3fff -> fragment offset != 0 OR more fragments */
> > +	if (ntohs(iph->frag_off) & 0x3fff)
> > +		fragment = true;
> > +
> 
> What about the more self documented and faster :
> 
> if (iph->frag_off & htons(IP_OFFSET | IP_MF))
> 	fragment = true;
> 
Nicer definitely. I''ll need to add the equivalent def for IP6_OFFSET to
use the same style of test there.

Thanks,

  Paul