Patchwork [V2,2/2] ipvs: Extend MTU check to account for IPv6 NAT defrag changes

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Submitter Jesper Dangaard Brouer
Date Aug. 28, 2012, 2:23 p.m.
Message ID <20120828142248.11475.15917.stgit@dragon>
Download mbox | patch
Permalink /patch/180502/
State Not Applicable
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Comments

Jesper Dangaard Brouer - Aug. 28, 2012, 2:23 p.m.
This patch is necessary, to make IPVS work, after Patrick McHardys
IPv6 NAT defragmentation changes.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
In V2: the tunnel mode is no longer a special case.

 net/netfilter/ipvs/ip_vs_xmit.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)


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Eric Dumazet - Aug. 28, 2012, 2:49 p.m.
On Tue, 2012-08-28 at 16:23 +0200, Jesper Dangaard Brouer wrote:
> This patch is necessary, to make IPVS work, after Patrick McHardys
> IPv6 NAT defragmentation changes.
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
> In V2: the tunnel mode is no longer a special case.
> 
>  net/netfilter/ipvs/ip_vs_xmit.c |    9 ++++++++-
>  1 files changed, 8 insertions(+), 1 deletions(-)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
> index 67a3978..56f6d5d 100644
> --- a/net/netfilter/ipvs/ip_vs_xmit.c
> +++ b/net/netfilter/ipvs/ip_vs_xmit.c
> @@ -88,7 +88,14 @@ __ip_vs_dst_check(struct ip_vs_dest *dest, u32 rtos)
>  static inline bool
>  __mtu_check_toobig_v6(const struct sk_buff *skb, u32 mtu)
>  {
> -	if (skb->len > mtu && !skb_is_gso(skb)) {
> +	if (IP6CB(skb)->frag_max_size) {
> +		/* frag_max_size tell us that, this packet have been
> +		 * defragmented by netfilter IPv6 conntrack module.
> +		 */
> +		if (IP6CB(skb)->frag_max_size > mtu)
> +			return true; /* largest fragment violate MTU */
> +	}
> +	else if (skb->len > mtu && !skb_is_gso(skb)) {
>  		return true; /* Packet size violate MTU size */
>  	}

Couldnt you use a single test ?

if (IP6CB(skb)->frag_max_size > mtu)
	return true;

if (skb->len > mtu && !skb_is_gso(skb))
	return true;



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Patrick McHardy - Aug. 28, 2012, 8:10 p.m.
On Tue, 28 Aug 2012, Jesper Dangaard Brouer wrote:

> This patch is necessary, to make IPVS work, after Patrick McHardys
> IPv6 NAT defragmentation changes.
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

I've folded this into the defragmentation patch to avoid temporary 
breakage. Thanks Jesper.
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Jesper Dangaard Brouer - Aug. 29, 2012, 7:02 a.m.
On Tue, 2012-08-28 at 07:49 -0700, Eric Dumazet wrote:
> On Tue, 2012-08-28 at 16:23 +0200, Jesper Dangaard Brouer wrote:
> > This patch is necessary, to make IPVS work, after Patrick McHardys
> > IPv6 NAT defragmentation changes.
> > 
> > Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> > ---
> > In V2: the tunnel mode is no longer a special case.
> > 
> >  net/netfilter/ipvs/ip_vs_xmit.c |    9 ++++++++-
> >  1 files changed, 8 insertions(+), 1 deletions(-)
> > 
> > diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
> > index 67a3978..56f6d5d 100644
> > --- a/net/netfilter/ipvs/ip_vs_xmit.c
> > +++ b/net/netfilter/ipvs/ip_vs_xmit.c
> > @@ -88,7 +88,14 @@ __ip_vs_dst_check(struct ip_vs_dest *dest, u32 rtos)
> >  static inline bool
> >  __mtu_check_toobig_v6(const struct sk_buff *skb, u32 mtu)
> >  {
> > -	if (skb->len > mtu && !skb_is_gso(skb)) {
> > +	if (IP6CB(skb)->frag_max_size) {
> > +		/* frag_max_size tell us that, this packet have been
> > +		 * defragmented by netfilter IPv6 conntrack module.
> > +		 */
> > +		if (IP6CB(skb)->frag_max_size > mtu)
> > +			return true; /* largest fragment violate MTU */
Implicit:         else
     			return false

(if it makes it more clear, not sure)
> > +	}
> > +	else if (skb->len > mtu && !skb_is_gso(skb)) {
> >  		return true; /* Packet size violate MTU size */
> >  	}
> 
> Couldnt you use a single test ?
> 
> if (IP6CB(skb)->frag_max_size > mtu)
> 	return true;
> 
> if (skb->len > mtu && !skb_is_gso(skb))
> 	return true;
> 

Nope, this will not work.

If (IP6CB(skb)->frag_max_size > 0) then we have a defragmented packet,
this means that skb->len cannot be used for MTU checking, because
skb->len is now the total length of all the fragments (which your
solution will fall-through to)

The unreadable version of the function is:

static inline bool
__mtu_check_toobig_v6_unreadable(const struct sk_buff *skb, u32 mtu)
{
       return !!((!IP6CB(skb)->frag_max_size &&
                  (skb->len > mtu && !skb_is_gso(skb)))
                 || IP6CB(skb)->frag_max_size > mtu);
}

Perhaps you like this version better (you seem to use constructions like
this), but I really dislike it, because I (personally) find it
harder/slower to read this kind of code, and I also believe its more
error prone when someone needs to extend this.
Eric Dumazet - Aug. 29, 2012, 8:43 a.m.
On Wed, 2012-08-29 at 09:02 +0200, Jesper Dangaard Brouer wrote:
> On Tue, 2012-08-28 at 07:49 -0700, Eric Dumazet wrote:
> > On Tue, 2012-08-28 at 16:23 +0200, Jesper Dangaard Brouer wrote:
> > > This patch is necessary, to make IPVS work, after Patrick McHardys
> > > IPv6 NAT defragmentation changes.
> > > 
> > > Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> > > ---
> > > In V2: the tunnel mode is no longer a special case.
> > > 
> > >  net/netfilter/ipvs/ip_vs_xmit.c |    9 ++++++++-
> > >  1 files changed, 8 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
> > > index 67a3978..56f6d5d 100644
> > > --- a/net/netfilter/ipvs/ip_vs_xmit.c
> > > +++ b/net/netfilter/ipvs/ip_vs_xmit.c
> > > @@ -88,7 +88,14 @@ __ip_vs_dst_check(struct ip_vs_dest *dest, u32 rtos)
> > >  static inline bool
> > >  __mtu_check_toobig_v6(const struct sk_buff *skb, u32 mtu)
> > >  {
> > > -	if (skb->len > mtu && !skb_is_gso(skb)) {
> > > +	if (IP6CB(skb)->frag_max_size) {
> > > +		/* frag_max_size tell us that, this packet have been
> > > +		 * defragmented by netfilter IPv6 conntrack module.
> > > +		 */
> > > +		if (IP6CB(skb)->frag_max_size > mtu)
> > > +			return true; /* largest fragment violate MTU */
> Implicit:         else
>      			return false
> 
> (if it makes it more clear, not sure)
> > > +	}
> > > +	else if (skb->len > mtu && !skb_is_gso(skb)) {
> > >  		return true; /* Packet size violate MTU size */
> > >  	}
> > 
> > Couldnt you use a single test ?
> > 
> > if (IP6CB(skb)->frag_max_size > mtu)
> > 	return true;
> > 
> > if (skb->len > mtu && !skb_is_gso(skb))
> > 	return true;
> > 
> 
> Nope, this will not work.
> 
> If (IP6CB(skb)->frag_max_size > 0) then we have a defragmented packet,
> this means that skb->len cannot be used for MTU checking, because
> skb->len is now the total length of all the fragments (which your
> solution will fall-through to)
> 

If the packet was not fragmented, its was a single frame.

But if this frame length is above mtu, packet is not too big ?

Sorry if its a stupid question.



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Jesper Dangaard Brouer - Aug. 29, 2012, 9:04 a.m.
On Wed, 2012-08-29 at 01:43 -0700, Eric Dumazet wrote:
> On Wed, 2012-08-29 at 09:02 +0200, Jesper Dangaard Brouer wrote:
> > On Tue, 2012-08-28 at 07:49 -0700, Eric Dumazet wrote:
> > > On Tue, 2012-08-28 at 16:23 +0200, Jesper Dangaard Brouer wrote:
> > > > This patch is necessary, to make IPVS work, after Patrick McHardys
> > > > IPv6 NAT defragmentation changes.
> > > > 
> > > > Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> > > > ---
> > > > In V2: the tunnel mode is no longer a special case.
> > > > 
> > > >  net/netfilter/ipvs/ip_vs_xmit.c |    9 ++++++++-
> > > >  1 files changed, 8 insertions(+), 1 deletions(-)
> > > > 
> > > > diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
> > > > index 67a3978..56f6d5d 100644
> > > > --- a/net/netfilter/ipvs/ip_vs_xmit.c
> > > > +++ b/net/netfilter/ipvs/ip_vs_xmit.c
> > > > @@ -88,7 +88,14 @@ __ip_vs_dst_check(struct ip_vs_dest *dest, u32 rtos)
> > > >  static inline bool
> > > >  __mtu_check_toobig_v6(const struct sk_buff *skb, u32 mtu)
> > > >  {
> > > > -	if (skb->len > mtu && !skb_is_gso(skb)) {
> > > > +	if (IP6CB(skb)->frag_max_size) {
> > > > +		/* frag_max_size tell us that, this packet have been
> > > > +		 * defragmented by netfilter IPv6 conntrack module.
> > > > +		 */
> > > > +		if (IP6CB(skb)->frag_max_size > mtu)
> > > > +			return true; /* largest fragment violate MTU */
> > Implicit:         else
> >      			return false
> > 
> > (if it makes it more clear, not sure)
> > > > +	}
> > > > +	else if (skb->len > mtu && !skb_is_gso(skb)) {
> > > >  		return true; /* Packet size violate MTU size */
> > > >  	}
> > > 
> > > Couldnt you use a single test ?
> > > 
> > > if (IP6CB(skb)->frag_max_size > mtu)
> > > 	return true;
> > > 
> > > if (skb->len > mtu && !skb_is_gso(skb))
> > > 	return true;
> > > 
> > 
> > Nope, this will not work.
> > 
> > If (IP6CB(skb)->frag_max_size > 0) then we have a defragmented packet,
> > this means that skb->len cannot be used for MTU checking, because
> > skb->len is now the total length of all the fragments (which your
> > solution will fall-through to)
> > 
> 
> If the packet was not fragmented, its was a single frame.
> 
> But if this frame length is above mtu, packet is not too big ?

Nope... not if its a defragmented/reassembled packet.

> Sorry if its a stupid question.

These changes have to be seen together with Patrick's patch:
 "netfilter: nf_conntrack_ipv6: improve fragmentation handling"
 http://thread.gmane.org/gmane.linux.network/241517/focus=241518

The IPv6 packet arriving have been defragmented/reassembled by the
nf_conntrack_ipv6 module.  Thus, they look like a normal, but big,
packet to us.   We let it through, because it will be re-fragmented
again later, but first we need to check if the largest fragment would
violate the MTU.

Hope it makes it more clear.


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Patch

diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
index 67a3978..56f6d5d 100644
--- a/net/netfilter/ipvs/ip_vs_xmit.c
+++ b/net/netfilter/ipvs/ip_vs_xmit.c
@@ -88,7 +88,14 @@  __ip_vs_dst_check(struct ip_vs_dest *dest, u32 rtos)
 static inline bool
 __mtu_check_toobig_v6(const struct sk_buff *skb, u32 mtu)
 {
-	if (skb->len > mtu && !skb_is_gso(skb)) {
+	if (IP6CB(skb)->frag_max_size) {
+		/* frag_max_size tell us that, this packet have been
+		 * defragmented by netfilter IPv6 conntrack module.
+		 */
+		if (IP6CB(skb)->frag_max_size > mtu)
+			return true; /* largest fragment violate MTU */
+	}
+	else if (skb->len > mtu && !skb_is_gso(skb)) {
 		return true; /* Packet size violate MTU size */
 	}
 	return false;