[PATCH net] ipv4: allow local fragmentation in ip_finish_output_gso()

[PATCH net] ipv4: allow local fragmentation in ip_finish_output_gso()

[Lance Richardson]

Some configurations (e.g. geneve interface with default
MTU of 1500 over an ethernet interface with 1500 MTU) result
in the transmission of packets that exceed the configured MTU.
While this should be considered to be a "bad" configuration,
it is still allowed and should not result in the sending
of packets that exceed the configured MTU.

Fix by dropping the assumption in ip_finish_output_gso() that
locally originated gso packets will never need fragmentation.
Basic testing using iperf (observing CPU usage and bandwidth)
have shown no measurable performance impact for traffic not
requiring fragmentation.

Fixes: c7ba65d7b649 ("net: ip: push gso skb forwarding handling down the stack")
Reported-by: Jan Tluka <jtluka@redhat.com>
Signed-off-by: Lance Richardson <lrichard@redhat.com>
---
 net/ipv4/ip_output.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 03e7f73..4971401 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -239,11 +239,9 @@ static int ip_finish_output_gso(struct net *net, struct sock *sk,
 	struct sk_buff *segs;
 	int ret = 0;
 
-	/* common case: fragmentation of segments is not allowed,
-	 * or seglen is <= mtu
+	/* common case: seglen is <= mtu
 	 */
-	if (((IPCB(skb)->flags & IPSKB_FRAG_SEGS) == 0) ||
-	      skb_gso_validate_mtu(skb, mtu))
+	if (skb_gso_validate_mtu(skb, mtu))
 		return ip_finish_output2(net, sk, skb);
 
 	/* Slowpath -  GSO segment length is exceeding the dst MTU.
-- 
2.5.5

Re: [PATCH net] ipv4: allow local fragmentation in ip_finish_output_gso()

[Hannes Frederic Sowa]

On Wed, Nov 2, 2016, at 17:29, Lance Richardson wrote:
> Some configurations (e.g. geneve interface with default
> MTU of 1500 over an ethernet interface with 1500 MTU) result
> in the transmission of packets that exceed the configured MTU.
> While this should be considered to be a "bad" configuration,
> it is still allowed and should not result in the sending
> of packets that exceed the configured MTU.
> 
> Fix by dropping the assumption in ip_finish_output_gso() that
> locally originated gso packets will never need fragmentation.
> Basic testing using iperf (observing CPU usage and bandwidth)
> have shown no measurable performance impact for traffic not
> requiring fragmentation.
> 
> Fixes: c7ba65d7b649 ("net: ip: push gso skb forwarding handling down the
> stack")
> Reported-by: Jan Tluka <jtluka@redhat.com>
> Signed-off-by: Lance Richardson <lrichard@redhat.com>

Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>

Re: [PATCH net] ipv4: allow local fragmentation in ip_finish_output_gso()

[Florian Westphal]

Lance Richardson <lrichard@redhat.com> wrote:
> Some configurations (e.g. geneve interface with default
> MTU of 1500 over an ethernet interface with 1500 MTU) result
> in the transmission of packets that exceed the configured MTU.
> While this should be considered to be a "bad" configuration,
> it is still allowed and should not result in the sending
> of packets that exceed the configured MTU.
> 
> Fix by dropping the assumption in ip_finish_output_gso() that
> locally originated gso packets will never need fragmentation.
> Basic testing using iperf (observing CPU usage and bandwidth)
> have shown no measurable performance impact for traffic not
> requiring fragmentation.
> 
> Fixes: c7ba65d7b649 ("net: ip: push gso skb forwarding handling down the stack")
> Reported-by: Jan Tluka <jtluka@redhat.com>
> Signed-off-by: Lance Richardson <lrichard@redhat.com>
> ---
>  net/ipv4/ip_output.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> index 03e7f73..4971401 100644
> --- a/net/ipv4/ip_output.c
> +++ b/net/ipv4/ip_output.c
> @@ -239,11 +239,9 @@ static int ip_finish_output_gso(struct net *net, struct sock *sk,
>  	struct sk_buff *segs;
>  	int ret = 0;
>  
> -	/* common case: fragmentation of segments is not allowed,
> -	 * or seglen is <= mtu
> +	/* common case: seglen is <= mtu
>  	 */
> -	if (((IPCB(skb)->flags & IPSKB_FRAG_SEGS) == 0) ||
> -	      skb_gso_validate_mtu(skb, mtu))
> +	if (skb_gso_validate_mtu(skb, mtu))

IPSKB_FRAG_SEGS is now useless and should be removed.

Re: [PATCH net] ipv4: allow local fragmentation in ip_finish_output_gso()

[Lance Richardson]

----- Original Message -----
> From: "Florian Westphal" <fw@strlen.de>
> To: "Lance Richardson" <lrichard@redhat.com>
> Cc: netdev@vger.kernel.org, fw@strlen.de, jtluka@redhat.com
> Sent: Wednesday, November 2, 2016 1:20:36 PM
> Subject: Re: [PATCH net] ipv4: allow local fragmentation in ip_finish_output_gso()
> 
> Lance Richardson <lrichard@redhat.com> wrote:
> > Some configurations (e.g. geneve interface with default
> > MTU of 1500 over an ethernet interface with 1500 MTU) result
> > in the transmission of packets that exceed the configured MTU.
> > While this should be considered to be a "bad" configuration,
> > it is still allowed and should not result in the sending
> > of packets that exceed the configured MTU.
> > 
> > Fix by dropping the assumption in ip_finish_output_gso() that
> > locally originated gso packets will never need fragmentation.
> > Basic testing using iperf (observing CPU usage and bandwidth)
> > have shown no measurable performance impact for traffic not
> > requiring fragmentation.
> > 
> > Fixes: c7ba65d7b649 ("net: ip: push gso skb forwarding handling down the
> > stack")
> > Reported-by: Jan Tluka <jtluka@redhat.com>
> > Signed-off-by: Lance Richardson <lrichard@redhat.com>
> > ---
> >  net/ipv4/ip_output.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> > index 03e7f73..4971401 100644
> > --- a/net/ipv4/ip_output.c
> > +++ b/net/ipv4/ip_output.c
> > @@ -239,11 +239,9 @@ static int ip_finish_output_gso(struct net *net,
> > struct sock *sk,
> >  	struct sk_buff *segs;
> >  	int ret = 0;
> >  
> > -	/* common case: fragmentation of segments is not allowed,
> > -	 * or seglen is <= mtu
> > +	/* common case: seglen is <= mtu
> >  	 */
> > -	if (((IPCB(skb)->flags & IPSKB_FRAG_SEGS) == 0) ||
> > -	      skb_gso_validate_mtu(skb, mtu))
> > +	if (skb_gso_validate_mtu(skb, mtu))
> 
> IPSKB_FRAG_SEGS is now useless and should be removed.
> 

Thanks, Florian, I've removed IPSKB_FRAG_SEGS in v2.

   Lance