Subject: Re: [PATCH net v3] ipv4: allow local fragmentation in ip_finish_output_gso()

From: Shmulik Ladkani <[email-masked]>
Date: Thu, 3 Nov 2016 09:42:39 +0200

On Wed,  2 Nov 2016 16:36:17 -0400
Lance Richardson <[email-masked]> wrote:

> -	/* 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);

OK.
Resembles https://patchwork.ozlabs.org/patch/644724/ ;)

> -	if (skb_iif && !(df & htons(IP_DF))) {
> -		/* Arrived from an ingress interface, got encapsulated, with
> -		 * fragmentation of encapulating frames allowed.
> -		 * If skb is gso, the resulting encapsulated network segments
> -		 * may exceed dst mtu.
> -		 * Allow IP Fragmentation of segments.
> -		 */
> -		IPCB(skb)->flags |= IPSKB_FRAG_SEGS;
> -	}

The only potential issue I see removing this, is that the KNOWLEDGE of
the forwarding/tunnel-bridging usecases (that require a
skb_gso_validate_mtu check) is lost.

Meaning, if one decides (as claimed in the past) that locally generated
traffic must NOT go through fragmentation validation, then no one
remembers those other valid usecases (which are very specific and not
trivial to imagine) that MUST go through it; Therefore it can easily get
broken.

Maybe we can elaborate in the comment above the call to
skb_gso_validate_mtu in ip_finish_output_gso?

Best,
Shmulik