Re: [PATCH] xfrm: Add pre-encap fragmentation for packet offload

[Steffen Klassert <steffen.klassert@secunet.com>]

On Sun, Nov 24, 2024 at 11:35:31AM +0200, Ilia Lin wrote:
> In packet offload mode the raw packets will be sent to the NiC,
> and will not return to the Network Stack. In event of crossing
> the MTU size after the encapsulation, the NiC HW may not be
> able to fragment the final packet.
> Adding mandatory pre-encapsulation fragmentation for both
> IPv4 and IPv6, if tunnel mode with packet offload is configured
> on the state.
> 
> Signed-off-by: Ilia Lin <ilia.lin@kernel.org>
> ---
>  net/ipv4/xfrm4_output.c | 31 +++++++++++++++++++++++++++++--
>  net/ipv6/xfrm6_output.c |  8 ++++++--
>  2 files changed, 35 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ipv4/xfrm4_output.c b/net/ipv4/xfrm4_output.c
> index 3cff51ba72bb0..a4271e0dd51bb 100644
> --- a/net/ipv4/xfrm4_output.c
> +++ b/net/ipv4/xfrm4_output.c
> @@ -14,17 +14,44 @@
>  #include <net/xfrm.h>
>  #include <net/icmp.h>
>  
> +static int __xfrm4_output_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
> +{
> +	return xfrm_output(sk, skb);
> +}
> +
>  static int __xfrm4_output(struct net *net, struct sock *sk, struct sk_buff *skb)
>  {
> -#ifdef CONFIG_NETFILTER
> -	struct xfrm_state *x = skb_dst(skb)->xfrm;
> +	struct dst_entry *dst = skb_dst(skb);
> +	struct xfrm_state *x = dst->xfrm;
> +	unsigned int mtu;
> +	bool toobig;
>  
> +#ifdef CONFIG_NETFILTER
>  	if (!x) {
>  		IPCB(skb)->flags |= IPSKB_REROUTED;
>  		return dst_output(net, sk, skb);
>  	}
>  #endif
>  
> +	if (x->props.mode != XFRM_MODE_TUNNEL || x->xso.type != XFRM_DEV_OFFLOAD_PACKET)
> +		goto skip_frag;
> +
> +	mtu = xfrm_state_mtu(x, dst_mtu(skb_dst(skb)));
> +
> +	toobig = skb->len > mtu && !skb_is_gso(skb);
> +
> +	if (!skb->ignore_df && toobig && skb->sk) {
> +		xfrm_local_error(skb, mtu);
> +		kfree_skb(skb);
> +		return -EMSGSIZE;
> +	}
> +
> +	if (toobig) {
> +		IPCB(skb)->frag_max_size = mtu;
> +		return ip_do_fragment(net, sk, skb, __xfrm4_output_finish);
> +	}

This would fragment the packet even if the DF bit is set.

Please no further packet offload stuff in generic code.