Re: [can-next,v2,6/6] can: gw: use can_gw_hops instead of sk_buff::csum_start

[Oliver Hartkopp]

On 28.01.26 17:12, Jakub Kicinski wrote:
> This is an AI-generated review of your patch. The human sending this
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>
> For local reproduction steps see https://netdev-ai.bots.linux.dev/ai-local.html
> ---
> can: gw: use can_gw_hops instead of sk_buff::csum_start
>
> This patch moves the hop counter storage from skb->csum_start to the CAN
> skb extension field can_skb_ext::can_gw_hops.
>
> > diff --git a/net/can/gw.c b/net/can/gw.c
> > index ad89a1913b34..2c134c9e5159 100644
> > --- a/net/can/gw.c
> > +++ b/net/can/gw.c
>
> [ ... ]
>
> > @@ -480,19 +480,8 @@ static void can_can_gw_rcv(struct sk_buff *skb, void *data)
> >   	/* Do not handle CAN frames routed more than 'max_hops' times.
> >   	 * In general we should never catch this delimiter which is intended
> >   	 * to cover a misconfiguration protection (e.g. circular CAN routes).
> > -	 *
> > -	 * The Controller Area Network controllers only accept CAN frames with
> > -	 * correct CRCs - which are not visible in the controller registers.
> > -	 * According to skbuff.h documentation the csum_start element for IP
> > -	 * checksums is undefined/unused when ip_summed == CHECKSUM_UNNECESSARY.
> > -	 * Only CAN skbs can be processed here which already have this property.
> >   	 */
> > -
> > -#define cgw_hops(skb) ((skb)->csum_start)
> > -
> > -	BUG_ON(skb->ip_summed != CHECKSUM_UNNECESSARY);
> > -
> > -	if (cgw_hops(skb) >= max_hops) {
> > +	if (csx->can_gw_hops >= max_hops) {
>              ^^^^^^^^^^^^^^^^
> Is csx->can_gw_hops initialized to zero for fresh CAN frames?
>
> The old code used skb->csum_start which was zeroed by skbuff_clear() during
> skb allocation. The new code reads from can_skb_ext::can_gw_hops, but looking
> at alloc_can_skb() in drivers/net/can/dev/skb.c, only can_iif is initialized:
>
>      csx = can_skb_ext_add(skb);
>      ...
>      csx->can_iif = dev->ifindex;
>
> The skb extension memory comes from kmem_cache_alloc() without __GFP_ZERO in
> __skb_ext_alloc(), so can_gw_hops will contain garbage from the slab.

Correct. It is not only a leftover of using csum_start but also of my 
first implementation attempt using another space in struct sk_buff which 
was also zero-initialized.

I'll add some initialization in can_skb_ext_add() for the next respin.

Excellent feedback!

Many thanks,
Oliver

> With default max_hops=1, if the garbage value is >= 1 (probability ~99.6%),
> the frame will be dropped at this check before any routing occurs.
>
> Should alloc_can_skb(), alloc_canfd_skb(), and alloc_canxl_skb() initialize
> csx->can_gw_hops = 0?
>
> >   		/* indicate deleted frames due to misconfiguration */
> >   		gwj->deleted_frames++;
> >   		return;
>
> [ ... ]