Re: [can-next,v2,6/6] can: gw: use can_gw_hops instead of sk_buff::csum_start
From: Jakub Kicinski <kuba@kernel.org>
Date: 2026-01-28 16:12:50
Also in:
linux-can, lkml
This is an AI-generated review of your patch. The human sending this email has considered the AI review valid, or at least plausible. 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.
quoted hunk ↗ jump to hunk
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
[ ... ]
quoted hunk ↗ jump to hunk
@@ -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.
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;
[ ... ] -- pw-bot: cr