linux-kernel - Re: [bpf, xdp] headroom - was: Re: Question about to KMSAN: uninit-value in can

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Message-ID: <20260106162306.0649424c@kernel.org>
Date: Tue, 6 Jan 2026 16:23:06 -0800
From: Jakub Kicinski <kuba@...nel.org>
To: Oliver Hartkopp <socketcan@...tkopp.net>
Cc: mkl@...gutronix.de, Prithvi <activprithvi@...il.com>, andrii@...nel.org,
 linux-can@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzkaller-bugs@...glegroups.com, netdev@...r.kernel.org
Subject: Re: [bpf, xdp] headroom - was: Re: Question about to KMSAN:
 uninit-value in can_receive

On Tue, 6 Jan 2026 13:04:41 +0100 Oliver Hartkopp wrote:
> When such skb is echo'ed back after successful transmission via 
> netif_rx() this leads to skb->skb_iif = skb->dev->ifindex;
> 
> To prevent a loopback the CAN frame must not be sent back to the 
> originating interface - even when it has been routed to different CAN 
> interfaces in the meantime (which always overwrites skb_iif).
> 
> Therefore we need to maintain the "real original" incoming interface.

Alternatively perhaps for this particular use case you could use
something like metadata_dst to mark the frame as forwarded / annotate
with the originating ifindex?