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

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Message-ID: <20260108071703.788c67ed@kernel.org>
Date: Thu, 8 Jan 2026 07:17:03 -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 Wed, 7 Jan 2026 16:34:13 +0100 Oliver Hartkopp wrote:
> > 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?  
> 
> I looked into it and the way how skb_dst is shared in the union behind 
> cb[] does not look very promising for skbs that wander up and down in 
> the network layer.

Maybe I'm misunderstanding, but skb_dst is only unioned with some
socket layer (TCP and sockmsg) fields, not with cb[]. It'd be
problematic if CAN gw frames had to traverse routing but I don't 
think they do?