Re: [PATCH net 8/9] batman-adv: mcast: linearize skbuff for packet generation

[Sven Eckelmann <sven@narfation.org>]

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https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260821094813.201800-1-sw%40simonwunderlich.de?part=8

[...]
> Discovered by: gpt-5-6-sol, opus-5 · Missed by: sashiko-gemini
> This isn't a bug introduced by this patch, but the new comment states a
> whole-skb requirement that the existing kernel-doc contradicts.
> 
> batadv_mcast_forw_scrape() documents only a prefix requirement:
> 
>  * Caller needs to ensure that the region from the skb data's start up
>  * to/including the to be removed bytes are linearized.
> 
> while its body asserts the entire skb is linear:
> 
> 	SKB_LINEAR_ASSERT(skb);
> 
> with, in include/linux/skbuff.h:
> 
> 	#define SKB_LINEAR_ASSERT(skb)  BUG_ON(skb_is_nonlinear(skb))
> 
> Similarly, batadv_mcast_forw_packet()'s kernel-doc lists the caller
> obligations (set network/transport header, free the skb) but does not
> mention linearity at all; only the in-body comment mentions it, and it
> also understates the assert:
> 
> 	/* (at least) TVLV part needs to be linearized */
> 	SKB_LINEAR_ASSERT(skb);
> 
> Should the kernel-doc of batadv_mcast_forw_scrape() and
> batadv_mcast_forw_packet() be updated to say the whole skb must be linear?
> 
> Along the same lines, the invariant this patch establishes is not recorded
> at either interface boundary: batadv_mcast_forw_push()'s kernel-doc does
> not mention that on success the skb is linear, and neither the
> batadv_mcast_forw_mcsend() kernel-doc nor its declaration in
> net/batman-adv/multicast.h mentions the precondition that
> batadv_mcast_forw_packet() enforces with BUG_ON().
> 
> No in-tree caller violates this today: batadv_recv_mcast_packet() unshares
> and linearizes, and the batadv_mcast_forw_mcsend() path is now covered by
> the skb_linearize() added here. Would documenting the requirement help
> avoid a future caller reintroducing the BUG()? 

Will definitely not deal with this documentation work this bugfix patch. Linus 
or anyone else interested can work on it: https://www.open-mesh.org/issues/468

Regards,
	Sven

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