Re: [PATCH net-next v2] seg6: reallocate the skb head on L2 encapsulation only when needed

[Yuya Kusakabe]

On Tue, Sep 8, 2026 at 5:24 PM netdev-bot+sashiko@xxxxxxxxxx wrote:
> Can that wrap to roughly 65534 and make the following
>
> 	memmove(skb_mac_header(skb), old_mac, skb->mac_len);
>
> write skb->mac_len bytes about 64 KB past skb->head?

Yes.  Forwarding plain ping6 traffic through a VLAN device with
reorder_hdr off on ingress reproduces it: skb->mac_header comes back as
65534 on a 704-byte head, and skb_mac_header_rebuild() writes 26 bytes
there.

> A similarly shaped window existed before this change, since the
> skb_cow_head() inside __seg6_do_srh_encap() never accounted for mac_len
> either.  Given that this patch takes over sizing the whole encapsulation
> up front, would it make sense to fix the amount here?

The window is not specific to the L2 modes, so I would rather fix it in
__seg6_do_srh_encap() and seg6_do_srh_encap_red() themselves.  Mode
encap reproduces it too, on unpatched net-next.  40475b63761a ("net:
ipv6: seg6_iptunnel: mitigate 2-realloc issue") replaced skb->mac_len
with dst_dev_overhead() in those two skb_cow_head() requests, and
dst_dev_overhead() is the smaller of the two whenever mac_len exceeds
LL_RESERVED_SPACE() of the egress device.  Asking for the larger of them
restores the guarantee without giving up the one 40475b63761a added.

I will send that against net, separately from this patch.

> This isn't a bug, but does this trailer match the form documented in
> Documentation/process/coding-assistants.rst?

No, it does not.  I will use "Assisted-by: LLM" from now on.