Re: [PATCH net] amt: do not offer software GSO on the amt device

[Eric Dumazet]

On Thu, Oct 1, 2026 at 12:27 AM Omar Ramadan <omar@xxxxxxxxxxxxx> wrote:
>
> Found by code inspection: an LLM-assisted review of drivers/net/amt.c
> while I was working on an IPv6 outer transport for amt.
>
> It has not been triggered. I have no reproducer, and I have not seen
> it in production or in testing; it comes from reading the code. The
> path is a GSO skb, for example from a UDP_SEGMENT sender or a GRO
> merged UDP flow, that is routed to a relay's amt device: it reaches
> amt_dev_xmit() unsegmented because the device offers
> NETIF_F_GSO_SOFTWARE, and the outer UDP layer later segments each
> per-tunnel copy at the inner gso_size.
>
> Testing: W=1 build with CONFIG_IPV6=y and =n, and
> tools/testing/selftests/net/amt.sh passes with the patch applied, but
> amt.sh never sends a GSO skb, so nothing exercises the changed path.

1)  Note that tx checksum offload is off by default on amt, so the bug
would not trigger by default.

2) Please take a look at Documentation/process/coding-assistants.rst

3) A better fix would be (for net-next) to call udp_tunnel_handle_offloads()
in amt_send_multicast_data() like other UDP tunnels do