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

On Thu, Oct 1, 2026 at 12:27 AM Omar Ramadan [off-list ref] 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