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

[Omar Ramadan <omar@blockcast.net>]

(Resending with the list and Cc's restored; my first copy went to
Eric only by mistake.)

On Thu, Oct 1, 2026, Eric Dumazet wrote:
> 1)  Note that tx checksum offload is off by default on amt, so the bug
> would not trigger by default.

You're right, and I should have checked that before sending. With
tx-checksum-ip-generic off, netdev_fix_features() drops GSO_UDP_L4
from amt's features, so validate_xmit_skb() segments any UDP GSO skb
before amt_dev_xmit() sees it. It takes "ethtool -K <amt> tx on" first.

The commit message, and my reply to the bot, also overstate the
symptom. Even with tx on, the per-tunnel copy keeps csum_start on the
inner UDP header, so where it is segmented in software,
__udp_gso_segment() rejects it and the copy is dropped, rather than
being cut into malformed AMT messages. That is from reading the code,
not from a reproducer.

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

Thanks, I have. Anything further I send that an LLM helped with will
carry an Assisted-by: tag and say what was and was not reproduced.

> 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

Agreed, please drop this patch. I'll do it that way for net-next,
with an amt.sh case that sends a UDP_SEGMENT burst through the relay
with tx on.

pw-bot: cr