Re: [PATCH net v4] net: qualcomm: rmnet: Update MTU handling during format changes

[Jakub Kicinski]

On Sat,  7 Nov 2026 23:57:27 -0800 Subash Abhinov Kasiviswanathan wrote:
> rmnet derives the MTU of its devices from the real device MTU minus the
> headroom needed by the egress data format. Several problems exist in
> how this is handled when the data format is set or changed:
> 
> - The headroom only accounts for the V4 egress checksum header and
>   ignores the V5 one, so devices using V5 checksum offload get an MTU
>   that is too large for the real device.
> - rmnet_newlink() sizes the first device from the port's data format
>   before the requested IFLA_RMNET_FLAGS have been applied, so the MTU
>   does not match the format that ends up being used.
> - A real device MTU smaller than the headroom underflows the unsigned
>   subtraction, and a real device with a jumbo MTU makes the creation of
>   the first device fail while resizing an existing one clamps it.
> - Existing endpoints are shrunk by writing dev->mtu directly. No MTU
>   notifiers run and userspace gets no link notification. The helper
>   also returns -1, and changelink flattens every failure into -EINVAL.
> - rmnet_changelink() publishes the new data format before resizing the
>   endpoints and reverts it on failure, so the datapath can observe a
>   format whose headers do not fit the MTUs. A failed resize can also
>   leave some endpoints shrunk, and a mux_id change requested in the
>   same call is kept even though the call returned an error.
> 
> Fix these by calculating the headroom from the requested data format,

These should probably be 5 separate fixes in a patch series?