Re: [PATCH net-next] mlxsw: spectrum_fid: use a dedicated list head pointer for sorted insert

[Ido Schimmel]

On Mon, May 25, 2026 at 03:17:59PM +0800, Maoyi Xie wrote:
> mlxsw_sp_fid_port_vid_list_add() inserts into a list sorted by
> local_port. It walks the list to find the first entry with a
> larger local_port, then inserts the new entry before it:
> 
>     list_add_tail(&port_vid->list, &tmp_port_vid->list);
> 
> If the loop falls through (the new local_port is the largest),
> tmp_port_vid runs off the end of the list. &tmp_port_vid->list
> then ends up at the list head itself (container_of() offsets
> cancel), and list_add_tail() inserts at the tail. So the code
> works today.
> 
> It is fragile though. Anyone who later adds a read of another
> field of tmp_port_vid will hit memory outside the list head.
> 
> Track the insertion point with a dedicated list_head pointer.
> Initialise insert_before to &fid->port_vid_list, set it to
> &tmp_port_vid->list only on early break, and pass insert_before
> to list_add_tail(). The cursor is no longer touched after the
> loop. Behaviour is unchanged.
> 
> Same shape as the Koschel cleanups from 2022 (e.g. 99d8ae4ec8a
> tracing, 2966a9918df clockevents, dc1acd5c946 dlm).
> 
> Signed-off-by: Maoyi Xie <maoyixie.tju@xxxxxxxxx>

Reviewed-by: Ido Schimmel <idosch@xxxxxxxxxx>