On Tue Aug 10 2021, Vladimir Oltean wrote:
> rtnl_fdb_dump() has logic to split a dump of PF_BRIDGE neighbors into
> multiple netlink skbs if the buffer provided by user space is too small
> (one buffer will typically handle a few hundred FDB entries).
>
> When the current buffer becomes full, nlmsg_put() in
> dsa_slave_port_fdb_do_dump() returns -EMSGSIZE and DSA saves the index
> of the last dumped FDB entry, returns to rtnl_fdb_dump() up to that
> point, and then the dump resumes on the same port with a new skb, and
> FDB entries up to the saved index are simply skipped.
>
> Since dsa_slave_port_fdb_do_dump() is pointed to by the "cb" passed to
> drivers, then drivers must check for the -EMSGSIZE error code returned
> by it. Otherwise, when a netlink skb becomes full, DSA will no longer
> save newly dumped FDB entries to it, but the driver will continue
> dumping. So FDB entries will be missing from the dump.
>
> Fix the broken backpressure by propagating the "cb" return code and
> allow rtnl_fdb_dump() to restart the FDB dump with a new skb.
>
> Fixes: e4b27ebc780f ("net: dsa: Add DSA driver for Hirschmann Hellcreek switches")
> Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>

Acked-by: Kurt Kanzenbach <kurt@linutronix.de>