Re: [PATCH net] can: j1939: fix lockless local-destination check

[Oleksij Rempel <o.rempel@pengutronix.de>]

Hi,

On Sun, Apr 19, 2026 at 10:06:14PM +0800, Shuhao Fu wrote:
> j1939_priv.ents[].nusers is documented as protected by priv->lock, and
> its updates already happen under that lock. j1939_can_recv() also reads
> it under read_lock_bh(). However, j1939_session_skb_queue() and
> j1939_tp_send() still read priv->ents[da].nusers without taking the
> lock.
> 
> Those transport-side checks decide whether to set J1939_ECU_LOCAL_DST, so
> they can race with j1939_local_ecu_get() and j1939_local_ecu_put() while
> userspace is binding or releasing sockets concurrently with TP traffic.
> This can misclassify TP/ETP sessions as local or remote and take the wrong
> transport path.
> 
> Fix both transport paths by routing the destination-locality check through
> a helper that reads ents[].nusers under read_lock_bh(&priv->lock).
> 
> Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
> Signed-off-by: Shuhao Fu <sfual@cse.ust.hk>

Thank you for your work. LGTM.
Tested-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Oleksij Rempel <o.rempel@pengutronix.de>

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