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Date:   Wed, 20 Jul 2022 21:13:57 +0200
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Fedor Pchelkin <pchelkin@...ras.ru>
Cc:     Robin van der Gracht <robin@...tonic.nl>,
        Oleksij Rempel <linux@...pel-privat.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Elenita Hinds <ecathinds@...il.com>,
        Kurt Van Dijck <dev.kurt@...dijck-laurijssen.be>,
        linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Alexey Khoroshilov <khoroshilov@...ras.ru>
Subject: Re: [PATCH] can: j1939: Remove unnecessary WARN_ON_ONCE in
 j1939_sk_queue_activate_next_locked()

Hi Fedor,

On Wed, Jul 20, 2022 at 02:06:45PM +0300, Fedor Pchelkin wrote:
> The purpose of WARN_ON_ONCE if the session with the same parameters
> has already been activated and is currently in active_session_list is
> not very clear. Is this warning implemented to indicate that userspace
> is doing something wrong?

yes.

> As far as I can see, there are two lists: active_session_list (which
> is for the whole device) and sk_session_queue (which is unique for
> each j1939_sock), and the situation when we have two sessions with
> the same type, addresses and destinations in two different
> sk_session_queues (owned by two different sockets) is actually highly
> probable - one is active and the other is willing to become active
> but the j1939_session_activate() does not let that happen. It is
> correct behaviour as I assume.

No. It is not typical use case and most probably it will create
problems. Are you working on some system where this use case is valid?

> Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
> 
> Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
> Signed-off-by: Fedor Pchelkin <pchelkin@...ras.ru>
> Signed-off-by: Alexey Khoroshilov <khoroshilov@...ras.ru>
>
> ---
>  net/can/j1939/socket.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/can/j1939/socket.c b/net/can/j1939/socket.c
> index f5ecfdcf57b2..be4b73afa16c 100644
> --- a/net/can/j1939/socket.c
> +++ b/net/can/j1939/socket.c
> @@ -178,7 +178,7 @@ static void j1939_sk_queue_activate_next_locked(struct j1939_session *session)
>  	if (!first)
>  		return;
>  
> -	if (WARN_ON_ONCE(j1939_session_activate(first))) {
> +	if (j1939_session_activate(first)) {
>  		first->err = -EBUSY;
>  		goto activate_next;
>  	} else {
> -- 
> 2.25.1
> 
> 

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