netdev - Re: [PATCH 1/1] can: skb: add and set local

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Message-ID: <20220511135522.2qvhtokb4j5qvr3j@pengutronix.de>
Date:   Wed, 11 May 2022 15:55:22 +0200
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Oliver Hartkopp <socketcan@...tkopp.net>
Cc:     Oleksij Rempel <o.rempel@...gutronix.de>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        Devid Antonio Filoni <devid.filoni@...uetechnologies.com>,
        kernel@...gutronix.de, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        David Jander <david@...tonic.nl>
Subject: Re: [PATCH 1/1] can: skb: add and set local_origin flag

On 11.05.2022 15:24:00, Oliver Hartkopp wrote:
> > Another use where skb->sk breaks is sending CAN frames with SO_TXTIME
> > with the sched_etf.
> > 
> > I have a patched version of cangen that uses SO_TXTIME. It attaches a
> > time to transmit to a CAN frame when sending it. If you send 10 frames,
> > each 100ms after the other and then exit the program, the first CAN
> > frames show up as TX'ed while the others (after closing the socket) show
> > up as RX'ed CAN frames in candump.
> 
> Hm, this could be an argument for the origin flag.
> 
> But I'm more scared about your described behaviour. What happens if
> the socket is still open?

SO_TXTIME makes an existing race window really, really, really wide.

The race window is between sendmsg() returning to user space and
can_put_echo_skb() -> can_create_echo_skb() -> can_skb_set_owner(). In
can_skb_set_owner() a reference to the socket is taken, if the socket is
not closed:

| https://elixir.bootlin.com/linux/v5.17.6/source/include/linux/can/skb.h#L75

If the socket closes _after_ sendmsg(), but _before_ the driver calls
can_put_echo_skb() the CAN frame will have no socket reference and show
up as RX'ed.

> There obviously must be some instance removing the sk reference, right??

No. That's all fine. We fixes that in:

| https://lore.kernel.org/all/20210226092456.27126-1-o.rempel@pengutronix.de/

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
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