linux-kernel - Re: [RFC PATCH net 1/2] net: introduce CAN specific pointer in the struct net

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Date:   Fri, 22 Jan 2021 12:47:51 +0100
From:   Marc Kleine-Budde <mkl@...gutronix.de>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Oleksij Rempel <o.rempel@...gutronix.de>,
        "David S. Miller" <davem@...emloft.net>,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        Robin van der Gracht <robin@...tonic.nl>,
        syzbot+5138c4dd15a0401bec7b@...kaller.appspotmail.com,
        kernel@...gutronix.de, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH net 1/2] net: introduce CAN specific pointer in the
 struct net_device

On Fri, Jan 15, 2021 at 04:07:23PM -0800, Jakub Kicinski wrote:
> On Fri, 15 Jan 2021 15:30:35 +0100 Oleksij Rempel wrote:
> > Since 20dd3850bcf8 ("can: Speed up CAN frame receiption by using
> > ml_priv") the CAN framework uses per device specific data in the AF_CAN
> > protocol. For this purpose the struct net_device->ml_priv is used. Later
> > the ml_priv usage in CAN was extended for other users, one of them being
> > CAN_J1939.
> > 
> > Later in the kernel ml_priv was converted to an union, used by other
> > drivers. E.g. the tun driver started storing it's stats pointer.
> > 
> > Since tun devices can claim to be a CAN device, CAN specific protocols
> > will wrongly interpret this pointer, which will cause system crashes.
> > Mostly this issue is visible in the CAN_J1939 stack.
> > 
> > To fix this issue, we request a dedicated CAN pointer within the
> > net_device struct.
> 
> No strong objection, others already added their pointers, but 
> I wonder if we can't save those couple of bytes by adding a
> ml_priv_type, to check instead of dev->type? And leave it 0
> for drivers using stats?

Sounds good.

If we want to save even more bytes, it might be possible, to move the
wireless and wpan pointers to ml_priv.

	struct wireless_dev	*ieee80211_ptr;
	struct wpan_dev		*ieee802154_ptr;

> That way other device types which are limited by all being 
> ARPHDR_ETHER can start using ml_priv as well?
> 
> > +#if IS_ENABLED(CONFIG_CAN)
> > +	struct can_ml_priv	*can;
> > +#endif
> 
> Perhaps put it next to all the _ptr fields?

Makes sense. Oleksij will resping the series.

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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