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

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Date:   Fri, 15 Jan 2021 16:07:23 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     mkl@...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, 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?

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?