netdev - Re: [PATCH 4/4] ipv6: symbol_get to access a sit symbol

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Message-ID: <20200518063043.GA19046@lst.de>
Date:   Mon, 18 May 2020 08:30:43 +0200
From:   Christoph Hellwig <hch@....de>
To:     David Miller <davem@...emloft.net>
Cc:     hch@....de, kuba@...nel.org, kuznet@....inr.ac.ru,
        yoshfuji@...ux-ipv6.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] ipv6: symbol_get to access a sit symbol

On Sat, May 16, 2020 at 01:55:48PM -0700, David Miller wrote:
> From: Christoph Hellwig <hch@....de>
> Date: Fri, 15 May 2020 08:33:24 +0200
> 
> > My initial plan was to add a ->tunnel_ctl method to the net_device_ops,
> > and lift the copy_{to,from}_user for SIOCADDTUNNEL, SIOCCHGTUNNEL,
> > SIOCDELTUNNEL and maybe SIOCGETTUNNEL to net/socket.c.  But that turned
> > out to have two problems:
> > 
> >  - first these ioctls names use SIOCDEVPRIVATE range, that can also
> >    be implemented by other drivers
> >  - the ip_tunnel_parm struture is only used by the ipv4 tunneling
> >    drivers (including sit), the "real" ipv6 tunnels use a
> >    ip6_tnl_parm or ip6_tnl_parm structure instead
> 
> Yes, this is the core of the problem, the user provided data's type
> is unknown until we are very deep in the call chains.
> 
> I wonder if there is some clever way to propagate this size value
> "up"?

As far as I can tell the only information vectors is the net_device
structure or its op vector.  But even then we have the problem that
other devices use the SIOCDEVPRIVATE range for something else.

I'll look into implenenting the tunnel_ctl method just for kernel
callers (plus maybe a generic helper for the ioctl), and we'll see if
you like that better.