linux-kernel - Re: [PATCH 1/2] usb: gadget: u_ether: Do not make UDC parent of the net device

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Date:   Fri, 10 Feb 2023 10:45:54 -0500
From:   Alan Stern <stern@...land.harvard.edu>
To:     Sascha Hauer <s.hauer@...gutronix.de>
Cc:     Paul Cercueil <paul@...pouillou.net>, linux-usb@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH 1/2] usb: gadget: u_ether: Do not make UDC parent of the
 net device

On Fri, Feb 10, 2023 at 03:49:41PM +0100, Sascha Hauer wrote:
> On Thu, Feb 09, 2023 at 10:05:35AM -0500, Alan Stern wrote:
> > Sascha:
> > 
> > When you first posted your original patch, I wondered if it was really 
> > the right thing to do.  Making the net device not be a child of the UDC 
> > device means you can (in theory) have strange behavior such as the 
> > kernel suspending the USB device controller while expecting the network 
> > interface to keep on working.
> > 
> > Is there a different way of solving the original problem?
> 
> I don't know which. One thing would be to couple the lifetime of the
> ethernet device to the lifetime of the UDC, but the result would look
> different to userspace, so wouldn't be ideal either.
> 
> Note the original reason doing this change was that we saw backtraces
> when doing a 'reboot -f', the 'rmmod dummy_hcd' was just an easy
> reproducer for the problem.
> 
> One other possibility might be to take a reference to the UDC while
> it is in use so that the module can't be rmmoded. Not sure if that fixes
> my original problem though.

Not being familiar with the networking code, I don't really understand 
the original problem.  Does the use-after-free error occur when you try 
to dereference a dev->parent pointer in the ethernet device?

If that's so, then taking a reference (i.e. get_device()) on the parent 
device should fix the problem.

If not, maybe you can give a more detailed guide as to what's going 
wrong.

Alan Stern