Re: [PATCH 1/2] usb: gadget: u_ether: Do not make UDC parent of the net device
From: Alan Stern
Date: Fri Feb 10 2023 - 10:46:00 EST
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