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

From: Sascha Hauer <s.hauer@pengutronix.de>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Paul Cercueil <paul@crapouillou.net>,
	linux-usb@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org, kernel@pengutronix.de
Subject: Re: [PATCH 1/2] usb: gadget: u_ether: Do not make UDC parent of the net device
Date: Fri, 10 Feb 2023 19:46:23 +0100	[thread overview]
Message-ID: <20230210184623.GC10447@pengutronix.de> (raw)
In-Reply-To: <Y+ZmsofmJPYRoUjW@rowland.harvard.edu>

On Fri, Feb 10, 2023 at 10:45:54AM -0500, Alan Stern wrote:
> 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.

I don't remember the details anymore. I'll do some more investigation
next week.

Sascha

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