linux-kernel - Re: debugging oops after disconnecting Nexio USB touchscreen

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Message-ID: <Pine.LNX.4.44L0.0912051211530.15558-100000@netrider.rowland.org>
Date:	Sat, 5 Dec 2009 12:16:13 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Andreas Mohr <andi@...as.de>
cc:	Ondrej Zary <linux@...nbow-software.org>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: debugging oops after disconnecting Nexio USB touchscreen

On Sat, 5 Dec 2009, Andreas Mohr wrote:

> Hi,
> 
> > Furthermore, the patch shows that the second-to-last argument to 
> > usb_fill_bulk_urb() -- the completion function -- is NULL.  That is 
> > strictly illegal and it should have caused an oops as soon as the URB 
> > was used.
> 
> Then there's definitely a WARN_ON or so missing in
> static inline void usb_fill_bulk_urb()

No there isn't.  That inline just fills in a bunch of fields.

You could argue that there is a WARN_ON missing in usb_submit_urb().  
I don't think one is necessary, but you might disagree.  Either way,
both of us missed the fact that right at the start of usb_submit_urb()
is a check for urb->complete being NULL; if it is NULL then the
submission simply fails (and there is no oops).

> And highly likely more checks in those areas that are causing my (and
> other people's) ftdi_sio tests and USB audio (MIPS mmap) to fail.
> Followup soon.

Sometimes having too many checks is worse than having too few, 
especially if the failure modes are relatively easy to handle.

Alan Stern

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