linux-kernel - Re: [PATCH] USB: don't read past config->interface[] if usb_control_msg() fails in usb_reset

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Date:	Tue, 9 Feb 2010 14:52:50 -0800
From:	Sarah Sharp <sarah.a.sharp@...ux.intel.com>
To:	Roel Kluin <roel.kluin@...il.com>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Greg Kroah-Hartman <gregkh@...e.de>, linux-usb@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] USB: don't read past config->interface[] if
 usb_control_msg() fails in usb_reset_configuration()

On Tue, Feb 09, 2010 at 11:01:24PM +0100, Roel Kluin wrote:
> After the loop `for (i = 0; i < config->desc.bNumInterfaces; i++)' if no
> break occurred, i equals config->desc.bNumInterfaces. so if
> usb_control_msg() failed then after goto reset_old_alts we read from
> config->interface[config->desc.bNumInterfaces].
> 
> Reported-by: "Juha Leppanen" <juha_motorsportcom@...kku.com>
> Signed-off-by: Roel Kluin <roel.kluin@...il.com>
> ---
> 
> >> You correctly identified a problem, but your fix is wrong -- or at 
> >> least, it is much too complicated. The proper fix goes like this:
> > 
> >  	/* If not, reinstate the old alternate settings */
> >  	if (retval < 0) {
> >  reset_old_alts:
> > -		for (; i >= 0; i--) {
> > +		for (i--; i >= 0; i--) {
> >  			struct usb_interface *intf = config->interface[i];
> >  			struct usb_host_interface *alt;
> 
> 
> Are you really sure this is better?

Yes.

> If usb_hcd_alloc_bandwidth() fails, in the loop _before_ the
> reset_old_alts label, don't we still have to reinstate the old
> alternate settings for that usb_interface config->interface[i]? This
> was what my initial patch tried to do.

No, you do not.  Take a look at usb_hcd_alloc_bandwidth() in
drivers/usb/core/hcd.c.  If an allocation fails for interface i when
the HCD's check_bandwidth() function is called, then
hcd->driver->reset_bandwidth is called.  We only need to clean up the
interfaces that have successfully been allocated bandwidth (0 to i - 1).

> Alternatively usb_hcd_alloc_bandwidth() could undo what it does if an
> error occurs there, then I think i could be decremented.

Yes, that's what usb_hcd_alloc_bandwidth() does at the reset label.

Sarah
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