linux-kernel - Re: [PATCH] usb: gadget: Check for NULL pointer in disconnect

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Message-ID: <20150324083927.GB2293@shlinux2>
Date:	Tue, 24 Mar 2015 16:39:29 +0800
From:	Peter Chen <peter.chen@...escale.com>
To:	Badhri Jagan Sridharan <badhri@...gle.com>
CC:	Felipe Balbi <balbi@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: gadget: Check for NULL pointer in disconnect

On Mon, Mar 23, 2015 at 04:00:37PM -0700, Badhri Jagan Sridharan wrote:
> > Do you really see some udc drivers call it after composite_unbind is
> > called? If it is, you may add dump_stack() to track that error.
> >
> > Besides this, function suspended_show is needed to add cdev NULL pointer
> > checking.
> 
> We see this happening occasionally in *not yet* upstreamed UDC
> code of some vendors (Yes, disconnect being called after unbind)

Does it a spurious interrupt?
I guess if you add struct usb_gadget_driver NULL pointer check check, it
will not occur:
	if (you_udc->driver)
		you_udc->driver->disconnect(&you_udc->gadget);
 
> After reviewing the entire composite.c file, I did notice that
> none of the functions check for NULL pointer when cdev is
> obtained from get_gadget_data.
> Is crashing/bringing down the whole kernel intentionally
> left to happen ?

Hmm, I don't think, the design may think it never occurs.

> Isn't printing a WARN/ERROR msg and
> returning not the preferable approach ?

I think BUG_ON is more suitable, let's see Felipe's comment.

> 
> On 03/22/2015 12:43 AM, Peter Chen wrote:
> >On Fri, Mar 20, 2015 at 04:40:52PM -0700, Badhri Jagan Sridharan wrote:
> >>Added a safety net to make sure that
> >>composite_disconnect does not end up disconneting
> >>a NULL device. Prevents NULL pointer crash.
> >>
> >>Signed-off-by: Badhri Jagan Sridharan <Badhri@...gle.com>
> >>---
> >>  drivers/usb/gadget/composite.c | 5 +++++
> >>  1 file changed, 5 insertions(+)
> >>
> >>diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
> >>index 13adfd1..90b37bd 100644
> >>--- a/drivers/usb/gadget/composite.c
> >>+++ b/drivers/usb/gadget/composite.c
> >>@@ -1823,6 +1823,11 @@ void composite_disconnect(struct usb_gadget *gadget)
> >>  	struct usb_composite_dev	*cdev = get_gadget_data(gadget);
> >>  	unsigned long			flags;
> >>
> >>+	if (!cdev) {
> >>+		WARN(1, "Trying to disconnect a NULL composite device\n");
> >>+		return;
> >>+	}
> >>+
> >
> >Do you really see some udc drivers call it after composite_unbind is
> >called? If it is, you may add dump_stack() to track that error.
> >
> >Besides this, function suspended_show is needed to add cdev NULL pointer
> >checking.
> >
> >>  	/* REVISIT:  should we have config and device level
> >>  	 * disconnect callbacks?
> >>  	 */
> >>--
> >>2.2.0.rc0.207.ga3a616c
> >>
> >>--
> >>To unsubscribe from this list: send the line "unsubscribe linux-usb" in
> >>the body of a message to majordomo@...r.kernel.org
> >>More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >

-- 

Best Regards,
Peter Chen
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