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From:   "zhuyan (M)" <zhuyan34@huawei.com>
To:     Alan Stern <stern@rowland.harvard.edu>
CC:     Greg KH <gregkh@linuxfoundation.org>,
        "anton@enomsg.org" <anton@enomsg.org>,
        "linux-usb@vger.kernel.org" <linux-usb@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "zhuyan (M)" <zhuyan34@huawei.com>
Subject: Re: [PATCH] usb:host: fix divide-by-zero in function fhci_queue_urb
Thread-Topic: [PATCH] usb:host: fix divide-by-zero in function fhci_queue_urb
Thread-Index: AdT05r4f+lcz9XnHRgGSjwdOkKXG4g==
Date:   Wed, 17 Apr 2019 07:09:41 +0000
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On Tue, 16 Apr 2019 11:07:56 -0400, Alan Stern wrote:

> On Tue, 16 Apr 2019, zhuyan (M) wrote:
> > On Tue, 16 Apr 2019 at 11:45:45 +0200, Greg KH wrote:
> > > On Tue, Apr 09, 2019 at 10:37:12PM +0800, zhuyan wrote:
> > > > In function fhci_queue_urb, the divisor of expression 
> > > > (urb->transfer_buffer_length % usb_maxpacket(urb->dev, urb->pipe,
> > > > usb_pipeout(urb->pipe))) may be zero.
> > > 
> > > How can you hit that?
> > > 
> > > > When it is zero, unexpected results may occur, so it is necessary 
> > > > to ensure that the divisor is not zero.
> > > > 
> > > > Signed-off-by: zhuyan <zhuyan34@huawei.com>
> > > 
> > > I need a "Full" name here, not just a single name.  Whatever you use to sign documents is good.
> > > 
> > > thanks,
> > > 
> > > greg k-h
> > 
> > In function usb_maxpacket, when ep is NULL, its return value is 0.  
> 
> fhci_queue_urb() shouldn't use urb->pipe to compute the maxpacket size 
> anyway.  It should use usb_endpoint_maxp(&urb->ep->desc).

Currently, fhci_queue_urb(), call usb_maxpacket() multiple times to calculate
 the maxpacket size. The usb_maxpacket() will call usb_endpoint_maxp() to 
compute the maxpacket size.

zhuyan

> > Then, in function fhci_queue_urb, the divisor of expression 
> > (urb->transfer_buffer_length % usb_maxpacket(urb->dev, urb->pipe, 
> > usb_pipeout(urb->pipe))) is zero. It will lead to unpredictable results.