linux-kernel - Re: [PATCH] usb: gadget: f_midi: Fixed a bug when buflen was smaller than wMaxPacketSize

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Date:	Thu, 10 Mar 2016 09:23:34 +0000
From:	Felipe Ferreri Tonello <eu@...ipetonello.com>
To:	Steve Calfee <stevecalfee@...il.com>
Cc:	USB list <linux-usb@...r.kernel.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Felipe Balbi <balbi@...nel.org>,
	Michal Nazarewicz <mina86@...a86.com>,
	Robert Baldyga <r.baldyga@...sung.com>
Subject: Re: [PATCH] usb: gadget: f_midi: Fixed a bug when buflen was smaller
 than wMaxPacketSize

Hi Steve,

On 09/03/16 22:43, Steve Calfee wrote:
> On Wed, Mar 9, 2016 at 11:39 AM, Felipe F. Tonello <eu@...ipetonello.com> wrote:
>> buflen by default (256) is smaller than wMaxPacketSize (512) in high-speed
>> devices.
>>
>> That caused the OUT endpoint to freeze if the host send any data packet of
>> length greater than 256 bytes.
>>
>> This is an example dump of what happended on that enpoint:
>> HOST:   [DATA][Length=260][...]
>> DEVICE: [NAK]
>> HOST:   [PING]
>> DEVICE: [NAK]
>> HOST:   [PING]
>> DEVICE: [NAK]
>> ...
>> HOST:   [PING]
>> DEVICE: [NAK]
>>
>> This patch fixes this problem by setting the minimum usb_request's buffer size
>> for the OUT endpoint as its wMaxPacketSize.
>>
>> Signed-off-by: Felipe F. Tonello <eu@...ipetonello.com>
>> ---
>>  drivers/usb/gadget/function/f_midi.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/usb/gadget/function/f_midi.c b/drivers/usb/gadget/function/f_midi.c
>> index 8475e3dc82d4..826ba641f29d 100644
>> --- a/drivers/usb/gadget/function/f_midi.c
>> +++ b/drivers/usb/gadget/function/f_midi.c
>> @@ -366,7 +366,9 @@ static int f_midi_set_alt(struct usb_function *f, unsigned intf, unsigned alt)
>>         /* allocate a bunch of read buffers and queue them all at once. */
>>         for (i = 0; i < midi->qlen && err == 0; i++) {
>>                 struct usb_request *req =
>> -                       midi_alloc_ep_req(midi->out_ep, midi->buflen);
>> +                       midi_alloc_ep_req(midi->out_ep,
>> +                               max_t(unsigned, midi->buflen,
>> +                                       bulk_out_desc.wMaxPacketSize));
>>                 if (req == NULL)
>>                         return -ENOMEM;
>>
> Won't you get a buffer overrun if midi->buflen is less than wMaxPacketSize?
> 

No, because of the *max_t(unsigned, midi->buflen,
bulk_out_desc.wMaxPacketSize)*.

Maybe that's not the most clear indentation but I had to break it in
order to avoid passing 80 columns.

-- 
Felipe

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