Hi Alan,
On 11/05/2013 07:38 AM, Alan Stern wrote:
> On Tue, 5 Nov 2013, David Cohen wrote:
>
>>>> + /*
>>>> + * Controller requires buffer size to be aligned to
>>>> + * maxpacketsize of an out endpoint.
>>>> + */
>>>> + if (gadget->quirk_ep_out_aligned_size && read) {
>>>> + /*
>>>> + * We pass 'orig_len' to usp_ep_align_maxpacketsize()
>>>> + * due to we're in a loop and 'len' may have been
>>>> + * changed.
>>>> + */
>>>> + len = usb_ep_align_maxpacketsize(ep->ep, orig_len);
>>>> + if (data && len > data_len) {
>>>> + kfree(data);
>>>> + data = NULL;
>>>> + data_len = 0;
>>>> + }
>>>> + }
>>>
>>> Since the value of orig_len never changes, there's no point calling
>>> usb_ep_align_maxpacketsize() inside the loop. You should call it only
>>> once, before the loop starts. Once you do that, you won't need
>>> orig_len at all.
>>
>> orig_len doesn't change but ep->ep does. If USB specs say max packet
>> size won't change even if ep does, than we can call it from outside the
>> loop.
>
> I'm not too familiar with this driver. It looks like the only way
> ep->ep can change is if the endpoint gets enabled while you're sitting
> inside the wait_event_interruptible() call.
>
> In fact, the whole structure of that loop looks peculiar. Why not
> acquire the mutex first and then do everything else?
I'm not 100% familiar with this driver too. I'd keep this change to
another patch.
>
> Does it even make sense for ep to change? Would this change be visible
> to the host? What if the host changes the alternate setting while this
> loop is running -- does it make sense for the userspace program to
> start a read or write under one altsetting but then have the read/write
> take place under a different altsetting?
It doesn't make sense to do so, but gadget driver allows it. If we just
ignore, it would be a security or instability issue possible to xploit
(for DWC3 and any other controller which may depend on this quirk).
Br, David Cohen
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