linux-kernel - RE: [PATCH] usb: gadget: f_fs: report error if excess data received

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Message-ID: <8737pmcvn0.fsf@linux.intel.com>
Date:	Fri, 13 May 2016 09:36:03 +0300
From:	Felipe Balbi <felipe.balbi@...ux.intel.com>
To:	"Du\, Changbin" <changbin.du@...el.com>,
	Al Viro <viro@...iv.linux.org.uk>
Cc:	"gregkh\@linuxfoundation.org" <gregkh@...uxfoundation.org>,
	"mina86\@mina86.com" <mina86@...a86.com>,
	"rui.silva\@linaro.org" <rui.silva@...aro.org>,
	"k.opasiak\@samsung.com" <k.opasiak@...sung.com>,
	"lars\@metafoo.de" <lars@...afoo.de>,
	"linux-usb\@vger.kernel.org" <linux-usb@...r.kernel.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] usb: gadget: f_fs: report error if excess data received


Hi,

"Du, Changbin" <changbin.du@...el.com> writes:
>> "Du, Changbin" <changbin.du@...el.com> writes:
>> >> right, and that was my point: if we copy more to userspace, then we have
>> >> a real big problem.
>> >>
>> > Yes, we drop the data because we userspace buffer is not enough this time.
>> > The problem here is that really can we just drop it silently? Maybe not.
>> 
>> Yeah, it probably deserves a pr_err() or pr_debug(), but host sending
>> more data than it should, is another problem altogether which needs to
>> be addressed at the host.
>> 
>> Adding a print to aid debugging is a good idea, but bailing out on the
>> peripheral side is not :-s
>> 
> Ok, if we think this is a problem at host side that the transfer is not device
> expected, then device side should not accept the data or deliver the
> transferred data to userspace. But now we take part of the data to userspace
> and says it is ok.
> Do you agree with this point?

We deliver to userspace the part userspace requested, right? So that's
okay. The USB details WRT e.g. babble or host trying to send more data
than expected, needs to be handled within the kernel.

> IMO, we expose usb transfer as a file on device side. But file read() doesn't
> have a requirement that "sorry, you cannot read so little! you need read all
> once, else we may drop data for you. :) ".

but that's not how read() semantics work. When userspace asks to read(x)
bytes, we have three possible outcomes:

i. We have x bytes to return, so we copy_to_user(x)

ii. We have y < x bytes to return, so we copy_to_user(y)

iii. We have y > x bytes to return, so we copy_to_user(x)

This is exactly how the kernel is behaving. The only "detail" we have is
that, for some reason, host is sending too much data. what I still don't
know is if this extra data is garbage or something userspace genuinely
cares about. Do you know the answer to this?

> And some library that may retry read() until get enough data (which is
> normal For a general read). Then sometimes the buffer size for
> sys_read may not as expected. This is why I think ioctl approach is
> more appropriate for usb transfer.

no, this won't change anything. Besides, it's a pointless discussion as
cannot break userspace ABI. GadgetFS and FunctionFS have been shipping
in kernel for many years.

-- 
balbi

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