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

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Date:	Mon, 16 May 2016 18:05:38 +0200
From:	Michal Nazarewicz <mina86@...a86.com>
To:	"Du\, Changbin" <changbin.du@...el.com>,
	Felipe Balbi <felipe.balbi@...ux.intel.com>,
	Alan Stern <stern@...land.harvard.edu>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	"gregkh\@linuxfoundation.org" <gregkh@...uxfoundation.org>,
	"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

So I’ve been looking at AIO handling in f_fs and either I’m stupid or
the code is broken.  Here’s part of ffs_user_copy_worker:

	int ret = io_data->req->status ? io_data->req->status :
					 io_data->req->actual;
	if (io_data->read && ret > 0) {
		use_mm(io_data->mm);
		ret = copy_to_iter(io_data->buf, ret, &io_data->data);
		if (iov_iter_count(&io_data->data))
			ret = -EFAULT;
		unuse_mm(io_data->mm);
	}

First of all, shouldn’t the copy_to_iter invocation be:

		if (copy_to_iter(io_data->buf, ret, &io_data->data))
                	ret = -EFAULT;

Second of all, if the request reads fewer bytes than user requested,
iov_iter_count(…) will be non-zero (namely it will be the difference
between user’s buffer size and data read).  This should not result in
EFAULT though.

So, am I going crazy? Or does this need to be fixed as well?

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ
«If at first you don’t succeed, give up skydiving»