usb: gadget: f_fs: printk error when excess data is dropped on read

Add a pr_err when host sent more data then the size of the buffer user
space gave us.  This may happen on UDCs which require OUT requests to
be aligned to max packet size.  The patch includes a description of the
situation.
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_fs.c

@@ -640,6 +640,44 @@ static void ffs_epfile_io_complete(struct usb_ep *_ep, struct usb_request *req)
 	}
 }
 
+static ssize_t ffs_copy_to_iter(void *data, int data_len, struct iov_iter *iter)
+{
+	ssize_t ret = copy_to_iter(data, data_len, iter);
+	if (likely(ret == data_len))
+		return ret;
+
+	if (unlikely(iov_iter_count(iter)))
+		return -EFAULT;
+
+	/*
+	 * Dear user space developer!
+	 *
+	 * TL;DR: To stop getting below error message in your kernel log, change
+	 * user space code using functionfs to align read buffers to a max
+	 * packet size.
+	 *
+	 * Some UDCs (e.g. dwc3) require request sizes to be a multiple of a max
+	 * packet size.  When unaligned buffer is passed to functionfs, it
+	 * internally uses a larger, aligned buffer so that such UDCs are happy.
+	 *
+	 * Unfortunately, this means that host may send more data than was
+	 * requested in read(2) system call.  f_fs doesn’t know what to do with
+	 * that excess data so it simply drops it.
+	 *
+	 * Was the buffer aligned in the first place, no such problem would
+	 * happen.
+	 *
+	 * This only affects OUT endpoints, i.e. reading data with a read(2),
+	 * aio_read(2) etc. system calls.  Writing data to an IN endpoint is not
+	 * affected.
+	 */
+	pr_err("functionfs read size %d > requested size %zd, dropping excess data. "
+	       "Align read buffer size to max packet size to avoid the problem.\n",
+	       data_len, ret);
+
+	return ret;
+}
+
 static void ffs_user_copy_worker(struct work_struct *work)
 {
 	struct ffs_io_data *io_data = container_of(work, struct ffs_io_data,
@@ -650,9 +688,7 @@ static void ffs_user_copy_worker(struct work_struct *work)
 
 	if (io_data->read && ret > 0) {
 		use_mm(io_data->mm);
-		ret = copy_to_iter(io_data->buf, ret, &io_data->data);
-		if (ret != io_data->req->actual && iov_iter_count(&io_data->data))
-			ret = -EFAULT;
+		ret = ffs_copy_to_iter(io_data->buf, ret, &io_data->data);
 		unuse_mm(io_data->mm);
 	}
 
@@ -803,18 +839,13 @@ static ssize_t ffs_epfile_io(struct file *file, struct ffs_io_data *io_data)
 			interrupted = ep->status < 0;
 		}
 
-		/*
-		 * XXX We may end up silently droping data here.  Since data_len
-		 * (i.e. req->length) may be bigger than len (after being
-		 * rounded up to maxpacketsize), we may end up with more data
-		 * then user space has space for.
-		 */
-		ret = interrupted ? -EINTR : ep->status;
-		if (io_data->read && ret > 0) {
-			ret = copy_to_iter(data, ret, &io_data->data);
-			if (!ret)
-				ret = -EFAULT;
-		}
+		if (interrupted)
+			ret = -EINTR;
+		else if (io_data->read && ep->status > 0)
+			ret = ffs_copy_to_iter(data, ep->status,
+					       &io_data->data);
+		else
+			ret = ep->status;
 		goto error_mutex;
 	} else if (!(req = usb_ep_alloc_request(ep->ep, GFP_KERNEL))) {
 		ret = -ENOMEM;