linux-kernel - Re: [PATCH v2 1/4] cdc-acm: reassemble fragmented notifications

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Date:   Fri, 24 Mar 2017 22:50:12 +0100
From:   Tobias Herzog <t-herzog@....de>
To:     Oliver Neukum <oneukum@...e.com>
Cc:     gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        linux-usb@...r.kernel.org
Subject: Re: [PATCH v2 1/4] cdc-acm: reassemble fragmented notifications

Hi Oliver,

thank you for your patience... :) I have a question to one of your
comments (see below).

Best regards,
Tobias

> Am Samstag, den 18.03.2017, 19:52 +0100 schrieb Tobias Herzog:
> > 
> > USB devices may have very limitited endpoint packet sizes, so that
> > notifications can not be transferred within one single usb packet.
> > Reassembling of multiple packages may be necessary.
> Hi,
> 
> almost perfect.
> A few new issue. Comments inline.
> 
> > 
> > 
> > +	struct usb_cdc_notification *dr = (struct
> > usb_cdc_notification *)buf;
> > +	unsigned char *data = (unsigned char *)(dr + 1);
> This is border line incorrect. It depends on the compiler not
> adding padding. Please make it
> 
> buf + sizeof(struct usb_cdc_notification)
> 
> > 
> > -	usb_mark_last_busy(acm->dev);
> > -
> > -	data = (unsigned char *)(dr + 1);
> >  	switch (dr->bNotificationType) {
> >  	case USB_CDC_NOTIFY_NETWORK_CONNECTION:
> >  		dev_dbg(&acm->control->dev,
> > @@ -363,8 +337,77 @@ static void acm_ctrl_irq(struct urb *urb)
> >  			__func__,
> >  			dr->bNotificationType, dr->wIndex,
> >  			dr->wLength, data[0], data[1]);
> > +	}
> > +}
> > +
> > +/* control interface reports status changes with "interrupt"
> > transfers */
> > +static void acm_ctrl_irq(struct urb *urb)
> > +{
> > +	struct acm *acm = urb->context;
> > +	struct usb_cdc_notification *dr = urb->transfer_buffer;
> > +	unsigned int current_size = urb->actual_length;
> > +	unsigned int expected_size, copy_size;
> > +	int retval;
> > +	int status = urb->status;
> > +
> > +	switch (status) {
> > +	case 0:
> > +		/* success */
> >  		break;
> > +	case -ECONNRESET:
> > +	case -ENOENT:
> > +	case -ESHUTDOWN:
> > +		/* this urb is terminated, clean up */
> > +		acm->nb_index = 0;
> > +		dev_dbg(&acm->control->dev,
> > +			"%s - urb shutting down with status:
> > %d\n",
> > +			__func__, status);
> > +		return;
> > +	default:
> > +		dev_dbg(&acm->control->dev,
> > +			"%s - nonzero urb status received: %d\n",
> > +			__func__, status);
> > +		goto exit;
> >  	}
> > +
> > +	usb_mark_last_busy(acm->dev);
> > +
> > +	if (acm->nb_index)
> > +		dr = (struct usb_cdc_notification *)acm-
> > >notification_buffer;
> > +
> > +	/* size = notification-header + (optional) data */
> > +	expected_size = sizeof(struct usb_cdc_notification) +
> > +					le16_to_cpu(dr->wLength);
> > +
> > +	if (current_size < expected_size) {
> > +		/* notification is transmitted fragmented,
> > reassemble */
> > +		if (acm->nb_size < expected_size) {
> > +			if (acm->nb_size) {
> > +				kfree(acm->notification_buffer);
> > +				acm->nb_size = 0;
> > +			}
> > +			acm->notification_buffer =
> > +					kmalloc(expected_size,
> > GFP_ATOMIC);
> Given how kmalloc works you'd better round this up to a power of two.
> 
> > 
> > +			if (!acm->notification_buffer)
> > +				goto exit;
> This is most subtle. Please add a comment that this prevents a double
> free if we get a disconnect()
I'm unsure if I got this right: Are you talking about the fact, that
the use of 'kmalloc' will make 'acm->notification_buffer' valid again
(i.e. NULL or pointing to a correctly allocated block), after it was
"invalidated" by 'kfree' (in case the previously allocated buffer was
too small)?
The 'goto exit'-thing for me is just not to use the buffer if
allocation fails. Or am I missing anything here?
> 
> > 
> > +			acm->nb_size = expected_size;
> > +		}
> 	Regards
> 		Oliver
>