Re: [PATCH] usb: core: Fix bandwidth for devices with invalid wBytesPerInterval

Greg KH
2026-04-02T14:09:54Z

On Thu, Apr 02, 2026 at 09:56:51AM -0400, Alan Stern wrote:
> On Thu, Apr 02, 2026 at 02:59:35PM +0800, Xuetao (kirin) wrote:
> > 2、Following Alan's suggestion in another email, should I check whether
> > wBytesPerInterval is a valid value and handle it in the
> > usb_parse_ss_endpoint_companion() ?
>
> Yes, IMO.
>
> > However, when parsing the device descriptor, we do not know whether the
> > actual data length transmitted by the peripheral is greater than
> > wBytesPerInterval.
>
> Note: wBytesPerInterval is in the endpoint descriptor, not the device
> descriptor.
>
> > Therefore, would it be sufficient to only add a check for whether
> > wBytesPerInterval is 0 in the existing flow, and if it is 0, set
> > wBytesPerInterval to cpu_to_le16(max_tx) by default?
> >
> > For example, modify it in the following way：
> >
> > if (le16_to_cpu(desc->wBytesPerInterval) > max_tx ||
> > le16_to_cpu(desc->wBytesPerInterval) == 0) {
> > dev_notice(ddev, "%s endpoint with wBytesPerInterval of %d in "
> > "config %d interface %d altsetting %d ep %d: "
> > "setting to %d\n",
> > usb_endpoint_xfer_isoc(&ep->desc) ? "Isoc" : "Int",
> > le16_to_cpu(desc->wBytesPerInterval),
> > cfgno, inum, asnum, ep->desc.bEndpointAddress,
> > max_tx);
> > ep->ss_ep_comp.wBytesPerInterval = cpu_to_le16(max_tx);
> > }
> >
> > Could you please give me some advice? Thanks.
>
> Try it and see if it fixes the problems you see with the network
> adapters.
>
> I saw the Greg said not to change the descriptors and just fail the
> device, but we already make this sort of change to correct other errors
> so there doesn't seem to be any reason not to do it here as well.
> Especially if it allows people to use devices that otherwise would not
> work.

I didn't realize this was on "real" devices, sorry. I thought this was
only a fuzzing thing. So yes, fix up the broken descriptor after
warning about it is the correct thing to do.

thanks,

greg k-h