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

[Greg KH]

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