linux-kernel - Re: [PATCH v2] usb: devio: Add ioctl to disallow detaching kernel USB drivers.

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Date:	Mon, 25 Jan 2016 10:21:21 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Bjørn Mork <bjorn@...k.no>
cc:	Emilio López <emilio.lopez@...labora.co.uk>,
	<gregkh@...uxfoundation.org>, <kborer@...il.com>,
	<k.opasiak@...sung.com>, <reillyg@...omium.org>,
	<keescook@...omium.org>, <linux-api@...r.kernel.org>,
	<linux-usb@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<jorgelo@...omium.org>, <dan.carpenter@...cle.com>
Subject: Re: [PATCH v2] usb: devio: Add ioctl to disallow detaching kernel
 USB drivers.

On Mon, 25 Jan 2016, Bjørn Mork wrote:

> I don't feel much like an expert here, but I can certainly make up an
> opinion anyway :)
> 
> Since 64bits kernels allow usb devio with interface numbers up to 63, I
> guess you need __u64 to avoid limiting the range?  Limiting will create
> all sorts of followup problems, so it's definitely easiest to just go
> with __u64.

But the Linux USB stack only allows up to 32 interfaces (see
include/linux/usb.h):

/* this maximum is arbitrary */
#define USB_MAXINTERFACES	32

So there's no point using a 64-bit value.

On the other hand, this value is supposed to be the same size as 
ps->ifclaimed, which is used as an argument to clear_bit(), set_bit(), 
and test_bit().  Those routines require unsigned long.

Alan Stern