Re: Exporting USB device ids from the kernel

From: Andrey Konovalov <andreyknvl@google.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>,
	USB list <linux-usb@vger.kernel.org>,
	Dmitry Vyukov <dvyukov@google.com>
Subject: Re: Exporting USB device ids from the kernel
Date: Mon, 18 Nov 2019 17:12:34 +0100	[thread overview]
Message-ID: <CAAeHK+xjUhR077goAHv=re78C6pzzSEBQxU+LZcOs0iCu2ZStg@mail.gmail.com> (raw)
In-Reply-To: <20191116084854.GA384892@kroah.com>

On Sat, Nov 16, 2019 at 9:49 AM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Fri, Nov 15, 2019 at 05:10:26PM +0100, Andrey Konovalov wrote:
> > On Fri, Nov 15, 2019 at 4:44 PM Alan Stern <stern@rowland.harvard.edu> wrote:
> > >
> > > On Fri, 15 Nov 2019, Andrey Konovalov wrote:
> > >
> > > > Hi Greg and Alan,
> > > >
> > > > For USB fuzzing it would be nice to be able to export usb_device_id
> > > > structs from the kernel to facilitate the fuzzer with generating USB
> > > > descriptors that match to actual drivers. The same is required for
> > > > hid_device_id structs, since those are matched separately by the
> > > > usbhid driver (are there other cases like this?).
> > > >
> > > > Currently I have a hacky patch [1] that walks all drivers for USB and
> > > > HID buses and then prints all device ids for those drivers into the
> > > > kernel log. Those are manually parsed and built into the fuzzer [2]
> > > > and then used to generate USB descriptors [3].
> > >
> > > There are so many different flags for those id structures, parsing and
> > > understanding them must be quite difficult.
> > >
> > > > I'm thinking of making a proper patch that will add a debugfs entry
> > > > like usb/drivers (and usb/hid_drivers?), that can be read to get
> > > > USB/HID device ids for all loaded drivers. Would that be acceptable?
> > > > Or should I use some other interface to do that?
> > >
> > > I can't think of a better way to get the information from a running
> > > kernel.
> > >
> > > There is another possibility, though.  If the drivers are built as
> > > modules, the information is already available to userspace tools via
> > > depmod.  You could get it from the modules.dep.bin file.  This has the
> > > advantage that it will work even for drivers that aren't currently
> > > loaded.
> >
> > This is the same thing Greg mentions above, right?
>
> Yes.
>
> > Would this work for drivers that are built into the kernel (as =y)?
>
> No, sorry.  There has not been any need to export that information to
> userspace as nothing has ever needed that.
>
> The only reason we exported that at all was to allow modules to
> auto-load to handle the device.

OK, I see. Ideally we would want to support both builtin drivers and
modules. I'll then implement the approach with exporting the ids
through debugfs. I'll send a patch once I have it.

Thanks!