Re: Extending USB_CONNECTINFO ioctl

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

On Tue, Jun 04, 2019 at 06:48:53PM +0200, Greg Kroah-Hartman wrote:
> On Tue, Jun 04, 2019 at 09:14:51AM -0700, Dmitry Torokhov wrote:
> > Hi Alan,
> > 
> > On Tue, Jun 04, 2019 at 10:17:25AM -0400, Alan Stern wrote:
> > > On Mon, 3 Jun 2019, Dmitry Torokhov wrote:
> > > 
> > > > Hi Alan, Greg,
> > > > 
> > > > When running software in a jailed environment where sysfs or udev is not
> > > > readily available and one can only have an FD to usbdevfs device passed
> > > > into the jail, there is a desire to allow libusb working. Alan recently
> > > > added USBDEVFS_GET_SPEED, but we are still missing bus number and list
> > > > of port numbers on the way to the root to be able to better identify the
> > > > device in question.
> > > > 
> > > > What do you think about adding a new ioctl:
> > > > 
> > > > struct usbdevfs_connectinfo_ex {
> > > > 	__u32 size;		/* size of the structure from the kernel POV */
> > > 
> > > This should be an input/output parameter.  That is, the original value
> > > says how big the userspace structure is, and the value stored by the
> > > kernel says how much space was actually filled in.  Or alternatively, 
> > > have two size fields (one for input and one for output).
> > 
> > The "incoming" size is encoded in the ioctl number (the 'len' argument
> > in USBDEVFS_CONNINFO_EX()), there is no reason to add it separately (or
> > make read/write).
> > 
> > > 
> > > > 	__u32 busnum;
> > > > 	__u32 devnum;
> > > > 	__u32 speed;		/* USB_SPEED_* form ch9.h */
> > > > 	u8 num_ports;		/* Number of entries in port_numbers array */
> > > > 	u8 port_numbers[31];	/* Current limit in USB3.0 spec is 7 */
> > > > };
> > > 
> > > Yeah, 31 is overkill.  Even the USB-2.0 spec limits the topology to 7
> > > tiers (including the root hub), meaning that only 6 port numbers are
> > > needed.
> > 
> > If you insist I can bring it down to 7, but this structure is short
> > lived, on stack, and there should not be many of them in the kernel, so
> > if we can avoid extending a particular field in the future I think it is
> > worth it.
> 
> I don't know what USB 4 has in it (and if I did, I couldn't mention it),
> but someone who can see those specs might want to take a look at them to
> just say "yeah, 31 looks like a good number" or "nope, make it a bit
> bigger please!"

OK, I believe 7 tiers should be enough, so I'll cut it down to 7 from
31.

Thanks.

-- 
Dmitry