Re: [PATCH] usbmon: Report device address assigned to USB device

[Pete Zaitcev <zaitcev@redhat.com>]

On Wed, 8 Jan 2020 16:55:13 +0100
Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:

> On Mon, Jan 06, 2020 at 10:37:17AM +0100, Tomasz Moń wrote:
> > Make USB device addresses match while sniffing USB communication
> > with usbmon and hardware USB sniffer (OpenVizsla) at the same time.
> > On xHCI root hubs the address is assigned by hardware and can be
> > different than devnum.
> 
> This breaks the userspace abi for matching up the devnum with the number
> that is listed by the kernel to userspace, making it really hard to
> match things up for xhci devices now :(
> 
> I understand the need to look at this data, but you can't do it in a way
> that will change things like this.

This sounds reasonable, although perhaps unfortunate. I acked Tomasz's
patch because I thought that XHCI is new, and thus it's no big deal
if its results are different.

Of course I rushed to examine the packet structure, but yes, there's
no space in the header, not without some trickery. For example, the
upper 8 bits of the ID are probably the same for all packets, so
it may be stuffed in there. Or, it might be possible to create an
extra header and attach it at the end of ISO descriptors.

Alan's suggestion of leaving the physical address in /sys appeals
the most to me, honestly. One thing though, libpcap will need to
rifle through /sys and then store that address, so its serialization
has to be changed no matter what. Unfortunately, I'm wholly ignorant
as to what syntax it uses and how extensible it is.

-- Pete