The dangers of productivity porn

A quick “hairball” post about how sometimes it’s better to just go with the flow. I like how xkcd made this point with a chart of whether it’s worth the time to fix something that’s bugging you.

I have a friend who is mechanical engineer. A few years ago he took me for a tour of his workplace that ended in his office. As I looked at his workstation, something leapt out at me. My friend had never changed his background screen. Whatever the computer came with by default, that’s what he was using. With this simple act of indifference, my friend taught me an important productivity lesson.

It’s great to be productive. It really is. But sometimes we chase productivity so much that it makes us, well, unproductive. It’s easy to read a lot about how to be more productive, but don’t forget that you have make that time up.

My wife keeps me grounded

This is a harmless “hairball” post I had as a draft.

Me: Hey, they added me to popurls.com!
My wife: Never heard of it. (pause) Had you heard of it before?
Me: Yeah.
Wife: Really?
Me: Yeah!
Wife: (with an extra helping of sarcasm) Really?
Me: Yes!
Wife: (dripping with condescension) You’re a very important man.

Playing with a USB Missile Launcher

This is the last half-finished “hairball” blog post about USB devices on Linux. I actually did manage to get a working program that controlled a USB foam missile launcher. Unfortunately, I didn’t document all the steps, so this blog post just sort of stops at some point.

I got a USB Missile Launcher for Christmas. The manufacturer, Dream Cheeky, provides software–but only for Windows XP. And I thought to myself, “wouldn’t it be fun to practice some USB reverse engineering skills?” Because another Christmas present was a USB protocol analyzer from Total Phase. I should note that plenty of other people have apparently already written drivers/software for USB missile launcher toys, but I wanted to poke around myself.

Total Phase makes a high-speed USB 2.0 protocol analyzer for $1200, or a regular-speed USB protocol analyzer for $400. Here’s a trick someone mentioned: if you get the cheaper protocol analyzer and need to work with a high-speed USB device, you may be able to plug the high-speed device into a low-speed USB hub to slow the device down.

I decided to start with ladyada’s excellent guide to hacking a Kinect by reverse engineering USB packets. So here’s what I did.

Step 1. Make sure the device works. It would suck to attempt to reverse engineer a broken device. I keep a Windows XP computer lying around, so I downloaded the software for it, installed the program, and plugged in the USB rocket launcher. After the install, XP wanted to restart, so I restarted the XP computer (unplugging my USB rocket launcher after the computer was off), then started the rocket launcher software back up, then plugged in the USB device. Sure enough, everything worked fine. The controls are: pan left/right, tilt up/down, and fire. Tip: the rocket launcher uses bursts of air, so don’t jam the foam rockets down hard on the launcher.

Step 2. Probe the device. I plugged the USB rocket launcher into a Linux machine running Ubuntu 10.04 (Lucid Lynx). I ran the command sudo lsusb -vv and the relevant info from the list of USB devices on my system was this:

Bus 002 Device 045: ID 0a81:0701 Chesen Electronics Corp. USB Missile Launcher
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               1.10
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0         8
  idVendor           0x0a81 Chesen Electronics Corp.
  idProduct          0x0701 USB Missile Launcher
  bcdDevice            0.01
  iManufacturer           1 Dream Link
  iProduct                2 USB Missile Launcher v1.0
  iSerial                 0 
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           34
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0xa0
      (Bus Powered)
      Remote Wakeup
    MaxPower              100mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           1
      bInterfaceClass         3 Human Interface Device
      bInterfaceSubClass      0 No Subclass
      bInterfaceProtocol      0 None
      iInterface              0 
        HID Device Descriptor:
          bLength                 9
          bDescriptorType        33
          bcdHID               1.00
          bCountryCode            0 Not supported
          bNumDescriptors         1
          bDescriptorType        34 Report
          wDescriptorLength      52
         Report Descriptors: 
           ** UNAVAILABLE **
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0001  1x 1 bytes
        bInterval              20
Device Status:     0x0000
  (Bus Powered)

Note that my Vendor ID = 0x0a81 and my Product ID = 0×0701. Also note that bNumEndpoints = 1. An endpoint is a channel for USB data communication. Then we get the Endpoint info:

      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0001  1x 1 bytes
        bInterval              20

According to ladyada’s write-up, the “IN” means that data goes IN to the computer from the device, and the “Interrupt” transfer type is good for sending large amounts of small data quickly (e.g. a USB mouse).

Step 3. Prepare your Linux system to talk to the device. First, let’s review ladyada’s steps, which is for Windows. She installs libusb-win32 and then runs a program called inf-wizard to make a driver shell. Then plugging the device into Windows will attach the LibUSB-win32 device driver. Next, she installed Python and PyUSB.

I wanted to stick with Linux. I didn’t need libusb-win32 or inf-wizard.exe, and I already had Python installed. So my next step was to download PyUSB, extract the zip into a directory, change into that directory, then run sudo python setup.py install in that directory to install PyUSB. Since you’re installing PyUSB system-wide, you do need to run that command with “sudo” to run it as root.

Step 4. Write a short program on your Linux machine to talk to the device. I made a file missile-launcher.py and executable access with “chmod ugo+rx missile-launcher.py” next. Here’s the short program I ended up with:

#!/usr/bin/python

import usb.core
import usb.util
import sys
 
# find our device
dev = usb.core.find(idVendor=0x0a81, idProduct=0x0701)
 
# was it found?
if dev is None:
    raise ValueError('Device not found')

# Linux kernel sets up a device driver for USB device, which you have
# to detach. Otherwise trying to interact with the device gives a
# 'Resource Busy' error.
try:
  dev.detach_kernel_driver(0)
except Exception, e:
  pass # already unregistered
 
# set the active configuration. With no arguments, the first
# configuration will be the active one
dev.set_configuration()
 
print "all done"

Note that my “idVendor=0x0a81, idProduct=0×0701″ parameters use the values I found from lsusb -vv. If you compare against ladyada’s short program you’ll notice one major difference. My code has these lines:

# Linux kernel sets up a device driver for USB device, which you have
# to detach. Otherwise trying to interact with the device gives a
# 'Resource Busy' error.
try:
  dev.detach_kernel_driver(0)
except Exception, e:
  pass # already unregistered

Ladyada’s PyUSB program for Windows didn’t have anything like that. But when I ran the program under Linux, I got the error message “usb.core.USBError: Resource busy”. It turns out that the Linux kernel tries to use a default kernel driver, and that prevents my program from talking to the device. Detaching the kernel driver lets me talk to the device just fine. I picked up this tip Ken Shirriff’s post about a USB panic button with Linux and Python. In theory you could also unbind the USB device from a command-line, but I prefer to do it right in my PyUSB program directly.

Note that you will need to run the python program as root, e.g. “sudo ./missile-launcher.py” or else you’ll get a warning message like “usb.core.USBError: Access denied (insufficient permissions)”.

At this point, you have a small working program that opens up a connection to the USB rocket launcher. If the USB rocket launcher isn’t plugged in, you’ll get a “Device not found” error, and if the USB device is plugged in, you’ll get an “all done” message and the program exits gracefully.

Step 5: Try to read from the USB device. In ladyada’s guide, she tried sending 0b11000000 = 0xC0 (“Read Vendor data from Device”) to a Kinect. I got no response from that, but I did get a response sending 0b10000000. That corresponds to sending:
- a ’1′ to read from the device
- a message type of ’00′ = standard. Ladyada got a response sending to ’10′ = vendor
- ’000′ (reserved bits, so always 0)
- ’00′ to say the recipient of the message is the device.
Then sending a request of 0 got back a result of two zero bytes:

bRequest  0
array('B', [0, 0])

Interestingly, running the same program again would get a “usb.core.USBError: Unknown error” response. At that point, I would unplug the USB device and then plug it back in to reset it. I didn’t get any other responses from trying to send message types of class or vendor (as opposed to standard), nor did I get any responses from try to send messages to the interface or endpoint (as opposed to the device). See ladyada’s guide for more details about fuzzing the device and what all the various bit fields mean.

Step 6: Set up the Linux computer to use the Total Phase Beagle. The CD worked nicely with HTML documentation on it. First, you copy some udev rules so that the device is writable by anyone when the Beagle is plugged in:


cd /media/Total Phase/drivers/linux/
sudo cp 99-totalphase.rules /etc/udev/rules.d/
sudo chmod 644 /etc/udev/rules.d/99-totalphase.rules

If you’ve already plugged in the Beagle, you’ll need to unplug it and plug it back in for these rules to fire. Next, you’ll need the Data Center software. You can get it off the CD, but I’d recommend downloading the latest software and user’s manual from the website instead. My CD had software version 4.20 for example and the website was up to 5.01. Extract the software zip file (either from online or the CD). Then follow the directions in the online manual (or user manual PDF). The directions according to the manual are

  1. Install the USB drivers and Data Center software. Copying the udev rules is enough for USB drivers on Linux. Unpacking the zip is all you need for the Data Center software, because the executable is self-contained.
  2. Plug the Beagle analyzer into the analysis machine. This was my Linux machine.
  3. Plug the Beagle analyzer into the bus to be analyzed. In this case, this was my XP computer. Don’t plug the USB missile launcher in yet though.
  4. Start the Data Center software. Run the program “Data Center” in the directory you extracted from the .zip file. Follow the rest of the instruction in the Quick Start section.

Linux USB device driver info

What, *another* half-finished blog post about Linux USB drivers? Yup.

Suppose you have a device and want a Linux device driver for it. There are a few steps you’ll need to take. One of the heroes in this area is Greg Kroah-Hartman. Greg wrote USBView, which is a Linux tool to enumerate a list of USB devices. He’s also done a massive amount of documentation as we’ll see below. One of his more eye-catching tricks is to walk a classroom through the process of writing a Linux driver for a USB thermometer live and in real-time. In addition to all the work he does for Linux in general, he recently announced a program to work with manufacturers and provide Linux drivers for new devices for free. That’s right, manufacturers get a free driver. From the original announcement:

All that is needed is some kind of specification that describes how your device works, or the email address of an engineer that is willing to answer questions every once in a while. A few sample devices might be good to have so that debugging doesn’t have to be done by email, but if necessary, that can be done.

In return, you will receive a complete and working Linux driver that is added to the main Linux kernel source tree. The driver will be written by some of the members of the Linux kernel developer community (over 1500 strong and growing).

That is majorly good karma for Linux and Greg. But if you’re not a manufacturer, here are the steps that you’d look into.

1. Get documentation of the USB protocol, or reverse engineer the protocol. It’s far easier if you can get documentation of the protocol. If you do need to reverse engineer the USB protocol, here are some tools that might help:

Windows tools: USB Snoopy let you do actions with your device and log the stream of USB information going downstream/outbound to the device, or upstream/inbound back to your computer. Snoopy Pro is a variant of the same code that evidently has some improvements. It appears that the preferred location for Snoopy Pro is here.

Linux tools:
- The previously mentioned USBView will show you devices that are currently plugged in.
- The usbutils package includes a bunch of handy console tools for USB, including lsusb, which shows you the USB devices that are currently plugged in. The output of lsusb looks like this:

Bus 001 Device 006: ID 2222:3061 MacAlly
Bus 001 Device 002: ID 0557:7000 ATEN International Co., Ltd
Bus 001 Device 003: ID 045e:00db Microsoft Corp.

2. Write the driver.

- It seems that the process of writing drivers in Linux is getting easier over time. The Linux Journal has documented this well. Compare this 2001 article by Greg Kroah-Hartman to Greg’s 2004 article on controlling a simple USB lamp device. Then see Greg’s follow-up article on writing a linux driver in user space. It turns out that you can use the libusb library to read/write with USB devices without ever mucking around in the kernel. This is possible because Linux provides a USB filesystem (called USBFS) that automatically mounts USB devices into the Linux directory tree. Note that libusb also works on BSDs, Mac/OSX computers, and that there is a Windows libusb port.

If you really want to delve into this deeply, there’s an O’Reilly book on Linux device drivers that you can buy as well.

Compile a simple USB program in Linux

Here’s another “hairball” post about USB devices and drivers on Linux. I wish some expert would write the definitive “here’s how to reverse-engineer a USB device and write a new USB driver” guide. I am definitely not that expert.

Once you reverse engineer a Windows USB device enough to know how it works, you’re ready to try talking to the device under Linux. Here are a few tips.

lsusb to list devices from a command-line.

On Ubuntu 7.10, usbview (use “sudo apt-get install usbview” to install it) is broken. It says “Can not open the file /proc/bus/usb/devices Verify that you have USB compiled into your kernel, have the USB core modules loaded, and have the usbdevfs filesystem mounted.” It turns out that Ubuntu decided to drop support for /proc/bus/usb devices. You can still find USB devices under /dev/bus/usb/ on Ubuntu, but they are special binary files.

usbmon is a Linux kernel module that lets you monitor USB events: http://people.redhat.com/zaitcev/linux/OLS05_zaitcev.pdf . The text file format is pretty terse (see the PDF for some details), but if you’ve installed Linux kernel source code, check Documentation/usb/usbmon.txt for more info. Note that USBMon is a tool with a graphical user interface (GUI) that is based on usbmon. I believe that usbmon operates by using instrumentation hooks in the usbcore Linux kernel module. usbmon normally outputs only text, although Eric Preston reported adding a binary interface to usbmon.

You can trace how talking to a USB device under Linux has gotten easier over the years by reading the sequence of articles that Greg Kroah-Hartman has written for Linux Journal over the years. Let’s examine a few of the articles:

October 2001: How to Write a Linux USB Device Driver. Greg walks through nitty gritty details for using a program usb-skeleton.c that he wrote. Aspiring writers of drivers have to learn quite a bit of kernel hacking.

April 2004: Writing a Simple USB Driver. Greg writes a simple driver for a USB lamp. The driver still sits in kernel space, but exposes some functionality to the user through sysfs. What the heck is sysfs? According to the Wikipedia page, “Sysfs is a virtual file system provided by the 2.6 Linux kernel and it replaced the old deprecated devfs which use in 2.4 series of stable kernels. Sysfs exports information about devices and drivers from the kernel device model to userspace, and is also used for configuration.” Pretty handy. This article is also notable because Greg mentions a couple ways to reverse-engineer an unknown USB protocol: 1) sniff the USB events with Windows software such as SnoopyPro, or 2) run Windows on top of Linux using virtualization software such as VMWare. When you plug in a USB device, VMWare can make the device available to the Windows instance. Windows happily talks with the USB device, unaware that because Linux is sitting between Windows and the USB device, Linux gets to see (and can dump) all the communication that is happening between Windows and the USB device.

June 2004: Writing a Real Driver — In User Space. Greg has now left the kernel behind for the ease of talking to a USB device from user space. This article covers three things that make like easier.

1. usbfs. Originally called usbdevfs, usbfs is a way for user space programs to read/write data to USB devices. The method of talking to the USB devices are ioctl (“input/output control”) function calls to special files, where each file represents a USB device. If you’ve use ioctl before, you’ll know that this is still a pretty down-to-the-metal way to talk to hardware.

2. libusb. This is where life really starts to get good. libusb is a library radically simplifies talking to USB devices. Even better, it runs on many types of UNIX besides Linux (including Mac OS X). You can read the official libusb documentation, but it lacks a good “talk to a simple device” example. I also enjoyed this overview of libusb.

3. Example source code. Is there anything better when you’re delving into a new library than example source code? Greg gives a short program that echoes his previous article. In just a few lines of code, he shows how to turn on an LED in a USB lamp.

There are a couple wrinkles with the sample code. First, there are at least a few errors in the code on the Linux Journal website. A C programmer can look at code like


usb_handle = usb_open(usb_dev);
if (usb_handle == NULL) {
fprintf(stderr,
goto exit;
}
usb_handle = usb_open(usb_dev);
if (usb_handle == NULL) {
fprintf(stderr, "Not able to claim the USB devicen");
goto exit;
}

and say “Hmm. That first fprintf doesn’t have a closing parenthesis. For that matter, why call usb_open() twice and overwrite usb_handle the second time?” So something got scrambled in the code listing. I recommend checking out this simple usbradio program as another short example of how to use libusb.

Second, Greg doesn’t really say how to compile the program. On Ubuntu 7.10 (Gutsy Gibbon), I had to install a few packages. I think I had to do “sudo apt-get install gcc build-essential libusb-dev” to get everything I needed. Then if your program is called talk-to-usb.c, you would compile the program with “gcc -o talk-to-usb talk-to-usb.c -lusb”. (The “-lusb” tells the compiler to use the libusb library when linking in code.)

August 2004: Snooping the USB Data Stream. This is a really fascinating article. Greg is given an unknown USB device and told to make it work on Linux. It turns out to be a crypto key and comes with working Linux code — but the license prohibits Greg from using the USB device as freely as he would like. This puts Greg back in the position of reverse-engineering a USB protocol. In the same way that Windows-running-atop-Linux-in-VMWare provides a way to monitor USB communication that takes place in usbfs, adding usbfs logging to Linux helps to see what commands are being sent from the too-restrictive Linux library for this USB device.

If you understood that last sentence, congratulations for paying such close attention. :) To continue, Greg walks the reader through a patch to the kernel to enable kernel-based logging of any USB communication through usbfs. He even adds a variable (controllable through sysfs) so that users can turn this USB logging on and off at will.

At this point (looking on the web, not in the kernel source), I noticed several reports of additional USB logging in Linux. There was Greg’s article. There’s usbmon, which is “a facility in kernel which is used to collect traces of I/O on the USB bus.” I noticed a patch by Paolo Abeni to add binary dumps to usbmon. Both usbmon and Paolo’s patch utilize debugfs, which is another Greg Kroah-Hartman invention to allow reading/writing of data to user space, especially debugging/binary data. As Greg puts it, “one line of code and you have a variable that can be read and written to from userspace.” And Eric Preston evidently added binary dumping of USB data to Linux as well, and then started a USB protocol dissector to view USB communication in ethereal/Wireshark, which normally is used to sniff on network communication.

Now we’ve gotten a lot of background, so we’re ready to look at an example actually monitoring USB communication in Linux. The best write-up I’ve seen is this one on USB snooping on Linux. It’s a little over a year old, but check out one of the comments:

The cool thing I have spotted is that the debugfs and usbmon module is already present under ubuntu. You will not need to compile or configure the kernel, just mount the debugfs, load the usbmon module, and the usbmon directory should appear under the /sys/kernel/debug directory.

That means that a stock Ubuntu 7.10 (Gutsy Gibbon) install can just run these commands:

sudo mount -t debugfs none_debugs /sys/kernel/debug
sudo modprobe usbmon
ls -l /sys/kernel/debug/usbmon/
total 0
-rw——- 1 root root 0 2008-02-17 00:59 0s
-rw——- 1 root root 0 2008-02-17 00:59 0t
-rw——- 1 root root 0 2008-02-17 00:59 0u
-rw——- 1 root root 0 2008-02-17 00:59 1s
-rw——- 1 root root 0 2008-02-17 00:59 1t
-rw——- 1 root root 0 2008-02-17 00:59 1u
-rw——- 1 root root 0 2008-02-17 00:59 2s
-rw——- 1 root root 0 2008-02-17 00:59 2t
-rw——- 1 root root 0 2008-02-17 00:59 2u

After that, you’re ready to cat those files, exercise the USB device, and observe the resulting USB traces to figure out what commands and data passed back and forth. In case you’re wondering, the filenames seem to be a number (0,1,2) followed by a letter (s,t,u). According to the usbmon documentation, the number corresponds to the bus numbers of the USB devices. The documentation also mentions that the ‘u’ files are a superset of the ‘t’ files with more detailed information. I don’t know what the ‘s’ files are.

Just as an aside, it looks like /proc/bus/usb/ is deprecated in Ubuntu 7.10 and is being replaced with /dev/bus/usb. Unfortunately, that breaks a few programs (usbview, virtualbox, and qemu). Instead of usbview, you can use lsusb to find the particulars of a specific USB device. Unfortunately, it sounds like /proc/bus/usb/devices is going away entirely, which is a shame because the file was easy-to-read for regular users. The bug report I linked to gives a workaround if you really want that file.

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