157 lines
5.8 KiB
Plaintext
157 lines
5.8 KiB
Plaintext
* Introduction
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The name "usbmon" in lowercase refers to a facility in kernel which is
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used to collect traces of I/O on the USB bus. This function is analogous
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to a packet socket used by network monitoring tools such as tcpdump(1)
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or Ethereal. Similarly, it is expected that a tool such as usbdump or
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USBMon (with uppercase letters) is used to examine raw traces produced
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by usbmon.
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The usbmon reports requests made by peripheral-specific drivers to Host
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Controller Drivers (HCD). So, if HCD is buggy, the traces reported by
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usbmon may not correspond to bus transactions precisely. This is the same
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situation as with tcpdump.
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* How to use usbmon to collect raw text traces
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Unlike the packet socket, usbmon has an interface which provides traces
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in a text format. This is used for two purposes. First, it serves as a
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common trace exchange format for tools while most sophisticated formats
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are finalized. Second, humans can read it in case tools are not available.
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To collect a raw text trace, execute following steps.
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1. Prepare
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Mount debugfs (it has to be enabled in your kernel configuration), and
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load the usbmon module (if built as module). The second step is skipped
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if usbmon is built into the kernel.
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# mount -t debugfs none_debugs /sys/kernel/debug
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# modprobe usbmon
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Verify that bus sockets are present.
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[root@lembas zaitcev]# ls /sys/kernel/debug/usbmon
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1s 1t 2s 2t 3s 3t 4s 4t
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[root@lembas zaitcev]#
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# ls /sys/kernel
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2. Find which bus connects to the desired device
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Run "cat /proc/bus/usb/devices", and find the T-line which corresponds to
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the device. Usually you do it by looking for the vendor string. If you have
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many similar devices, unplug one and compare two /proc/bus/usb/devices outputs.
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The T-line will have a bus number. Example:
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T: Bus=03 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 0
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D: Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
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P: Vendor=0557 ProdID=2004 Rev= 1.00
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S: Manufacturer=ATEN
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S: Product=UC100KM V2.00
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Bus=03 means it's bus 3.
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3. Start 'cat'
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# cat /sys/kernel/debug/usbmon/3t > /tmp/1.mon.out
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This process will be reading until killed. Naturally, the output can be
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redirected to a desirable location. This is preferred, because it is going
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to be quite long.
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4. Perform the desired operation on the USB bus
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This is where you do something that creates the traffic: plug in a flash key,
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copy files, control a webcam, etc.
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5. Kill cat
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Usually it's done with a keyboard interrupt (Control-C).
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At this point the output file (/tmp/1.mon.out in this example) can be saved,
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sent by e-mail, or inspected with a text editor. In the last case make sure
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that the file size is not excessive for your favourite editor.
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* Raw text data format
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The '0t' type data consists of a stream of events, such as URB submission,
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URB callback, submission error. Every event is a text line, which consists
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of whitespace separated words. The number of position of words may depend
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on the event type, but there is a set of words, common for all types.
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Here is the list of words, from left to right:
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- URB Tag. This is used to identify URBs is normally a kernel mode address
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of the URB structure in hexadecimal.
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- Timestamp in microseconds, a decimal number. The timestamp's resolution
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depends on available clock, and so it can be much worse than a microsecond
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(if the implementation uses jiffies, for example).
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- Event Type. This type refers to the format of the event, not URB type.
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Available types are: S - submission, C - callback, E - submission error.
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- "Pipe". The pipe concept is deprecated. This is a composite word, used to
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be derived from information in pipes. It consists of three fields, separated
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by colons: URB type and direction, Device address, Endpoint number.
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Type and direction are encoded with two bytes in the following manner:
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Ci Co Control input and output
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Zi Zo Isochronous input and output
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Ii Io Interrupt input and output
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Bi Bo Bulk input and output
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Device address and Endpoint number are decimal numbers with leading zeroes
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or 3 and 2 positions, correspondingly.
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- URB Status. This field makes no sense for submissions, but is present
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to help scripts with parsing. In error case, it contains the error code.
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- Data Length. This is the actual length in the URB.
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- Data tag. The usbmon may not always capture data, even if length is nonzero.
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Only if tag is '=', the data words are present.
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- Data words follow, in big endian hexadecimal format. Notice that they are
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not machine words, but really just a byte stream split into words to make
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it easier to read. Thus, the last word may contain from one to four bytes.
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The length of collected data is limited and can be less than the data length
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report in Data Length word.
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Here is an example of code to read the data stream in a well known programming
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language:
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class ParsedLine {
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int data_len; /* Available length of data */
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byte data[];
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void parseData(StringTokenizer st) {
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int availwords = st.countTokens();
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data = new byte[availwords * 4];
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data_len = 0;
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while (st.hasMoreTokens()) {
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String data_str = st.nextToken();
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int len = data_str.length() / 2;
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int i;
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for (i = 0; i < len; i++) {
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data[data_len] = Byte.parseByte(
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data_str.substring(i*2, i*2 + 2),
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16);
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data_len++;
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}
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}
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}
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}
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This format is obviously deficient. For example, the setup packet for control
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transfers is not delivered. This will change in the future.
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Examples:
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An input control transfer to get a port status:
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d74ff9a0 2640288196 S Ci:001:00 -115 4 <
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d74ff9a0 2640288202 C Ci:001:00 0 4 = 01010100
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An output bulk transfer to send a SCSI command 0x5E in a 31-byte Bulk wrapper
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to a storage device at address 5:
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dd65f0e8 4128379752 S Bo:005:02 -115 31 = 55534243 5e000000 00000000 00000600 00000000 00000000 00000000 000000
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dd65f0e8 4128379808 C Bo:005:02 0 31 >
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* Raw binary format and API
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TBD
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