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Possible License(s): CC-BY-SA-3.0, GPL-2.0, LGPL-2.0, AGPL-1.0
  1* Introduction
  3The name "usbmon" in lowercase refers to a facility in kernel which is
  4used to collect traces of I/O on the USB bus. This function is analogous
  5to a packet socket used by network monitoring tools such as tcpdump(1)
  6or Ethereal. Similarly, it is expected that a tool such as usbdump or
  7USBMon (with uppercase letters) is used to examine raw traces produced
  8by usbmon.
 10The usbmon reports requests made by peripheral-specific drivers to Host
 11Controller Drivers (HCD). So, if HCD is buggy, the traces reported by
 12usbmon may not correspond to bus transactions precisely. This is the same
 13situation as with tcpdump.
 15* How to use usbmon to collect raw text traces
 17Unlike the packet socket, usbmon has an interface which provides traces
 18in a text format. This is used for two purposes. First, it serves as a
 19common trace exchange format for tools while more sophisticated formats
 20are finalized. Second, humans can read it in case tools are not available.
 22To collect a raw text trace, execute following steps.
 241. Prepare
 26Mount debugfs (it has to be enabled in your kernel configuration), and
 27load the usbmon module (if built as module). The second step is skipped
 28if usbmon is built into the kernel.
 30# mount -t debugfs none_debugs /sys/kernel/debug
 31# modprobe usbmon
 34Verify that bus sockets are present.
 36# ls /sys/kernel/debug/usb/usbmon
 370s  0u  1s  1t  1u  2s  2t  2u  3s  3t  3u  4s  4t  4u
 40Now you can choose to either use the socket '0u' (to capture packets on all
 41buses), and skip to step #3, or find the bus used by your device with step #2.
 42This allows to filter away annoying devices that talk continuously.
 442. Find which bus connects to the desired device
 46Run "cat /proc/bus/usb/devices", and find the T-line which corresponds to
 47the device. Usually you do it by looking for the vendor string. If you have
 48many similar devices, unplug one and compare two /proc/bus/usb/devices outputs.
 49The T-line will have a bus number. Example:
 51T:  Bus=03 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12  MxCh= 0
 52D:  Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
 53P:  Vendor=0557 ProdID=2004 Rev= 1.00
 54S:  Manufacturer=ATEN
 55S:  Product=UC100KM V2.00
 57Bus=03 means it's bus 3.
 593. Start 'cat'
 61# cat /sys/kernel/debug/usb/usbmon/3u > /tmp/1.mon.out
 63to listen on a single bus, otherwise, to listen on all buses, type:
 65# cat /sys/kernel/debug/usb/usbmon/0u > /tmp/1.mon.out
 67This process will be reading until killed. Naturally, the output can be
 68redirected to a desirable location. This is preferred, because it is going
 69to be quite long.
 714. Perform the desired operation on the USB bus
 73This is where you do something that creates the traffic: plug in a flash key,
 74copy files, control a webcam, etc.
 765. Kill cat
 78Usually it's done with a keyboard interrupt (Control-C).
 80At this point the output file (/tmp/1.mon.out in this example) can be saved,
 81sent by e-mail, or inspected with a text editor. In the last case make sure
 82that the file size is not excessive for your favourite editor.
 84* Raw text data format
 86Two formats are supported currently: the original, or '1t' format, and
 87the '1u' format. The '1t' format is deprecated in kernel 2.6.21. The '1u'
 88format adds a few fields, such as ISO frame descriptors, interval, etc.
 89It produces slightly longer lines, but otherwise is a perfect superset
 90of '1t' format.
 92If it is desired to recognize one from the other in a program, look at the
 93"address" word (see below), where '1u' format adds a bus number. If 2 colons
 94are present, it's the '1t' format, otherwise '1u'.
 96Any text format data consists of a stream of events, such as URB submission,
 97URB callback, submission error. Every event is a text line, which consists
 98of whitespace separated words. The number or position of words may depend
 99on the event type, but there is a set of words, common for all types.
101Here is the list of words, from left to right:
103- URB Tag. This is used to identify URBs, and is normally an in-kernel address
104  of the URB structure in hexadecimal, but can be a sequence number or any
105  other unique string, within reason.
107- Timestamp in microseconds, a decimal number. The timestamp's resolution
108  depends on available clock, and so it can be much worse than a microsecond
109  (if the implementation uses jiffies, for example).
111- Event Type. This type refers to the format of the event, not URB type.
112  Available types are: S - submission, C - callback, E - submission error.
114- "Address" word (formerly a "pipe"). It consists of four fields, separated by
115  colons: URB type and direction, Bus number, Device address, Endpoint number.
116  Type and direction are encoded with two bytes in the following manner:
117    Ci Co   Control input and output
118    Zi Zo   Isochronous input and output
119    Ii Io   Interrupt input and output
120    Bi Bo   Bulk input and output
121  Bus number, Device address, and Endpoint are decimal numbers, but they may
122  have leading zeros, for the sake of human readers.
124- URB Status word. This is either a letter, or several numbers separated
125  by colons: URB status, interval, start frame, and error count. Unlike the
126  "address" word, all fields save the status are optional. Interval is printed
127  only for interrupt and isochronous URBs. Start frame is printed only for
128  isochronous URBs. Error count is printed only for isochronous callback
129  events.
131  The status field is a decimal number, sometimes negative, which represents
132  a "status" field of the URB. This field makes no sense for submissions, but
133  is present anyway to help scripts with parsing. When an error occurs, the
134  field contains the error code.
136  In case of a submission of a Control packet, this field contains a Setup Tag
137  instead of an group of numbers. It is easy to tell whether the Setup Tag is
138  present because it is never a number. Thus if scripts find a set of numbers
139  in this word, they proceed to read Data Length (except for isochronous URBs).
140  If they find something else, like a letter, they read the setup packet before
141  reading the Data Length or isochronous descriptors.
143- Setup packet, if present, consists of 5 words: one of each for bmRequestType,
144  bRequest, wValue, wIndex, wLength, as specified by the USB Specification 2.0.
145  These words are safe to decode if Setup Tag was 's'. Otherwise, the setup
146  packet was present, but not captured, and the fields contain filler.
148- Number of isochronous frame descriptors and descriptors themselves.
149  If an Isochronous transfer event has a set of descriptors, a total number
150  of them in an URB is printed first, then a word per descriptor, up to a
151  total of 5. The word consists of 3 colon-separated decimal numbers for
152  status, offset, and length respectively. For submissions, initial length
153  is reported. For callbacks, actual length is reported.
155- Data Length. For submissions, this is the requested length. For callbacks,
156  this is the actual length.
158- Data tag. The usbmon may not always capture data, even if length is nonzero.
159  The data words are present only if this tag is '='.
161- Data words follow, in big endian hexadecimal format. Notice that they are
162  not machine words, but really just a byte stream split into words to make
163  it easier to read. Thus, the last word may contain from one to four bytes.
164  The length of collected data is limited and can be less than the data length
165  report in Data Length word.
167Here is an example of code to read the data stream in a well known programming
170class ParsedLine {
171	int data_len;		/* Available length of data */
172	byte data[];
174	void parseData(StringTokenizer st) {
175		int availwords = st.countTokens();
176		data = new byte[availwords * 4];
177		data_len = 0;
178		while (st.hasMoreTokens()) {
179			String data_str = st.nextToken();
180			int len = data_str.length() / 2;
181			int i;
182			int b;	// byte is signed, apparently?! XXX
183			for (i = 0; i < len; i++) {
184				// data[data_len] = Byte.parseByte(
185				//     data_str.substring(i*2, i*2 + 2),
186				//     16);
187				b = Integer.parseInt(
188				     data_str.substring(i*2, i*2 + 2),
189				     16);
190				if (b >= 128)
191					b *= -1;
192				data[data_len] = (byte) b;
193				data_len++;
194			}
195		}
196	}
201An input control transfer to get a port status.
203d5ea89a0 3575914555 S Ci:1:001:0 s a3 00 0000 0003 0004 4 <
204d5ea89a0 3575914560 C Ci:1:001:0 0 4 = 01050000
206An output bulk transfer to send a SCSI command 0x5E in a 31-byte Bulk wrapper
207to a storage device at address 5:
209dd65f0e8 4128379752 S Bo:1:005:2 -115 31 = 55534243 5e000000 00000000 00000600 00000000 00000000 00000000 000000
210dd65f0e8 4128379808 C Bo:1:005:2 0 31 >
212* Raw binary format and API
214The overall architecture of the API is about the same as the one above,
215only the events are delivered in binary format. Each event is sent in
216the following structure (its name is made up, so that we can refer to it):
218struct usbmon_packet {
219	u64 id;			/*  0: URB ID - from submission to callback */
220	unsigned char type;	/*  8: Same as text; extensible. */
221	unsigned char xfer_type; /*    ISO (0), Intr, Control, Bulk (3) */
222	unsigned char epnum;	/*     Endpoint number and transfer direction */
223	unsigned char devnum;	/*     Device address */
224	u16 busnum;		/* 12: Bus number */
225	char flag_setup;	/* 14: Same as text */
226	char flag_data;		/* 15: Same as text; Binary zero is OK. */
227	s64 ts_sec;		/* 16: gettimeofday */
228	s32 ts_usec;		/* 24: gettimeofday */
229	int status;		/* 28: */
230	unsigned int length;	/* 32: Length of data (submitted or actual) */
231	unsigned int len_cap;	/* 36: Delivered length */
232	union {			/* 40: */
233		unsigned char setup[SETUP_LEN];	/* Only for Control S-type */
234		struct iso_rec {		/* Only for ISO */
235			int error_count;
236			int numdesc;
237		} iso;
238	} s;
239	int interval;		/* 48: Only for Interrupt and ISO */
240	int start_frame;	/* 52: For ISO */
241	unsigned int xfer_flags; /* 56: copy of URB's transfer_flags */
242	unsigned int ndesc;	/* 60: Actual number of ISO descriptors */
243};				/* 64 total length */
245These events can be received from a character device by reading with read(2),
246with an ioctl(2), or by accessing the buffer with mmap. However, read(2)
247only returns first 48 bytes for compatibility reasons.
249The character device is usually called /dev/usbmonN, where N is the USB bus
250number. Number zero (/dev/usbmon0) is special and means "all buses".
251Note that specific naming policy is set by your Linux distribution.
253If you create /dev/usbmon0 by hand, make sure that it is owned by root
254and has mode 0600. Otherwise, unpriviledged users will be able to snoop
255keyboard traffic.
257The following ioctl calls are available, with MON_IOC_MAGIC 0x92:
259 MON_IOCQ_URB_LEN, defined as _IO(MON_IOC_MAGIC, 1)
261This call returns the length of data in the next event. Note that majority of
262events contain no data, so if this call returns zero, it does not mean that
263no events are available.
265 MON_IOCG_STATS, defined as _IOR(MON_IOC_MAGIC, 3, struct mon_bin_stats)
267The argument is a pointer to the following structure:
269struct mon_bin_stats {
270	u32 queued;
271	u32 dropped;
274The member "queued" refers to the number of events currently queued in the
275buffer (and not to the number of events processed since the last reset).
277The member "dropped" is the number of events lost since the last call
280 MON_IOCT_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 4)
282This call sets the buffer size. The argument is the size in bytes.
283The size may be rounded down to the next chunk (or page). If the requested
284size is out of [unspecified] bounds for this kernel, the call fails with
287 MON_IOCQ_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 5)
289This call returns the current size of the buffer in bytes.
291 MON_IOCX_GET, defined as _IOW(MON_IOC_MAGIC, 6, struct mon_get_arg)
292 MON_IOCX_GETX, defined as _IOW(MON_IOC_MAGIC, 10, struct mon_get_arg)
294These calls wait for events to arrive if none were in the kernel buffer,
295then return the first event. The argument is a pointer to the following
298struct mon_get_arg {
299	struct usbmon_packet *hdr;
300	void *data;
301	size_t alloc;		/* Length of data (can be zero) */
304Before the call, hdr, data, and alloc should be filled. Upon return, the area
305pointed by hdr contains the next event structure, and the data buffer contains
306the data, if any. The event is removed from the kernel buffer.
308The MON_IOCX_GET copies 48 bytes to hdr area, MON_IOCX_GETX copies 64 bytes.
310 MON_IOCX_MFETCH, defined as _IOWR(MON_IOC_MAGIC, 7, struct mon_mfetch_arg)
312This ioctl is primarily used when the application accesses the buffer
313with mmap(2). Its argument is a pointer to the following structure:
315struct mon_mfetch_arg {
316	uint32_t *offvec;	/* Vector of events fetched */
317	uint32_t nfetch;	/* Number of events to fetch (out: fetched) */
318	uint32_t nflush;	/* Number of events to flush */
321The ioctl operates in 3 stages.
323First, it removes and discards up to nflush events from the kernel buffer.
324The actual number of events discarded is returned in nflush.
326Second, it waits for an event to be present in the buffer, unless the pseudo-
327device is open with O_NONBLOCK.
329Third, it extracts up to nfetch offsets into the mmap buffer, and stores
330them into the offvec. The actual number of event offsets is stored into
331the nfetch.
333 MON_IOCH_MFLUSH, defined as _IO(MON_IOC_MAGIC, 8)
335This call removes a number of events from the kernel buffer. Its argument
336is the number of events to remove. If the buffer contains fewer events
337than requested, all events present are removed, and no error is reported.
338This works when no events are available too.
342The ioctl FIONBIO may be implemented in the future, if there's a need.
344In addition to ioctl(2) and read(2), the special file of binary API can
345be polled with select(2) and poll(2). But lseek(2) does not work.
347* Memory-mapped access of the kernel buffer for the binary API
349The basic idea is simple:
351To prepare, map the buffer by getting the current size, then using mmap(2).
352Then, execute a loop similar to the one written in pseudo-code below:
354   struct mon_mfetch_arg fetch;
355   struct usbmon_packet *hdr;
356   int nflush = 0;
357   for (;;) {
358      fetch.offvec = vec; // Has N 32-bit words
359      fetch.nfetch = N;   // Or less than N
360      fetch.nflush = nflush;
361      ioctl(fd, MON_IOCX_MFETCH, &fetch);   // Process errors, too
362      nflush = fetch.nfetch;       // This many packets to flush when done
363      for (i = 0; i < nflush; i++) {
364         hdr = (struct ubsmon_packet *) &mmap_area[vec[i]];
365         if (hdr->type == '@')     // Filler packet
366            continue;
367         caddr_t data = &mmap_area[vec[i]] + 64;
368         process_packet(hdr, data);
369      }
370   }
372Thus, the main idea is to execute only one ioctl per N events.
374Although the buffer is circular, the returned headers and data do not cross
375the end of the buffer, so the above pseudo-code does not need any gathering.