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Based on kernel version 3.13. Page generated on 2014-01-20 22:04 EST.

1	* Introduction
2	
3	The name "usbmon" in lowercase refers to a facility in kernel which is
4	used to collect traces of I/O on the USB bus. This function is analogous
5	to a packet socket used by network monitoring tools such as tcpdump(1)
6	or Ethereal. Similarly, it is expected that a tool such as usbdump or
7	USBMon (with uppercase letters) is used to examine raw traces produced
8	by usbmon.
9	
10	The usbmon reports requests made by peripheral-specific drivers to Host
11	Controller Drivers (HCD). So, if HCD is buggy, the traces reported by
12	usbmon may not correspond to bus transactions precisely. This is the same
13	situation as with tcpdump.
14	
15	Two APIs are currently implemented: "text" and "binary". The binary API
16	is available through a character device in /dev namespace and is an ABI.
17	The text API is deprecated since 2.6.35, but available for convenience.
18	
19	* How to use usbmon to collect raw text traces
20	
21	Unlike the packet socket, usbmon has an interface which provides traces
22	in a text format. This is used for two purposes. First, it serves as a
23	common trace exchange format for tools while more sophisticated formats
24	are finalized. Second, humans can read it in case tools are not available.
25	
26	To collect a raw text trace, execute following steps.
27	
28	1. Prepare
29	
30	Mount debugfs (it has to be enabled in your kernel configuration), and
31	load the usbmon module (if built as module). The second step is skipped
32	if usbmon is built into the kernel.
33	
34	# mount -t debugfs none_debugs /sys/kernel/debug
35	# modprobe usbmon
36	#
37	
38	Verify that bus sockets are present.
39	
40	# ls /sys/kernel/debug/usb/usbmon
41	0s  0u  1s  1t  1u  2s  2t  2u  3s  3t  3u  4s  4t  4u
42	#
43	
44	Now you can choose to either use the socket '0u' (to capture packets on all
45	buses), and skip to step #3, or find the bus used by your device with step #2.
46	This allows to filter away annoying devices that talk continuously.
47	
48	2. Find which bus connects to the desired device
49	
50	Run "cat /sys/kernel/debug/usb/devices", and find the T-line which corresponds
51	to the device. Usually you do it by looking for the vendor string. If you have
52	many similar devices, unplug one and compare the two
53	/sys/kernel/debug/usb/devices outputs. The T-line will have a bus number.
54	Example:
55	
56	T:  Bus=03 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12  MxCh= 0
57	D:  Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
58	P:  Vendor=0557 ProdID=2004 Rev= 1.00
59	S:  Manufacturer=ATEN
60	S:  Product=UC100KM V2.00
61	
62	"Bus=03" means it's bus 3. Alternatively, you can look at the output from
63	"lsusb" and get the bus number from the appropriate line. Example:
64	
65	Bus 003 Device 002: ID 0557:2004 ATEN UC100KM V2.00
66	
67	3. Start 'cat'
68	
69	# cat /sys/kernel/debug/usb/usbmon/3u > /tmp/1.mon.out
70	
71	to listen on a single bus, otherwise, to listen on all buses, type:
72	
73	# cat /sys/kernel/debug/usb/usbmon/0u > /tmp/1.mon.out
74	
75	This process will be reading until killed. Naturally, the output can be
76	redirected to a desirable location. This is preferred, because it is going
77	to be quite long.
78	
79	4. Perform the desired operation on the USB bus
80	
81	This is where you do something that creates the traffic: plug in a flash key,
82	copy files, control a webcam, etc.
83	
84	5. Kill cat
85	
86	Usually it's done with a keyboard interrupt (Control-C).
87	
88	At this point the output file (/tmp/1.mon.out in this example) can be saved,
89	sent by e-mail, or inspected with a text editor. In the last case make sure
90	that the file size is not excessive for your favourite editor.
91	
92	* Raw text data format
93	
94	Two formats are supported currently: the original, or '1t' format, and
95	the '1u' format. The '1t' format is deprecated in kernel 2.6.21. The '1u'
96	format adds a few fields, such as ISO frame descriptors, interval, etc.
97	It produces slightly longer lines, but otherwise is a perfect superset
98	of '1t' format.
99	
100	If it is desired to recognize one from the other in a program, look at the
101	"address" word (see below), where '1u' format adds a bus number. If 2 colons
102	are present, it's the '1t' format, otherwise '1u'.
103	
104	Any text format data consists of a stream of events, such as URB submission,
105	URB callback, submission error. Every event is a text line, which consists
106	of whitespace separated words. The number or position of words may depend
107	on the event type, but there is a set of words, common for all types.
108	
109	Here is the list of words, from left to right:
110	
111	- URB Tag. This is used to identify URBs, and is normally an in-kernel address
112	  of the URB structure in hexadecimal, but can be a sequence number or any
113	  other unique string, within reason.
114	
115	- Timestamp in microseconds, a decimal number. The timestamp's resolution
116	  depends on available clock, and so it can be much worse than a microsecond
117	  (if the implementation uses jiffies, for example).
118	
119	- Event Type. This type refers to the format of the event, not URB type.
120	  Available types are: S - submission, C - callback, E - submission error.
121	
122	- "Address" word (formerly a "pipe"). It consists of four fields, separated by
123	  colons: URB type and direction, Bus number, Device address, Endpoint number.
124	  Type and direction are encoded with two bytes in the following manner:
125	    Ci Co   Control input and output
126	    Zi Zo   Isochronous input and output
127	    Ii Io   Interrupt input and output
128	    Bi Bo   Bulk input and output
129	  Bus number, Device address, and Endpoint are decimal numbers, but they may
130	  have leading zeros, for the sake of human readers.
131	
132	- URB Status word. This is either a letter, or several numbers separated
133	  by colons: URB status, interval, start frame, and error count. Unlike the
134	  "address" word, all fields save the status are optional. Interval is printed
135	  only for interrupt and isochronous URBs. Start frame is printed only for
136	  isochronous URBs. Error count is printed only for isochronous callback
137	  events.
138	
139	  The status field is a decimal number, sometimes negative, which represents
140	  a "status" field of the URB. This field makes no sense for submissions, but
141	  is present anyway to help scripts with parsing. When an error occurs, the
142	  field contains the error code.
143	
144	  In case of a submission of a Control packet, this field contains a Setup Tag
145	  instead of an group of numbers. It is easy to tell whether the Setup Tag is
146	  present because it is never a number. Thus if scripts find a set of numbers
147	  in this word, they proceed to read Data Length (except for isochronous URBs).
148	  If they find something else, like a letter, they read the setup packet before
149	  reading the Data Length or isochronous descriptors.
150	
151	- Setup packet, if present, consists of 5 words: one of each for bmRequestType,
152	  bRequest, wValue, wIndex, wLength, as specified by the USB Specification 2.0.
153	  These words are safe to decode if Setup Tag was 's'. Otherwise, the setup
154	  packet was present, but not captured, and the fields contain filler.
155	
156	- Number of isochronous frame descriptors and descriptors themselves.
157	  If an Isochronous transfer event has a set of descriptors, a total number
158	  of them in an URB is printed first, then a word per descriptor, up to a
159	  total of 5. The word consists of 3 colon-separated decimal numbers for
160	  status, offset, and length respectively. For submissions, initial length
161	  is reported. For callbacks, actual length is reported.
162	
163	- Data Length. For submissions, this is the requested length. For callbacks,
164	  this is the actual length.
165	
166	- Data tag. The usbmon may not always capture data, even if length is nonzero.
167	  The data words are present only if this tag is '='.
168	
169	- Data words follow, in big endian hexadecimal format. Notice that they are
170	  not machine words, but really just a byte stream split into words to make
171	  it easier to read. Thus, the last word may contain from one to four bytes.
172	  The length of collected data is limited and can be less than the data length
173	  reported in the Data Length word. In the case of an Isochronous input (Zi)
174	  completion where the received data is sparse in the buffer, the length of
175	  the collected data can be greater than the Data Length value (because Data
176	  Length counts only the bytes that were received whereas the Data words
177	  contain the entire transfer buffer).
178	
179	Examples:
180	
181	An input control transfer to get a port status.
182	
183	d5ea89a0 3575914555 S Ci:1:001:0 s a3 00 0000 0003 0004 4 <
184	d5ea89a0 3575914560 C Ci:1:001:0 0 4 = 01050000
185	
186	An output bulk transfer to send a SCSI command 0x28 (READ_10) in a 31-byte
187	Bulk wrapper to a storage device at address 5:
188	
189	dd65f0e8 4128379752 S Bo:1:005:2 -115 31 = 55534243 ad000000 00800000 80010a28 20000000 20000040 00000000 000000
190	dd65f0e8 4128379808 C Bo:1:005:2 0 31 >
191	
192	* Raw binary format and API
193	
194	The overall architecture of the API is about the same as the one above,
195	only the events are delivered in binary format. Each event is sent in
196	the following structure (its name is made up, so that we can refer to it):
197	
198	struct usbmon_packet {
199		u64 id;			/*  0: URB ID - from submission to callback */
200		unsigned char type;	/*  8: Same as text; extensible. */
201		unsigned char xfer_type; /*    ISO (0), Intr, Control, Bulk (3) */
202		unsigned char epnum;	/*     Endpoint number and transfer direction */
203		unsigned char devnum;	/*     Device address */
204		u16 busnum;		/* 12: Bus number */
205		char flag_setup;	/* 14: Same as text */
206		char flag_data;		/* 15: Same as text; Binary zero is OK. */
207		s64 ts_sec;		/* 16: gettimeofday */
208		s32 ts_usec;		/* 24: gettimeofday */
209		int status;		/* 28: */
210		unsigned int length;	/* 32: Length of data (submitted or actual) */
211		unsigned int len_cap;	/* 36: Delivered length */
212		union {			/* 40: */
213			unsigned char setup[SETUP_LEN];	/* Only for Control S-type */
214			struct iso_rec {		/* Only for ISO */
215				int error_count;
216				int numdesc;
217			} iso;
218		} s;
219		int interval;		/* 48: Only for Interrupt and ISO */
220		int start_frame;	/* 52: For ISO */
221		unsigned int xfer_flags; /* 56: copy of URB's transfer_flags */
222		unsigned int ndesc;	/* 60: Actual number of ISO descriptors */
223	};				/* 64 total length */
224	
225	These events can be received from a character device by reading with read(2),
226	with an ioctl(2), or by accessing the buffer with mmap. However, read(2)
227	only returns first 48 bytes for compatibility reasons.
228	
229	The character device is usually called /dev/usbmonN, where N is the USB bus
230	number. Number zero (/dev/usbmon0) is special and means "all buses".
231	Note that specific naming policy is set by your Linux distribution.
232	
233	If you create /dev/usbmon0 by hand, make sure that it is owned by root
234	and has mode 0600. Otherwise, unpriviledged users will be able to snoop
235	keyboard traffic.
236	
237	The following ioctl calls are available, with MON_IOC_MAGIC 0x92:
238	
239	 MON_IOCQ_URB_LEN, defined as _IO(MON_IOC_MAGIC, 1)
240	
241	This call returns the length of data in the next event. Note that majority of
242	events contain no data, so if this call returns zero, it does not mean that
243	no events are available.
244	
245	 MON_IOCG_STATS, defined as _IOR(MON_IOC_MAGIC, 3, struct mon_bin_stats)
246	
247	The argument is a pointer to the following structure:
248	
249	struct mon_bin_stats {
250		u32 queued;
251		u32 dropped;
252	};
253	
254	The member "queued" refers to the number of events currently queued in the
255	buffer (and not to the number of events processed since the last reset).
256	
257	The member "dropped" is the number of events lost since the last call
258	to MON_IOCG_STATS.
259	
260	 MON_IOCT_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 4)
261	
262	This call sets the buffer size. The argument is the size in bytes.
263	The size may be rounded down to the next chunk (or page). If the requested
264	size is out of [unspecified] bounds for this kernel, the call fails with
265	-EINVAL.
266	
267	 MON_IOCQ_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 5)
268	
269	This call returns the current size of the buffer in bytes.
270	
271	 MON_IOCX_GET, defined as _IOW(MON_IOC_MAGIC, 6, struct mon_get_arg)
272	 MON_IOCX_GETX, defined as _IOW(MON_IOC_MAGIC, 10, struct mon_get_arg)
273	
274	These calls wait for events to arrive if none were in the kernel buffer,
275	then return the first event. The argument is a pointer to the following
276	structure:
277	
278	struct mon_get_arg {
279		struct usbmon_packet *hdr;
280		void *data;
281		size_t alloc;		/* Length of data (can be zero) */
282	};
283	
284	Before the call, hdr, data, and alloc should be filled. Upon return, the area
285	pointed by hdr contains the next event structure, and the data buffer contains
286	the data, if any. The event is removed from the kernel buffer.
287	
288	The MON_IOCX_GET copies 48 bytes to hdr area, MON_IOCX_GETX copies 64 bytes.
289	
290	 MON_IOCX_MFETCH, defined as _IOWR(MON_IOC_MAGIC, 7, struct mon_mfetch_arg)
291	
292	This ioctl is primarily used when the application accesses the buffer
293	with mmap(2). Its argument is a pointer to the following structure:
294	
295	struct mon_mfetch_arg {
296		uint32_t *offvec;	/* Vector of events fetched */
297		uint32_t nfetch;	/* Number of events to fetch (out: fetched) */
298		uint32_t nflush;	/* Number of events to flush */
299	};
300	
301	The ioctl operates in 3 stages.
302	
303	First, it removes and discards up to nflush events from the kernel buffer.
304	The actual number of events discarded is returned in nflush.
305	
306	Second, it waits for an event to be present in the buffer, unless the pseudo-
307	device is open with O_NONBLOCK.
308	
309	Third, it extracts up to nfetch offsets into the mmap buffer, and stores
310	them into the offvec. The actual number of event offsets is stored into
311	the nfetch.
312	
313	 MON_IOCH_MFLUSH, defined as _IO(MON_IOC_MAGIC, 8)
314	
315	This call removes a number of events from the kernel buffer. Its argument
316	is the number of events to remove. If the buffer contains fewer events
317	than requested, all events present are removed, and no error is reported.
318	This works when no events are available too.
319	
320	 FIONBIO
321	
322	The ioctl FIONBIO may be implemented in the future, if there's a need.
323	
324	In addition to ioctl(2) and read(2), the special file of binary API can
325	be polled with select(2) and poll(2). But lseek(2) does not work.
326	
327	* Memory-mapped access of the kernel buffer for the binary API
328	
329	The basic idea is simple:
330	
331	To prepare, map the buffer by getting the current size, then using mmap(2).
332	Then, execute a loop similar to the one written in pseudo-code below:
333	
334	   struct mon_mfetch_arg fetch;
335	   struct usbmon_packet *hdr;
336	   int nflush = 0;
337	   for (;;) {
338	      fetch.offvec = vec; // Has N 32-bit words
339	      fetch.nfetch = N;   // Or less than N
340	      fetch.nflush = nflush;
341	      ioctl(fd, MON_IOCX_MFETCH, &fetch);   // Process errors, too
342	      nflush = fetch.nfetch;       // This many packets to flush when done
343	      for (i = 0; i < nflush; i++) {
344	         hdr = (struct ubsmon_packet *) &mmap_area[vec[i]];
345	         if (hdr->type == '@')     // Filler packet
346	            continue;
347	         caddr_t data = &mmap_area[vec[i]] + 64;
348	         process_packet(hdr, data);
349	      }
350	   }
351	
352	Thus, the main idea is to execute only one ioctl per N events.
353	
354	Although the buffer is circular, the returned headers and data do not cross
355	the end of the buffer, so the above pseudo-code does not need any gathering.
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