1 ======
2 usbmon
3 ======
4
5 Introduction
6 ============
7
8 The name "usbmon" in lowercase refers to a facility in kernel which is
9 used to collect traces of I/O on the USB bus. This function is analogous
10 to a packet socket used by network monitoring tools such as tcpdump(1)
11 or Ethereal. Similarly, it is expected that a tool such as usbdump or
12 USBMon (with uppercase letters) is used to examine raw traces produced
13 by usbmon.
14
15 The usbmon reports requests made by peripheral-specific drivers to Host
16 Controller Drivers (HCD). So, if HCD is buggy, the traces reported by
17 usbmon may not correspond to bus transactions precisely. This is the same
18 situation as with tcpdump.
19
20 Two APIs are currently implemented: "text" and "binary". The binary API
21 is available through a character device in /dev namespace and is an ABI.
22 The text API is deprecated since 2.6.35, but available for convenience.
23
24 How to use usbmon to collect raw text traces
25 ============================================
26
27 Unlike the packet socket, usbmon has an interface which provides traces
28 in a text format. This is used for two purposes. First, it serves as a
29 common trace exchange format for tools while more sophisticated formats
30 are finalized. Second, humans can read it in case tools are not available.
31
32 To collect a raw text trace, execute following steps.
33
34 1. Prepare
35 ----------
36
37 Mount debugfs (it has to be enabled in your kernel configuration), and
38 load the usbmon module (if built as module). The second step is skipped
39 if usbmon is built into the kernel::
40
41 # mount -t debugfs none_debugs /sys/kernel/debug
42 # modprobe usbmon
43 #
44
45 Verify that bus sockets are present::
46
47 # ls /sys/kernel/debug/usb/usbmon
48 0s 0u 1s 1t 1u 2s 2t 2u 3s 3t 3u 4s 4t 4u
49 #
50
51 Now you can choose to either use the socket '0u' (to capture packets on all
52 buses), and skip to step #3, or find the bus used by your device with step #2.
53 This allows to filter away annoying devices that talk continuously.
54
55 2. Find which bus connects to the desired device
56 ------------------------------------------------
57
58 Run "cat /sys/kernel/debug/usb/devices", and find the T-line which corresponds
59 to the device. Usually you do it by looking for the vendor string. If you have
60 many similar devices, unplug one and compare the two
61 /sys/kernel/debug/usb/devices outputs. The T-line will have a bus number.
62
63 Example::
64
65 T: Bus=03 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 0
66 D: Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
67 P: Vendor=0557 ProdID=2004 Rev= 1.00
68 S: Manufacturer=ATEN
69 S: Product=UC100KM V2.00
70
71 "Bus=03" means it's bus 3. Alternatively, you can look at the output from
72 "lsusb" and get the bus number from the appropriate line. Example:
73
74 Bus 003 Device 002: ID 0557:2004 ATEN UC100KM V2.00
75
76 3. Start 'cat'
77 --------------
78
79 ::
80
81 # cat /sys/kernel/debug/usb/usbmon/3u > /tmp/1.mon.out
82
83 to listen on a single bus, otherwise, to listen on all buses, type::
84
85 # cat /sys/kernel/debug/usb/usbmon/0u > /tmp/1.mon.out
86
87 This process will read until it is killed. Naturally, the output can be
88 redirected to a desirable location. This is preferred, because it is going
89 to be quite long.
90
91 4. Perform the desired operation on the USB bus
92 -----------------------------------------------
93
94 This is where you do something that creates the traffic: plug in a flash key,
95 copy files, control a webcam, etc.
96
97 5. Kill cat
98 -----------
99
100 Usually it's done with a keyboard interrupt (Control-C).
101
102 At this point the output file (/tmp/1.mon.out in this example) can be saved,
103 sent by e-mail, or inspected with a text editor. In the last case make sure
104 that the file size is not excessive for your favourite editor.
105
106 Raw text data format
107 ====================
108
109 Two formats are supported currently: the original, or '1t' format, and
110 the '1u' format. The '1t' format is deprecated in kernel 2.6.21. The '1u'
111 format adds a few fields, such as ISO frame descriptors, interval, etc.
112 It produces slightly longer lines, but otherwise is a perfect superset
113 of '1t' format.
114
115 If it is desired to recognize one from the other in a program, look at the
116 "address" word (see below), where '1u' format adds a bus number. If 2 colons
117 are present, it's the '1t' format, otherwise '1u'.
118
119 Any text format data consists of a stream of events, such as URB submission,
120 URB callback, submission error. Every event is a text line, which consists
121 of whitespace separated words. The number or position of words may depend
122 on the event type, but there is a set of words, common for all types.
123
124 Here is the list of words, from left to right:
125
126 - URB Tag. This is used to identify URBs, and is normally an in-kernel address
127 of the URB structure in hexadecimal, but can be a sequence number or any
128 other unique string, within reason.
129
130 - Timestamp in microseconds, a decimal number. The timestamp's resolution
131 depends on available clock, and so it can be much worse than a microsecond
132 (if the implementation uses jiffies, for example).
133
134 - Event Type. This type refers to the format of the event, not URB type.
135 Available types are: S - submission, C - callback, E - submission error.
136
137 - "Address" word (formerly a "pipe"). It consists of four fields, separated by
138 colons: URB type and direction, Bus number, Device address, Endpoint number.
139 Type and direction are encoded with two bytes in the following manner:
140
141 == == =============================
142 Ci Co Control input and output
143 Zi Zo Isochronous input and output
144 Ii Io Interrupt input and output
145 Bi Bo Bulk input and output
146 == == =============================
147
148 Bus number, Device address, and Endpoint are decimal numbers, but they may
149 have leading zeros, for the sake of human readers.
150
151 - URB Status word. This is either a letter, or several numbers separated
152 by colons: URB status, interval, start frame, and error count. Unlike the
153 "address" word, all fields save the status are optional. Interval is printed
154 only for interrupt and isochronous URBs. Start frame is printed only for
155 isochronous URBs. Error count is printed only for isochronous callback
156 events.
157
158 The status field is a decimal number, sometimes negative, which represents
159 a "status" field of the URB. This field makes no sense for submissions, but
160 is present anyway to help scripts with parsing. When an error occurs, the
161 field contains the error code.
162
163 In case of a submission of a Control packet, this field contains a Setup Tag
164 instead of an group of numbers. It is easy to tell whether the Setup Tag is
165 present because it is never a number. Thus if scripts find a set of numbers
166 in this word, they proceed to read Data Length (except for isochronous URBs).
167 If they find something else, like a letter, they read the setup packet before
168 reading the Data Length or isochronous descriptors.
169
170 - Setup packet, if present, consists of 5 words: one of each for bmRequestType,
171 bRequest, wValue, wIndex, wLength, as specified by the USB Specification 2.0.
172 These words are safe to decode if Setup Tag was 's'. Otherwise, the setup
173 packet was present, but not captured, and the fields contain filler.
174
175 - Number of isochronous frame descriptors and descriptors themselves.
176 If an Isochronous transfer event has a set of descriptors, a total number
177 of them in an URB is printed first, then a word per descriptor, up to a
178 total of 5. The word consists of 3 colon-separated decimal numbers for
179 status, offset, and length respectively. For submissions, initial length
180 is reported. For callbacks, actual length is reported.
181
182 - Data Length. For submissions, this is the requested length. For callbacks,
183 this is the actual length.
184
185 - Data tag. The usbmon may not always capture data, even if length is nonzero.
186 The data words are present only if this tag is '='.
187
188 - Data words follow, in big endian hexadecimal format. Notice that they are
189 not machine words, but really just a byte stream split into words to make
190 it easier to read. Thus, the last word may contain from one to four bytes.
191 The length of collected data is limited and can be less than the data length
192 reported in the Data Length word. In the case of an Isochronous input (Zi)
193 completion where the received data is sparse in the buffer, the length of
194 the collected data can be greater than the Data Length value (because Data
195 Length counts only the bytes that were received whereas the Data words
196 contain the entire transfer buffer).
197
198 Examples:
199
200 An input control transfer to get a port status::
201
202 d5ea89a0 3575914555 S Ci:1:001:0 s a3 00 0000 0003 0004 4 <
203 d5ea89a0 3575914560 C Ci:1:001:0 0 4 = 01050000
204
205 An output bulk transfer to send a SCSI command 0x28 (READ_10) in a 31-byte
206 Bulk wrapper to a storage device at address 5::
207
208 dd65f0e8 4128379752 S Bo:1:005:2 -115 31 = 55534243 ad000000 00800000 80010a28 20000000 20000040 00000000 000000
209 dd65f0e8 4128379808 C Bo:1:005:2 0 31 >
210
211 Raw binary format and API
212 =========================
213
214 The overall architecture of the API is about the same as the one above,
215 only the events are delivered in binary format. Each event is sent in
216 the following structure (its name is made up, so that we can refer to it)::
217
218 struct 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 */
244
245 These events can be received from a character device by reading with read(2),
246 with an ioctl(2), or by accessing the buffer with mmap. However, read(2)
247 only returns first 48 bytes for compatibility reasons.
248
249 The character device is usually called /dev/usbmonN, where N is the USB bus
250 number. Number zero (/dev/usbmon0) is special and means "all buses".
251 Note that specific naming policy is set by your Linux distribution.
252
253 If you create /dev/usbmon0 by hand, make sure that it is owned by root
254 and has mode 0600. Otherwise, unprivileged users will be able to snoop
255 keyboard traffic.
256
257 The following ioctl calls are available, with MON_IOC_MAGIC 0x92:
258
259 MON_IOCQ_URB_LEN, defined as _IO(MON_IOC_MAGIC, 1)
260
261 This call returns the length of data in the next event. Note that majority of
262 events contain no data, so if this call returns zero, it does not mean that
263 no events are available.
264
265 MON_IOCG_STATS, defined as _IOR(MON_IOC_MAGIC, 3, struct mon_bin_stats)
266
267 The argument is a pointer to the following structure::
268
269 struct mon_bin_stats {
270 u32 queued;
271 u32 dropped;
272 };
273
274 The member "queued" refers to the number of events currently queued in the
275 buffer (and not to the number of events processed since the last reset).
276
277 The member "dropped" is the number of events lost since the last call
278 to MON_IOCG_STATS.
279
280 MON_IOCT_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 4)
281
282 This call sets the buffer size. The argument is the size in bytes.
283 The size may be rounded down to the next chunk (or page). If the requested
284 size is out of [unspecified] bounds for this kernel, the call fails with
285 -EINVAL.
286
287 MON_IOCQ_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 5)
288
289 This call returns the current size of the buffer in bytes.
290
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)
293
294 These calls wait for events to arrive if none were in the kernel buffer,
295 then return the first event. The argument is a pointer to the following
296 structure::
297
298 struct mon_get_arg {
299 struct usbmon_packet *hdr;
300 void *data;
301 size_t alloc; /* Length of data (can be zero) */
302 };
303
304 Before the call, hdr, data, and alloc should be filled. Upon return, the area
305 pointed by hdr contains the next event structure, and the data buffer contains
306 the data, if any. The event is removed from the kernel buffer.
307
308 The MON_IOCX_GET copies 48 bytes to hdr area, MON_IOCX_GETX copies 64 bytes.
309
310 MON_IOCX_MFETCH, defined as _IOWR(MON_IOC_MAGIC, 7, struct mon_mfetch_arg)
311
312 This ioctl is primarily used when the application accesses the buffer
313 with mmap(2). Its argument is a pointer to the following structure::
314
315 struct 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 */
319 };
320
321 The ioctl operates in 3 stages.
322
323 First, it removes and discards up to nflush events from the kernel buffer.
324 The actual number of events discarded is returned in nflush.
325
326 Second, it waits for an event to be present in the buffer, unless the pseudo-
327 device is open with O_NONBLOCK.
328
329 Third, it extracts up to nfetch offsets into the mmap buffer, and stores
330 them into the offvec. The actual number of event offsets is stored into
331 the nfetch.
332
333 MON_IOCH_MFLUSH, defined as _IO(MON_IOC_MAGIC, 8)
334
335 This call removes a number of events from the kernel buffer. Its argument
336 is the number of events to remove. If the buffer contains fewer events
337 than requested, all events present are removed, and no error is reported.
338 This works when no events are available too.
339
340 FIONBIO
341
342 The ioctl FIONBIO may be implemented in the future, if there's a need.
343
344 In addition to ioctl(2) and read(2), the special file of binary API can
345 be polled with select(2) and poll(2). But lseek(2) does not work.
346
347 * Memory-mapped access of the kernel buffer for the binary API
348
349 The basic idea is simple:
350
351 To prepare, map the buffer by getting the current size, then using mmap(2).
352 Then, execute a loop similar to the one written in pseudo-code below::
353
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 }
371
372 Thus, the main idea is to execute only one ioctl per N events.
373
374 Although the buffer is circular, the returned headers and data do not cross
375 the end of the buffer, so the above pseudo-code does not need any gathering.
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