linux/Documentation/usb/usbmon.txt
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   1* Introduction
   2
   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.
   9
  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.
  14
  15Two APIs are currently implemented: "text" and "binary". The binary API
  16is available through a character device in /dev namespace and is an ABI.
  17The text API is deprecated since 2.6.35, but available for convenience.
  18
  19* How to use usbmon to collect raw text traces
  20
  21Unlike the packet socket, usbmon has an interface which provides traces
  22in a text format. This is used for two purposes. First, it serves as a
  23common trace exchange format for tools while more sophisticated formats
  24are finalized. Second, humans can read it in case tools are not available.
  25
  26To collect a raw text trace, execute following steps.
  27
  281. Prepare
  29
  30Mount debugfs (it has to be enabled in your kernel configuration), and
  31load the usbmon module (if built as module). The second step is skipped
  32if usbmon is built into the kernel.
  33
  34# mount -t debugfs none_debugs /sys/kernel/debug
  35# modprobe usbmon
  36#
  37
  38Verify that bus sockets are present.
  39
  40# ls /sys/kernel/debug/usb/usbmon
  410s  0u  1s  1t  1u  2s  2t  2u  3s  3t  3u  4s  4t  4u
  42#
  43
  44Now you can choose to either use the socket '0u' (to capture packets on all
  45buses), and skip to step #3, or find the bus used by your device with step #2.
  46This allows to filter away annoying devices that talk continuously.
  47
  482. Find which bus connects to the desired device
  49
  50Run "cat /sys/kernel/debug/usb/devices", and find the T-line which corresponds
  51to the device. Usually you do it by looking for the vendor string. If you have
  52many similar devices, unplug one and compare the two
  53/sys/kernel/debug/usb/devices outputs. The T-line will have a bus number.
  54Example:
  55
  56T:  Bus=03 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#=  2 Spd=12  MxCh= 0
  57D:  Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
  58P:  Vendor=0557 ProdID=2004 Rev= 1.00
  59S:  Manufacturer=ATEN
  60S:  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
  65Bus 003 Device 002: ID 0557:2004 ATEN UC100KM V2.00
  66
  673. Start 'cat'
  68
  69# cat /sys/kernel/debug/usb/usbmon/3u > /tmp/1.mon.out
  70
  71to 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
  75This process will be reading until killed. Naturally, the output can be
  76redirected to a desirable location. This is preferred, because it is going
  77to be quite long.
  78
  794. Perform the desired operation on the USB bus
  80
  81This is where you do something that creates the traffic: plug in a flash key,
  82copy files, control a webcam, etc.
  83
  845. Kill cat
  85
  86Usually it's done with a keyboard interrupt (Control-C).
  87
  88At this point the output file (/tmp/1.mon.out in this example) can be saved,
  89sent by e-mail, or inspected with a text editor. In the last case make sure
  90that the file size is not excessive for your favourite editor.
  91
  92* Raw text data format
  93
  94Two formats are supported currently: the original, or '1t' format, and
  95the '1u' format. The '1t' format is deprecated in kernel 2.6.21. The '1u'
  96format adds a few fields, such as ISO frame descriptors, interval, etc.
  97It produces slightly longer lines, but otherwise is a perfect superset
  98of '1t' format.
  99
 100If 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
 102are present, it's the '1t' format, otherwise '1u'.
 103
 104Any text format data consists of a stream of events, such as URB submission,
 105URB callback, submission error. Every event is a text line, which consists
 106of whitespace separated words. The number or position of words may depend
 107on the event type, but there is a set of words, common for all types.
 108
 109Here 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
 179Examples:
 180
 181An input control transfer to get a port status.
 182
 183d5ea89a0 3575914555 S Ci:1:001:0 s a3 00 0000 0003 0004 4 <
 184d5ea89a0 3575914560 C Ci:1:001:0 0 4 = 01050000
 185
 186An output bulk transfer to send a SCSI command 0x28 (READ_10) in a 31-byte
 187Bulk wrapper to a storage device at address 5:
 188
 189dd65f0e8 4128379752 S Bo:1:005:2 -115 31 = 55534243 ad000000 00800000 80010a28 20000000 20000040 00000000 000000
 190dd65f0e8 4128379808 C Bo:1:005:2 0 31 >
 191
 192* Raw binary format and API
 193
 194The overall architecture of the API is about the same as the one above,
 195only the events are delivered in binary format. Each event is sent in
 196the following structure (its name is made up, so that we can refer to it):
 197
 198struct 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
 225These events can be received from a character device by reading with read(2),
 226with an ioctl(2), or by accessing the buffer with mmap. However, read(2)
 227only returns first 48 bytes for compatibility reasons.
 228
 229The character device is usually called /dev/usbmonN, where N is the USB bus
 230number. Number zero (/dev/usbmon0) is special and means "all buses".
 231Note that specific naming policy is set by your Linux distribution.
 232
 233If you create /dev/usbmon0 by hand, make sure that it is owned by root
 234and has mode 0600. Otherwise, unpriviledged users will be able to snoop
 235keyboard traffic.
 236
 237The following ioctl calls are available, with MON_IOC_MAGIC 0x92:
 238
 239 MON_IOCQ_URB_LEN, defined as _IO(MON_IOC_MAGIC, 1)
 240
 241This call returns the length of data in the next event. Note that majority of
 242events contain no data, so if this call returns zero, it does not mean that
 243no events are available.
 244
 245 MON_IOCG_STATS, defined as _IOR(MON_IOC_MAGIC, 3, struct mon_bin_stats)
 246
 247The argument is a pointer to the following structure:
 248
 249struct mon_bin_stats {
 250        u32 queued;
 251        u32 dropped;
 252};
 253
 254The member "queued" refers to the number of events currently queued in the
 255buffer (and not to the number of events processed since the last reset).
 256
 257The member "dropped" is the number of events lost since the last call
 258to MON_IOCG_STATS.
 259
 260 MON_IOCT_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 4)
 261
 262This call sets the buffer size. The argument is the size in bytes.
 263The size may be rounded down to the next chunk (or page). If the requested
 264size 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
 269This 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
 274These calls wait for events to arrive if none were in the kernel buffer,
 275then return the first event. The argument is a pointer to the following
 276structure:
 277
 278struct mon_get_arg {
 279        struct usbmon_packet *hdr;
 280        void *data;
 281        size_t alloc;           /* Length of data (can be zero) */
 282};
 283
 284Before the call, hdr, data, and alloc should be filled. Upon return, the area
 285pointed by hdr contains the next event structure, and the data buffer contains
 286the data, if any. The event is removed from the kernel buffer.
 287
 288The 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
 292This ioctl is primarily used when the application accesses the buffer
 293with mmap(2). Its argument is a pointer to the following structure:
 294
 295struct 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
 301The ioctl operates in 3 stages.
 302
 303First, it removes and discards up to nflush events from the kernel buffer.
 304The actual number of events discarded is returned in nflush.
 305
 306Second, it waits for an event to be present in the buffer, unless the pseudo-
 307device is open with O_NONBLOCK.
 308
 309Third, it extracts up to nfetch offsets into the mmap buffer, and stores
 310them into the offvec. The actual number of event offsets is stored into
 311the nfetch.
 312
 313 MON_IOCH_MFLUSH, defined as _IO(MON_IOC_MAGIC, 8)
 314
 315This call removes a number of events from the kernel buffer. Its argument
 316is the number of events to remove. If the buffer contains fewer events
 317than requested, all events present are removed, and no error is reported.
 318This works when no events are available too.
 319
 320 FIONBIO
 321
 322The ioctl FIONBIO may be implemented in the future, if there's a need.
 323
 324In addition to ioctl(2) and read(2), the special file of binary API can
 325be 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
 329The basic idea is simple:
 330
 331To prepare, map the buffer by getting the current size, then using mmap(2).
 332Then, 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
 352Thus, the main idea is to execute only one ioctl per N events.
 353
 354Although the buffer is circular, the returned headers and data do not cross
 355the end of the buffer, so the above pseudo-code does not need any gathering.
 356
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