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-* Introduction
-
-The name "usbmon" in lowercase refers to a facility in kernel which is
-used to collect traces of I/O on the USB bus. This function is analogous
-to a packet socket used by network monitoring tools such as tcpdump(1)
-or Ethereal. Similarly, it is expected that a tool such as usbdump or
-USBMon (with uppercase letters) is used to examine raw traces produced
-by usbmon.
-
-The usbmon reports requests made by peripheral-specific drivers to Host
-Controller Drivers (HCD). So, if HCD is buggy, the traces reported by
-usbmon may not correspond to bus transactions precisely. This is the same
-situation as with tcpdump.
-
-Two APIs are currently implemented: "text" and "binary". The binary API
-is available through a character device in /dev namespace and is an ABI.
-The text API is deprecated since 2.6.35, but available for convenience.
-
-* How to use usbmon to collect raw text traces
-
-Unlike the packet socket, usbmon has an interface which provides traces
-in a text format. This is used for two purposes. First, it serves as a
-common trace exchange format for tools while more sophisticated formats
-are finalized. Second, humans can read it in case tools are not available.
-
-To collect a raw text trace, execute following steps.
-
-1. Prepare
-
-Mount debugfs (it has to be enabled in your kernel configuration), and
-load the usbmon module (if built as module). The second step is skipped
-if usbmon is built into the kernel.
-
-# mount -t debugfs none_debugs /sys/kernel/debug
-# modprobe usbmon
-#
-
-Verify that bus sockets are present.
-
-# ls /sys/kernel/debug/usb/usbmon
-0s 0u 1s 1t 1u 2s 2t 2u 3s 3t 3u 4s 4t 4u
-#
-
-Now you can choose to either use the socket '0u' (to capture packets on all
-buses), and skip to step #3, or find the bus used by your device with step #2.
-This allows to filter away annoying devices that talk continuously.
-
-2. Find which bus connects to the desired device
-
-Run "cat /sys/kernel/debug/usb/devices", and find the T-line which corresponds
-to the device. Usually you do it by looking for the vendor string. If you have
-many similar devices, unplug one and compare the two
-/sys/kernel/debug/usb/devices outputs. The T-line will have a bus number.
-Example:
-
-T: Bus=03 Lev=01 Prnt=01 Port=00 Cnt=01 Dev#= 2 Spd=12 MxCh= 0
-D: Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs= 1
-P: Vendor=0557 ProdID=2004 Rev= 1.00
-S: Manufacturer=ATEN
-S: Product=UC100KM V2.00
-
-"Bus=03" means it's bus 3. Alternatively, you can look at the output from
-"lsusb" and get the bus number from the appropriate line. Example:
-
-Bus 003 Device 002: ID 0557:2004 ATEN UC100KM V2.00
-
-3. Start 'cat'
-
-# cat /sys/kernel/debug/usb/usbmon/3u > /tmp/1.mon.out
-
-to listen on a single bus, otherwise, to listen on all buses, type:
-
-# cat /sys/kernel/debug/usb/usbmon/0u > /tmp/1.mon.out
-
-This process will be reading until killed. Naturally, the output can be
-redirected to a desirable location. This is preferred, because it is going
-to be quite long.
-
-4. Perform the desired operation on the USB bus
-
-This is where you do something that creates the traffic: plug in a flash key,
-copy files, control a webcam, etc.
-
-5. Kill cat
-
-Usually it's done with a keyboard interrupt (Control-C).
-
-At this point the output file (/tmp/1.mon.out in this example) can be saved,
-sent by e-mail, or inspected with a text editor. In the last case make sure
-that the file size is not excessive for your favourite editor.
-
-* Raw text data format
-
-Two formats are supported currently: the original, or '1t' format, and
-the '1u' format. The '1t' format is deprecated in kernel 2.6.21. The '1u'
-format adds a few fields, such as ISO frame descriptors, interval, etc.
-It produces slightly longer lines, but otherwise is a perfect superset
-of '1t' format.
-
-If it is desired to recognize one from the other in a program, look at the
-"address" word (see below), where '1u' format adds a bus number. If 2 colons
-are present, it's the '1t' format, otherwise '1u'.
-
-Any text format data consists of a stream of events, such as URB submission,
-URB callback, submission error. Every event is a text line, which consists
-of whitespace separated words. The number or position of words may depend
-on the event type, but there is a set of words, common for all types.
-
-Here is the list of words, from left to right:
-
-- URB Tag. This is used to identify URBs, and is normally an in-kernel address
- of the URB structure in hexadecimal, but can be a sequence number or any
- other unique string, within reason.
-
-- Timestamp in microseconds, a decimal number. The timestamp's resolution
- depends on available clock, and so it can be much worse than a microsecond
- (if the implementation uses jiffies, for example).
-
-- Event Type. This type refers to the format of the event, not URB type.
- Available types are: S - submission, C - callback, E - submission error.
-
-- "Address" word (formerly a "pipe"). It consists of four fields, separated by
- colons: URB type and direction, Bus number, Device address, Endpoint number.
- Type and direction are encoded with two bytes in the following manner:
- Ci Co Control input and output
- Zi Zo Isochronous input and output
- Ii Io Interrupt input and output
- Bi Bo Bulk input and output
- Bus number, Device address, and Endpoint are decimal numbers, but they may
- have leading zeros, for the sake of human readers.
-
-- URB Status word. This is either a letter, or several numbers separated
- by colons: URB status, interval, start frame, and error count. Unlike the
- "address" word, all fields save the status are optional. Interval is printed
- only for interrupt and isochronous URBs. Start frame is printed only for
- isochronous URBs. Error count is printed only for isochronous callback
- events.
-
- The status field is a decimal number, sometimes negative, which represents
- a "status" field of the URB. This field makes no sense for submissions, but
- is present anyway to help scripts with parsing. When an error occurs, the
- field contains the error code.
-
- In case of a submission of a Control packet, this field contains a Setup Tag
- instead of an group of numbers. It is easy to tell whether the Setup Tag is
- present because it is never a number. Thus if scripts find a set of numbers
- in this word, they proceed to read Data Length (except for isochronous URBs).
- If they find something else, like a letter, they read the setup packet before
- reading the Data Length or isochronous descriptors.
-
-- Setup packet, if present, consists of 5 words: one of each for bmRequestType,
- bRequest, wValue, wIndex, wLength, as specified by the USB Specification 2.0.
- These words are safe to decode if Setup Tag was 's'. Otherwise, the setup
- packet was present, but not captured, and the fields contain filler.
-
-- Number of isochronous frame descriptors and descriptors themselves.
- If an Isochronous transfer event has a set of descriptors, a total number
- of them in an URB is printed first, then a word per descriptor, up to a
- total of 5. The word consists of 3 colon-separated decimal numbers for
- status, offset, and length respectively. For submissions, initial length
- is reported. For callbacks, actual length is reported.
-
-- Data Length. For submissions, this is the requested length. For callbacks,
- this is the actual length.
-
-- Data tag. The usbmon may not always capture data, even if length is nonzero.
- The data words are present only if this tag is '='.
-
-- Data words follow, in big endian hexadecimal format. Notice that they are
- not machine words, but really just a byte stream split into words to make
- it easier to read. Thus, the last word may contain from one to four bytes.
- The length of collected data is limited and can be less than the data length
- reported in the Data Length word. In the case of an Isochronous input (Zi)
- completion where the received data is sparse in the buffer, the length of
- the collected data can be greater than the Data Length value (because Data
- Length counts only the bytes that were received whereas the Data words
- contain the entire transfer buffer).
-
-Examples:
-
-An input control transfer to get a port status.
-
-d5ea89a0 3575914555 S Ci:1:001:0 s a3 00 0000 0003 0004 4 <
-d5ea89a0 3575914560 C Ci:1:001:0 0 4 = 01050000
-
-An output bulk transfer to send a SCSI command 0x28 (READ_10) in a 31-byte
-Bulk wrapper to a storage device at address 5:
-
-dd65f0e8 4128379752 S Bo:1:005:2 -115 31 = 55534243 ad000000 00800000 80010a28 20000000 20000040 00000000 000000
-dd65f0e8 4128379808 C Bo:1:005:2 0 31 >
-
-* Raw binary format and API
-
-The overall architecture of the API is about the same as the one above,
-only the events are delivered in binary format. Each event is sent in
-the following structure (its name is made up, so that we can refer to it):
-
-struct usbmon_packet {
- u64 id; /* 0: URB ID - from submission to callback */
- unsigned char type; /* 8: Same as text; extensible. */
- unsigned char xfer_type; /* ISO (0), Intr, Control, Bulk (3) */
- unsigned char epnum; /* Endpoint number and transfer direction */
- unsigned char devnum; /* Device address */
- u16 busnum; /* 12: Bus number */
- char flag_setup; /* 14: Same as text */
- char flag_data; /* 15: Same as text; Binary zero is OK. */
- s64 ts_sec; /* 16: gettimeofday */
- s32 ts_usec; /* 24: gettimeofday */
- int status; /* 28: */
- unsigned int length; /* 32: Length of data (submitted or actual) */
- unsigned int len_cap; /* 36: Delivered length */
- union { /* 40: */
- unsigned char setup[SETUP_LEN]; /* Only for Control S-type */
- struct iso_rec { /* Only for ISO */
- int error_count;
- int numdesc;
- } iso;
- } s;
- int interval; /* 48: Only for Interrupt and ISO */
- int start_frame; /* 52: For ISO */
- unsigned int xfer_flags; /* 56: copy of URB's transfer_flags */
- unsigned int ndesc; /* 60: Actual number of ISO descriptors */
-}; /* 64 total length */
-
-These events can be received from a character device by reading with read(2),
-with an ioctl(2), or by accessing the buffer with mmap. However, read(2)
-only returns first 48 bytes for compatibility reasons.
-
-The character device is usually called /dev/usbmonN, where N is the USB bus
-number. Number zero (/dev/usbmon0) is special and means "all buses".
-Note that specific naming policy is set by your Linux distribution.
-
-If you create /dev/usbmon0 by hand, make sure that it is owned by root
-and has mode 0600. Otherwise, unpriviledged users will be able to snoop
-keyboard traffic.
-
-The following ioctl calls are available, with MON_IOC_MAGIC 0x92:
-
- MON_IOCQ_URB_LEN, defined as _IO(MON_IOC_MAGIC, 1)
-
-This call returns the length of data in the next event. Note that majority of
-events contain no data, so if this call returns zero, it does not mean that
-no events are available.
-
- MON_IOCG_STATS, defined as _IOR(MON_IOC_MAGIC, 3, struct mon_bin_stats)
-
-The argument is a pointer to the following structure:
-
-struct mon_bin_stats {
- u32 queued;
- u32 dropped;
-};
-
-The member "queued" refers to the number of events currently queued in the
-buffer (and not to the number of events processed since the last reset).
-
-The member "dropped" is the number of events lost since the last call
-to MON_IOCG_STATS.
-
- MON_IOCT_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 4)
-
-This call sets the buffer size. The argument is the size in bytes.
-The size may be rounded down to the next chunk (or page). If the requested
-size is out of [unspecified] bounds for this kernel, the call fails with
--EINVAL.
-
- MON_IOCQ_RING_SIZE, defined as _IO(MON_IOC_MAGIC, 5)
-
-This call returns the current size of the buffer in bytes.
-
- MON_IOCX_GET, defined as _IOW(MON_IOC_MAGIC, 6, struct mon_get_arg)
- MON_IOCX_GETX, defined as _IOW(MON_IOC_MAGIC, 10, struct mon_get_arg)
-
-These calls wait for events to arrive if none were in the kernel buffer,
-then return the first event. The argument is a pointer to the following
-structure:
-
-struct mon_get_arg {
- struct usbmon_packet *hdr;
- void *data;
- size_t alloc; /* Length of data (can be zero) */
-};
-
-Before the call, hdr, data, and alloc should be filled. Upon return, the area
-pointed by hdr contains the next event structure, and the data buffer contains
-the data, if any. The event is removed from the kernel buffer.
-
-The MON_IOCX_GET copies 48 bytes to hdr area, MON_IOCX_GETX copies 64 bytes.
-
- MON_IOCX_MFETCH, defined as _IOWR(MON_IOC_MAGIC, 7, struct mon_mfetch_arg)
-
-This ioctl is primarily used when the application accesses the buffer
-with mmap(2). Its argument is a pointer to the following structure:
-
-struct mon_mfetch_arg {
- uint32_t *offvec; /* Vector of events fetched */
- uint32_t nfetch; /* Number of events to fetch (out: fetched) */
- uint32_t nflush; /* Number of events to flush */
-};
-
-The ioctl operates in 3 stages.
-
-First, it removes and discards up to nflush events from the kernel buffer.
-The actual number of events discarded is returned in nflush.
-
-Second, it waits for an event to be present in the buffer, unless the pseudo-
-device is open with O_NONBLOCK.
-
-Third, it extracts up to nfetch offsets into the mmap buffer, and stores
-them into the offvec. The actual number of event offsets is stored into
-the nfetch.
-
- MON_IOCH_MFLUSH, defined as _IO(MON_IOC_MAGIC, 8)
-
-This call removes a number of events from the kernel buffer. Its argument
-is the number of events to remove. If the buffer contains fewer events
-than requested, all events present are removed, and no error is reported.
-This works when no events are available too.
-
- FIONBIO
-
-The ioctl FIONBIO may be implemented in the future, if there's a need.
-
-In addition to ioctl(2) and read(2), the special file of binary API can
-be polled with select(2) and poll(2). But lseek(2) does not work.
-
-* Memory-mapped access of the kernel buffer for the binary API
-
-The basic idea is simple:
-
-To prepare, map the buffer by getting the current size, then using mmap(2).
-Then, execute a loop similar to the one written in pseudo-code below:
-
- struct mon_mfetch_arg fetch;
- struct usbmon_packet *hdr;
- int nflush = 0;
- for (;;) {
- fetch.offvec = vec; // Has N 32-bit words
- fetch.nfetch = N; // Or less than N
- fetch.nflush = nflush;
- ioctl(fd, MON_IOCX_MFETCH, &fetch); // Process errors, too
- nflush = fetch.nfetch; // This many packets to flush when done
- for (i = 0; i < nflush; i++) {
- hdr = (struct ubsmon_packet *) &mmap_area[vec[i]];
- if (hdr->type == '@') // Filler packet
- continue;
- caddr_t data = &mmap_area[vec[i]] + 64;
- process_packet(hdr, data);
- }
- }
-
-Thus, the main idea is to execute only one ioctl per N events.
-
-Although the buffer is circular, the returned headers and data do not cross
-the end of the buffer, so the above pseudo-code does not need any gathering.