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diff --git a/Documentation/usb/usbmon.rst b/Documentation/usb/usbmon.rst new file mode 100644 index 000000000..6d5ec1e62 --- /dev/null +++ b/Documentation/usb/usbmon.rst @@ -0,0 +1,375 @@ +====== +usbmon +====== + +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 read until it is 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, unprivileged 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. |