Linux USB: fix incorrect values for the packet length.

[libpcap] / pcap-usb-linux.c

/*
 * Copyright (c) 2006 Paolo Abeni (Italy)
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote
 * products derived from this software without specific prior written
 * permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * USB sniffing API implementation for Linux platform
 * By Paolo Abeni <paolo.abeni@email.it>
 * Modifications: Kris Katterjohn <katterjohn@gmail.com>
 *
 */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include "pcap-int.h"
#include "pcap-usb-linux.h"
#include "pcap-usb-linux-common.h"
#include "pcap/usb.h"

#include "extract.h"

#ifdef NEED_STRERROR_H
#include "strerror.h"
#endif

#include <errno.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <limits.h>
#include <string.h>
#include <dirent.h>
#include <byteswap.h>
#include <netinet/in.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/utsname.h>
#ifdef HAVE_LINUX_USBDEVICE_FS_H
/*
 * We might need <linux/compiler.h> to define __user for
 * <linux/usbdevice_fs.h>.
 */
#ifdef HAVE_LINUX_COMPILER_H
#include <linux/compiler.h>
#endif /* HAVE_LINUX_COMPILER_H */
#include <linux/usbdevice_fs.h>
#endif /* HAVE_LINUX_USBDEVICE_FS_H */

#include "diag-control.h"

#define USB_IFACE "usbmon"

#define USBMON_DEV_PREFIX "usbmon"
#define USBMON_DEV_PREFIX_LEN   (sizeof USBMON_DEV_PREFIX - 1)
#define USB_LINE_LEN 4096

#if __BYTE_ORDER == __LITTLE_ENDIAN
#define htols(s) s
#define htoll(l) l
#define htol64(ll) ll
#else
#define htols(s) bswap_16(s)
#define htoll(l) bswap_32(l)
#define htol64(ll) bswap_64(ll)
#endif

struct mon_bin_stats {
        uint32_t queued;
        uint32_t dropped;
};

struct mon_bin_get {
        pcap_usb_header *hdr;
        void *data;
        size_t data_len;   /* Length of data (can be zero) */
};

struct mon_bin_mfetch {
        int32_t *offvec;   /* Vector of events fetched */
        int32_t nfetch;    /* Number of events to fetch (out: fetched) */
        int32_t nflush;    /* Number of events to flush */
};

#define MON_IOC_MAGIC 0x92

#define MON_IOCQ_URB_LEN _IO(MON_IOC_MAGIC, 1)
#define MON_IOCX_URB  _IOWR(MON_IOC_MAGIC, 2, struct mon_bin_hdr)
#define MON_IOCG_STATS _IOR(MON_IOC_MAGIC, 3, struct mon_bin_stats)
#define MON_IOCT_RING_SIZE _IO(MON_IOC_MAGIC, 4)
#define MON_IOCQ_RING_SIZE _IO(MON_IOC_MAGIC, 5)
#define MON_IOCX_GET   _IOW(MON_IOC_MAGIC, 6, struct mon_bin_get)
#define MON_IOCX_MFETCH _IOWR(MON_IOC_MAGIC, 7, struct mon_bin_mfetch)
#define MON_IOCH_MFLUSH _IO(MON_IOC_MAGIC, 8)

#define MON_BIN_SETUP   0x1 /* setup hdr is present*/
#define MON_BIN_SETUP_ZERO      0x2 /* setup buffer is not available */
#define MON_BIN_DATA_ZERO       0x4 /* data buffer is not available */
#define MON_BIN_ERROR   0x8

/*
 * Private data for capturing on Linux USB.
 */
struct pcap_usb_linux {
        u_char *mmapbuf;        /* memory-mapped region pointer */
        size_t mmapbuflen;      /* size of region */
        int bus_index;
        u_int packets_read;
};

/* forward declaration */
static int usb_activate(pcap_t *);
static int usb_stats_linux_bin(pcap_t *, struct pcap_stat *);
static int usb_read_linux_bin(pcap_t *, int , pcap_handler , u_char *);
static int usb_read_linux_mmap(pcap_t *, int , pcap_handler , u_char *);
static int usb_inject_linux(pcap_t *, const void *, int);
static int usb_setdirection_linux(pcap_t *, pcap_direction_t);
static void usb_cleanup_linux_mmap(pcap_t *);

/* facility to add an USB device to the device list*/
static int
usb_dev_add(pcap_if_list_t *devlistp, int n, char *err_str)
{
        char dev_name[10];
        char dev_descr[30];
        snprintf(dev_name, 10, USB_IFACE"%d", n);
        /*
         * XXX - is there any notion of "up" and "running"?
         */
        if (n == 0) {
                /*
                 * As this refers to all buses, there's no notion of
                 * "connected" vs. "disconnected", as that's a property
                 * that would apply to a particular USB interface.
                 */
                if (add_dev(devlistp, dev_name,
                    PCAP_IF_CONNECTION_STATUS_NOT_APPLICABLE,
                    "Raw USB traffic, all USB buses", err_str) == NULL)
                        return -1;
        } else {
                /*
                 * XXX - is there a way to determine whether anything's
                 * plugged into this bus interface or not, and set
                 * PCAP_IF_CONNECTION_STATUS_CONNECTED or
                 * PCAP_IF_CONNECTION_STATUS_DISCONNECTED?
                 */
                snprintf(dev_descr, 30, "Raw USB traffic, bus number %d", n);
                if (add_dev(devlistp, dev_name, 0, dev_descr, err_str) == NULL)
                        return -1;
        }

        return 0;
}

int
usb_findalldevs(pcap_if_list_t *devlistp, char *err_str)
{
        struct dirent* data;
        int ret = 0;
        DIR* dir;
        int n;
        char* name;

        /*
         * We require 2.6.27 or later kernels, so we have binary-mode support.
         * The devices are of the form /dev/usbmon{N}.
         * Open /dev and scan it.
         */
        dir = opendir("/dev");
        if (dir != NULL) {
                while ((ret == 0) && ((data = readdir(dir)) != 0)) {
                        name = data->d_name;

                        /*
                         * Is this a usbmon device?
                         */
                        if (strncmp(name, USBMON_DEV_PREFIX,
                            USBMON_DEV_PREFIX_LEN) != 0)
                                continue;       /* no */

                        /*
                         * What's the device number?
                         */
                        if (sscanf(&name[USBMON_DEV_PREFIX_LEN], "%d", &n) == 0)
                                continue;       /* failed */

                        ret = usb_dev_add(devlistp, n, err_str);
                }

                closedir(dir);
        }
        return 0;
}

/*
 * Matches what's in mon_bin.c in the Linux kernel.
 */
#define MIN_RING_SIZE   (8*1024)
#define MAX_RING_SIZE   (1200*1024)

static int
usb_set_ring_size(pcap_t* handle, int header_size)
{
        /*
         * A packet from binary usbmon has:
         *
         *  1) a fixed-length header, of size header_size;
         *  2) descriptors, for isochronous transfers;
         *  3) the payload.
         *
         * The kernel buffer has a size, defaulting to 300KB, with a
         * minimum of 8KB and a maximum of 1200KB.  The size is set with
         * the MON_IOCT_RING_SIZE ioctl; the size passed in is rounded up
         * to a page size.
         *
         * No more than {buffer size}/5 bytes worth of payload is saved.
         * Therefore, if we subtract the fixed-length size from the
         * snapshot length, we have the biggest payload we want (we
         * don't worry about the descriptors - if we have descriptors,
         * we'll just discard the last bit of the payload to get it
         * to fit).  We multiply that result by 5 and set the buffer
         * size to that value.
         */
        int ring_size;

        if (handle->snapshot < header_size)
                handle->snapshot = header_size;
        /* The maximum snapshot size is small enough that this won't overflow */
        ring_size = (handle->snapshot - header_size) * 5;

        /*
         * Will this get an error?
         * (There's no wqy to query the minimum or maximum, so we just
         * copy the value from the kernel source.  We don't round it
         * up to a multiple of the page size.)
         */
        if (ring_size > MAX_RING_SIZE) {
                /*
                 * Yes.  Lower the ring size to the maximum, and set the
                 * snapshot length to the value that would give us a
                 * maximum-size ring.
                 */
                ring_size = MAX_RING_SIZE;
                handle->snapshot = header_size + (MAX_RING_SIZE/5);
        } else if (ring_size < MIN_RING_SIZE) {
                /*
                 * Yes.  Raise the ring size to the minimum, but leave
                 * the snapshot length unchanged, so we show the
                 * callback no more data than specified by the
                 * snapshot length.
                 */
                ring_size = MIN_RING_SIZE;
        }

        if (ioctl(handle->fd, MON_IOCT_RING_SIZE, ring_size) == -1) {
                pcap_fmt_errmsg_for_errno(handle->errbuf, PCAP_ERRBUF_SIZE,
                    errno, "Can't set ring size from fd %d", handle->fd);
                return -1;
        }
        return ring_size;
}

static
int usb_mmap(pcap_t* handle)
{
        struct pcap_usb_linux *handlep = handle->priv;
        int len;

        /*
         * Attempt to set the ring size as appropriate for the snapshot
         * length, reducing the snapshot length if that'd make the ring
         * bigger than the kernel supports.
         */
        len = usb_set_ring_size(handle, (int)sizeof(pcap_usb_header_mmapped));
        if (len == -1) {
                /* Failed.  Fall back on non-memory-mapped access. */
                return 0;
        }

        handlep->mmapbuflen = len;
        handlep->mmapbuf = mmap(0, handlep->mmapbuflen, PROT_READ,
            MAP_SHARED, handle->fd, 0);
        if (handlep->mmapbuf == MAP_FAILED) {
                /*
                 * Failed.  We don't treat that as a fatal error, we
                 * just try to fall back on non-memory-mapped access.
                 */
                return 0;
        }
        return 1;
}

#ifdef HAVE_LINUX_USBDEVICE_FS_H

#define CTRL_TIMEOUT    (5*1000)        /* milliseconds */

#define USB_DIR_IN              0x80
#define USB_TYPE_STANDARD       0x00
#define USB_RECIP_DEVICE        0x00

#define USB_REQ_GET_DESCRIPTOR  6

#define USB_DT_DEVICE           1
#define USB_DT_CONFIG           2

#define USB_DEVICE_DESCRIPTOR_SIZE      18
#define USB_CONFIG_DESCRIPTOR_SIZE      9

/* probe the descriptors of the devices attached to the bus */
/* the descriptors will end up in the captured packet stream */
/* and be decoded by external apps like wireshark */
/* without these identifying probes packet data can't be fully decoded */
static void
probe_devices(int bus)
{
        struct usbdevfs_ctrltransfer ctrl;
        struct dirent* data;
        int ret = 0;
        char busdevpath[sizeof("/dev/bus/usb/000/") + NAME_MAX];
        DIR* dir;
        uint8_t descriptor[USB_DEVICE_DESCRIPTOR_SIZE];
        uint8_t configdesc[USB_CONFIG_DESCRIPTOR_SIZE];

        /* scan usb bus directories for device nodes */
        snprintf(busdevpath, sizeof(busdevpath), "/dev/bus/usb/%03d", bus);
        dir = opendir(busdevpath);
        if (!dir)
                return;

        while ((ret >= 0) && ((data = readdir(dir)) != 0)) {
                int fd;
                char* name = data->d_name;

                if (name[0] == '.')
                        continue;

                snprintf(busdevpath, sizeof(busdevpath), "/dev/bus/usb/%03d/%s", bus, data->d_name);

                fd = open(busdevpath, O_RDWR);
                if (fd == -1)
                        continue;

                /*
                 * Sigh.  Different kernels have different member names
                 * for this structure.
                 */
#ifdef HAVE_STRUCT_USBDEVFS_CTRLTRANSFER_BREQUESTTYPE
                ctrl.bRequestType = USB_DIR_IN | USB_TYPE_STANDARD | USB_RECIP_DEVICE;
                ctrl.bRequest = USB_REQ_GET_DESCRIPTOR;
                ctrl.wValue = USB_DT_DEVICE << 8;
                ctrl.wIndex = 0;
                ctrl.wLength = sizeof(descriptor);
#else
                ctrl.requesttype = USB_DIR_IN | USB_TYPE_STANDARD | USB_RECIP_DEVICE;
                ctrl.request = USB_REQ_GET_DESCRIPTOR;
                ctrl.value = USB_DT_DEVICE << 8;
                ctrl.index = 0;
                ctrl.length = sizeof(descriptor);
#endif
                ctrl.data = descriptor;
                ctrl.timeout = CTRL_TIMEOUT;

                ret = ioctl(fd, USBDEVFS_CONTROL, &ctrl);

                /* Request CONFIGURATION descriptor alone to know wTotalLength */
#ifdef HAVE_STRUCT_USBDEVFS_CTRLTRANSFER_BREQUESTTYPE
                ctrl.wValue = USB_DT_CONFIG << 8;
                ctrl.wLength = sizeof(configdesc);
#else
                ctrl.value = USB_DT_CONFIG << 8;
                ctrl.length = sizeof(configdesc);
#endif
                ctrl.data = configdesc;
                ret = ioctl(fd, USBDEVFS_CONTROL, &ctrl);
                if (ret >= 0) {
                        uint16_t wtotallength;
                        wtotallength = EXTRACT_LE_U_2(&configdesc[2]);
#ifdef HAVE_STRUCT_USBDEVFS_CTRLTRANSFER_BREQUESTTYPE
                        ctrl.wLength = wtotallength;
#else
                        ctrl.length = wtotallength;
#endif
                        ctrl.data = malloc(wtotallength);
                        if (ctrl.data) {
                                ret = ioctl(fd, USBDEVFS_CONTROL, &ctrl);
                                free(ctrl.data);
                        }
                }
                close(fd);
        }
        closedir(dir);
}
#endif /* HAVE_LINUX_USBDEVICE_FS_H */

pcap_t *
usb_create(const char *device, char *ebuf, int *is_ours)
{
        const char *cp;
        char *cpend;
        long devnum;
        pcap_t *p;

        /* Does this look like a USB monitoring device? */
        cp = strrchr(device, '/');
        if (cp == NULL)
                cp = device;
        /* Does it begin with USB_IFACE? */
        if (strncmp(cp, USB_IFACE, sizeof USB_IFACE - 1) != 0) {
                /* Nope, doesn't begin with USB_IFACE */
                *is_ours = 0;
                return NULL;
        }
        /* Yes - is USB_IFACE followed by a number? */
        cp += sizeof USB_IFACE - 1;
        devnum = strtol(cp, &cpend, 10);
        if (cpend == cp || *cpend != '\0') {
                /* Not followed by a number. */
                *is_ours = 0;
                return NULL;
        }
        if (devnum < 0) {
                /* Followed by a non-valid number. */
                *is_ours = 0;
                return NULL;
        }

        /* OK, it's probably ours. */
        *is_ours = 1;

        p = PCAP_CREATE_COMMON(ebuf, struct pcap_usb_linux);
        if (p == NULL)
                return (NULL);

        p->activate_op = usb_activate;
        return (p);
}

static int
usb_activate(pcap_t* handle)
{
        struct pcap_usb_linux *handlep = handle->priv;
        char            full_path[USB_LINE_LEN];

        /*
         * Turn a negative snapshot value (invalid), a snapshot value of
         * 0 (unspecified), or a value bigger than the normal maximum
         * value, into the maximum allowed value.
         *
         * If some application really *needs* a bigger snapshot
         * length, we should just increase MAXIMUM_SNAPLEN.
         */
        if (handle->snapshot <= 0 || handle->snapshot > MAXIMUM_SNAPLEN)
                handle->snapshot = MAXIMUM_SNAPLEN;

        /* Initialize some components of the pcap structure. */
        handle->bufsize = handle->snapshot;
        handle->offset = 0;
        handle->linktype = DLT_USB_LINUX;

        handle->inject_op = usb_inject_linux;
        handle->setfilter_op = install_bpf_program; /* no kernel filtering */
        handle->setdirection_op = usb_setdirection_linux;
        handle->set_datalink_op = NULL; /* can't change data link type */
        handle->getnonblock_op = pcap_getnonblock_fd;
        handle->setnonblock_op = pcap_setnonblock_fd;

        /*get usb bus index from device name */
        if (sscanf(handle->opt.device, USB_IFACE"%d", &handlep->bus_index) != 1)
        {
                snprintf(handle->errbuf, PCAP_ERRBUF_SIZE,
                        "Can't get USB bus index from %s", handle->opt.device);
                return PCAP_ERROR;
        }

        /*
         * We require 2.6.27 or later kernels, so we have binary-mode support.
         * Try to open the binary interface.
         */
        snprintf(full_path, USB_LINE_LEN, "/dev/"USBMON_DEV_PREFIX"%d",
            handlep->bus_index);
        handle->fd = open(full_path, O_RDONLY, 0);
        if (handle->fd < 0)
        {
                /*
                 * The attempt failed; why?
                 */
                switch (errno) {

                case ENOENT:
                        /*
                         * The device doesn't exist.
                         * That could either mean that there's
                         * no support for monitoring USB buses
                         * (which probably means "the usbmon
                         * module isn't loaded") or that there
                         * is but that *particular* device
                         * doesn't exist (no "scan all buses"
                         * device if the bus index is 0, no
                         * such bus if the bus index isn't 0).
                         *
                         * For now, don't provide an error message;
                         * if we can determine what the particular
                         * problem is, we should report that.
                         */
                        handle->errbuf[0] = '\0';
                        return PCAP_ERROR_NO_SUCH_DEVICE;

                case EACCES:
                        /*
                         * We didn't have permission to open it.
                         */
DIAG_OFF_FORMAT_TRUNCATION
                        snprintf(handle->errbuf, PCAP_ERRBUF_SIZE,
                            "Attempt to open %s failed with EACCES - root privileges may be required",
                            full_path);
DIAG_ON_FORMAT_TRUNCATION
                        return PCAP_ERROR_PERM_DENIED;

                default:
                        /*
                         * Something went wrong.
                         */
                        pcap_fmt_errmsg_for_errno(handle->errbuf,
                            PCAP_ERRBUF_SIZE, errno,
                            "Can't open USB bus file %s", full_path);
                        return PCAP_ERROR;
                }
        }

        if (handle->opt.rfmon)
        {
                /*
                 * Monitor mode doesn't apply to USB devices.
                 */
                close(handle->fd);
                return PCAP_ERROR_RFMON_NOTSUP;
        }

        /* try to use fast mmap access */
        if (usb_mmap(handle))
        {
                /* We succeeded. */
                handle->linktype = DLT_USB_LINUX_MMAPPED;
                handle->stats_op = usb_stats_linux_bin;
                handle->read_op = usb_read_linux_mmap;
                handle->cleanup_op = usb_cleanup_linux_mmap;
#ifdef HAVE_LINUX_USBDEVICE_FS_H
                probe_devices(handlep->bus_index);
#endif

                /*
                 * "handle->fd" is a real file, so
                 * "select()" and "poll()" work on it.
                 */
                handle->selectable_fd = handle->fd;
                return 0;
        }

        /*
         * We failed; try plain binary interface access.
         *
         * Attempt to set the ring size as appropriate for
         * the snapshot length, reducing the snapshot length
         * if that'd make the ring bigger than the kernel
         * supports.
         */
        if (usb_set_ring_size(handle, (int)sizeof(pcap_usb_header)) == -1) {
                /* Failed. */
                close(handle->fd);
                return PCAP_ERROR;
        }
        handle->stats_op = usb_stats_linux_bin;
        handle->read_op = usb_read_linux_bin;
#ifdef HAVE_LINUX_USBDEVICE_FS_H
        probe_devices(handlep->bus_index);
#endif

        /*
         * "handle->fd" is a real file, so "select()" and "poll()"
         * work on it.
         */
        handle->selectable_fd = handle->fd;

        /* for plain binary access and text access we need to allocate the read
         * buffer */
        handle->buffer = malloc(handle->bufsize);
        if (!handle->buffer) {
                pcap_fmt_errmsg_for_errno(handle->errbuf, PCAP_ERRBUF_SIZE,
                    errno, "malloc");
                close(handle->fd);
                return PCAP_ERROR;
        }
        return 0;
}

static int
usb_inject_linux(pcap_t *handle, const void *buf _U_, int size _U_)
{
        snprintf(handle->errbuf, PCAP_ERRBUF_SIZE,
            "Packet injection is not supported on USB devices");
        return (-1);
}

static int
usb_setdirection_linux(pcap_t *p, pcap_direction_t d)
{
        /*
         * It's guaranteed, at this point, that d is a valid
         * direction value.
         */
        p->direction = d;
        return 0;
}

static int
usb_stats_linux_bin(pcap_t *handle, struct pcap_stat *stats)
{
        struct pcap_usb_linux *handlep = handle->priv;
        int ret;
        struct mon_bin_stats st;
        ret = ioctl(handle->fd, MON_IOCG_STATS, &st);
        if (ret < 0)
        {
                pcap_fmt_errmsg_for_errno(handle->errbuf, PCAP_ERRBUF_SIZE,
                    errno, "Can't read stats from fd %d", handle->fd);
                return -1;
        }

        stats->ps_recv = handlep->packets_read + st.queued;
        stats->ps_drop = st.dropped;
        stats->ps_ifdrop = 0;
        return 0;
}

/*
 * see <linux-kernel-source>/Documentation/usb/usbmon.txt and
 * <linux-kernel-source>/drivers/usb/mon/mon_bin.c binary ABI
 */
static int
usb_read_linux_bin(pcap_t *handle, int max_packets _U_, pcap_handler callback, u_char *user)
{
        struct pcap_usb_linux *handlep = handle->priv;
        struct mon_bin_get info;
        int ret;
        struct pcap_pkthdr pkth;
        u_int clen = handle->snapshot - sizeof(pcap_usb_header);

        /* the usb header is going to be part of 'packet' data*/
        info.hdr = (pcap_usb_header*) handle->buffer;
        info.data = (u_char *)handle->buffer + sizeof(pcap_usb_header);
        info.data_len = clen;

        /* ignore interrupt system call errors */
        do {
                ret = ioctl(handle->fd, MON_IOCX_GET, &info);
                if (handle->break_loop)
                {
                        handle->break_loop = 0;
                        return -2;
                }
        } while ((ret == -1) && (errno == EINTR));
        if (ret < 0)
        {
                if (errno == EAGAIN)
                        return 0;       /* no data there */

                pcap_fmt_errmsg_for_errno(handle->errbuf, PCAP_ERRBUF_SIZE,
                    errno, "Can't read from fd %d", handle->fd);
                return -1;
        }

        /*
         * info.hdr->data_len is the number of bytes of isochronous
         * descriptors (if any) plus the number of bytes of data
         * provided.  There are no isochronous descriptors here,
         * because we're using the old 48-byte header.
         *
         * If info.hdr->data_flag is non-zero, there's no URB data;
         * info.hdr->urb_len is the size of the buffer into which
         * data is to be placed; it does not represent the amount
         * of data transferred.  If info.hdr->data_flag is zero,
         * there is URB data, and info.hdr->urb_len is the number
         * of bytes transmitted or received; it doesn't include
         * isochronous descriptors.
         *
         * The kernel may give us more data than the snaplen; if it did,
         * reduce the data length so that the total number of bytes we
         * tell our client we have is not greater than the snaplen.
         */
        if (info.hdr->data_len < clen)
                clen = info.hdr->data_len;
        info.hdr->data_len = clen;
        pkth.caplen = sizeof(pcap_usb_header) + clen;
        if (info.hdr->data_flag) {
                /*
                 * No data; just base the on-the-wire length on
                 * info.hdr->data_len (so that it's >= the captured
                 * length).
                 */
                pkth.len = sizeof(pcap_usb_header) + info.hdr->data_len;
        } else {
                /*
                 * We got data; base the on-the-wire length on
                 * info.hdr->urb_len, so that it includes data
                 * discarded by the USB monitor device due to
                 * its buffer being too small.
                 */
                pkth.len = sizeof(pcap_usb_header) + info.hdr->urb_len;
        }
        pkth.ts.tv_sec = (time_t)info.hdr->ts_sec;
        pkth.ts.tv_usec = info.hdr->ts_usec;

        if (handle->fcode.bf_insns == NULL ||
            pcap_filter(handle->fcode.bf_insns, handle->buffer,
              pkth.len, pkth.caplen)) {
                handlep->packets_read++;
                callback(user, &pkth, handle->buffer);
                return 1;
        }

        return 0;       /* didn't pass filter */
}

/*
 * see <linux-kernel-source>/Documentation/usb/usbmon.txt and
 * <linux-kernel-source>/drivers/usb/mon/mon_bin.c binary ABI
 */
#define VEC_SIZE 32
static int
usb_read_linux_mmap(pcap_t *handle, int max_packets, pcap_handler callback, u_char *user)
{
        struct pcap_usb_linux *handlep = handle->priv;
        struct mon_bin_mfetch fetch;
        int32_t vec[VEC_SIZE];
        struct pcap_pkthdr pkth;
        u_char *bp;
        pcap_usb_header_mmapped* hdr;
        int nflush = 0;
        int packets = 0;
        u_int clen, max_clen;

        max_clen = handle->snapshot - sizeof(pcap_usb_header_mmapped);

        for (;;) {
                int i, ret;
                int limit;

                if (PACKET_COUNT_IS_UNLIMITED(max_packets)) {
                        /*
                         * There's no limit on the number of packets
                         * to process, so try to fetch VEC_SIZE packets.
                         */
                        limit = VEC_SIZE;
                } else {
                        /*
                         * Try to fetch as many packets as we have left
                         * to process, or VEC_SIZE packets, whichever
                         * is less.
                         *
                         * At this point, max_packets > 0 (otherwise,
                         * PACKET_COUNT_IS_UNLIMITED(max_packets)
                         * would be true) and max_packets > packets
                         * (packet starts out as 0, and the test
                         * at the bottom of the loop exits if
                         * max_packets <= packets), so limit is
                         * guaranteed to be > 0.
                         */
                        limit = max_packets - packets;
                        if (limit > VEC_SIZE)
                                limit = VEC_SIZE;
                }

                /*
                 * Try to fetch as many events as possible, up to
                 * the limit, and flush the events we've processed
                 * earlier (nflush) - MON_IOCX_MFETCH does both
                 * (presumably to reduce the number of system
                 * calls in loops like this).
                 */
                fetch.offvec = vec;
                fetch.nfetch = limit;
                fetch.nflush = nflush;
                /* ignore interrupt system call errors */
                do {
                        ret = ioctl(handle->fd, MON_IOCX_MFETCH, &fetch);
                        if (handle->break_loop)
                        {
                                handle->break_loop = 0;
                                return -2;
                        }
                } while ((ret == -1) && (errno == EINTR));
                if (ret < 0)
                {
                        if (errno == EAGAIN)
                                return 0;       /* no data there */

                        pcap_fmt_errmsg_for_errno(handle->errbuf,
                            PCAP_ERRBUF_SIZE, errno, "Can't mfetch fd %d",
                            handle->fd);
                        return -1;
                }

                /* keep track of processed events, we will flush them later */
                nflush = fetch.nfetch;
                for (i=0; i<fetch.nfetch; ++i) {
                        /*
                         * XXX - we can't check break_loop here, as
                         * we read the indices of packets into a
                         * local variable, so if we're later called
                         * to fetch more packets, those packets will
                         * not be seen - and won't be flushed, either.
                         *
                         * Instead, we would have to keep the array
                         * of indices in our private data, along
                         * with the count of packets to flush - or
                         * would have to flush the already-processed
                         * packets if we break out of the loop here.
                         */

                        /* Get a pointer to this packet's buffer */
                        bp = &handlep->mmapbuf[vec[i]];

                        /* That begins with a metadata header */
                        hdr = (pcap_usb_header_mmapped*) bp;

                        /* discard filler */
                        if (hdr->event_type == '@')
                                continue;

                        /*
                         * hdr->data_len is the number of bytes of
                         * isochronous descriptors (if any) plus the
                         * number of bytes of data provided.
                         *
                         * If hdr->data_flag is non-zero, there's no
                         * URB data; hdr->urb_len is the size of the
                         * buffer into which data is to be placed; it does
                         * not represent the amount of data transferred.
                         * If hdr->data_flag is zero, there is URB data,
                         * and hdr->urb_len is the number of bytes
                         * transmitted or received; it doesn't include
                         * isochronous descriptors.
                         *
                         * The kernel may give us more data than the
                         * snaplen; if it did, reduce the data length
                         * so that the total number of bytes we
                         * tell our client we have is not greater than
                         * the snaplen.
                         */
                        clen = max_clen;
                        if (hdr->data_len < clen)
                                clen = hdr->data_len;
                        pkth.caplen = sizeof(pcap_usb_header_mmapped) + clen;
                        set_linux_usb_mmapped_length(&pkth, bp);
                        pkth.ts.tv_sec = (time_t)hdr->ts_sec;
                        pkth.ts.tv_usec = hdr->ts_usec;

                        if (handle->fcode.bf_insns == NULL ||
                            pcap_filter(handle->fcode.bf_insns, (u_char*) hdr,
                              pkth.len, pkth.caplen)) {
                                handlep->packets_read++;
                                callback(user, &pkth, (u_char*) hdr);
                                packets++;
                        }
                }

                /*
                 * If max_packets specifiesg "unlimited", we stop after
                 * the first chunk.
                 */
                if (PACKET_COUNT_IS_UNLIMITED(max_packets) ||
                    (packets >= max_packets))
                        break;
        }

        /* flush pending events*/
        if (ioctl(handle->fd, MON_IOCH_MFLUSH, nflush) == -1) {
                pcap_fmt_errmsg_for_errno(handle->errbuf, PCAP_ERRBUF_SIZE,
                    errno, "Can't mflush fd %d", handle->fd);
                return -1;
        }
        return packets;
}

static void
usb_cleanup_linux_mmap(pcap_t* handle)
{
        struct pcap_usb_linux *handlep = handle->priv;

        /* if we have a memory-mapped buffer, unmap it */
        if (handlep->mmapbuf != NULL) {
                munmap(handlep->mmapbuf, handlep->mmapbuflen);
                handlep->mmapbuf = NULL;
        }
        pcap_cleanup_live_common(handle);
}