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* inc version number to force fusd upgrade
* added helpful error messages

/*
 *
 * Copyright (c) 2003 The Regents of the University of California.  All 
 * rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 *
 * - Neither the name of the University nor the names of its
 *   contributors may be used to endorse or promote products derived
 *   from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
 *
 */
 

/*
 * fusd userspace library: functions that know how to properly talk
 * to the fusd kernel module
 *
 * authors: jelson and girod
 *
 * $Id: libfusd.c,v 1.71 2004/04/26 06:25:26 girod Exp $
 */

char libfusd_c_id[] = "$Id: libfusd.c,v 1.71 2004/04/26 06:25:26 girod Exp $";

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/uio.h>
#include <sys/ioctl.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <time.h>
#include <sys/poll.h>

#include "fusd.h"
#include "fusd_msg.h"

#define MIN(a, b) ((a) < (b) ? (a) : (b))

/* maximum number of messages processed by a single call to fusd_dispatch */
#define MAX_MESSAGES_PER_DISPATCH 40

/* used for fusd_run */
static fd_set fusd_fds;

/* default prefix of devices (often "/dev/") */
char *dev_root = DEFAULT_DEV_ROOT;

/* 
 * fusd_fops_set is an array that keeps track of the file operations
 * struct for each fusd fd.
 */
static fusd_file_operations_t fusd_fops_set[FD_SETSIZE];
fusd_file_operations_t null_fops = { NULL };

/*
 * accessor macros
 */
#define FUSD_GET_FOPS(fd) \
  (fusd_fops_set + (fd))

#define FUSD_SET_FOPS(fd,fi) \
  (fusd_fops_set[(fd)]=(*fi))

#define FUSD_FD_VALID(fd) \
  (((fd)>=0) && \
   ((fd)<FD_SETSIZE) && \
   (memcmp(FUSD_GET_FOPS(fd), &null_fops, sizeof(fusd_file_operations_t))))


/*
 * fusd_init
 * 
 * this is called automatically before the first
 * register call 
 */
void fusd_init()
{
  static int fusd_init_needed = 1;

  if (fusd_init_needed) {
    int i;
    struct stat statbuf;

    fusd_init_needed = 0;

    for (i = 0; i < FD_SETSIZE; i++)
      FUSD_SET_FOPS(i, &null_fops);
    FD_ZERO(&fusd_fds);

    /* test to see if devfs is actually mounted where we think it is */
    if (stat(FUSD_CONTROL_DEVNAME, &statbuf) < 0) 
      fprintf(stderr, "libfusd: FUSD is not installed!\n");
  }
}


/*
 *  FUSD status handling helper functions
 */

int fusd_status_process(fusd_status_context_t *ctx)
{
  int retval = STATUS_ERR;

  int status = read(ctx->fd, ctx->buf+ctx->len, sizeof(ctx->buf)-ctx->len);
  if (status > 0) {
    int count;
    
    ctx->len += status;
    count = ctx->len / sizeof(fusd_status_t);
    if (ctx->verbose)
      fprintf(stderr, "Got %d records (%d).. processing\n", count, ctx->len);
    ctx->len = ctx->len % sizeof(fusd_status_t);

    if (count > 0) {
      fusd_status_t *fs = (fusd_status_t *)ctx->buf;
      if (fs->full_mode && !ctx->full_mode) {
	ctx->full_mode = 1;
	if (ctx->begin_full_cb) ctx->begin_full_cb(ctx->private_data);
      }
      retval = ctx->new_data_cb((fusd_status_t *)ctx->buf, count, ctx->private_data);	  
      memmove(ctx->buf, ctx->buf+(count*sizeof(fusd_status_t)), ctx->len);
    }
  }

  else if (status == 0) {
    if (ctx->full_mode) {
      if (ctx->end_full_cb) ctx->end_full_cb(ctx->private_data);
      ctx->full_mode = 0;
    }
    retval = STATUS_DONE;
  }

  return retval;
}


int fusd_status_wait(fusd_status_context_t *ctx)
{
  while (1) {
    /* poll */
    struct pollfd pfd = {
      fd: ctx->fd,
      events: POLLIN
    };
    int status = poll(&pfd, 1, -1);

    /* error? */
    if (status < 0) {
      if (errno && (errno != EINTR))
	return -1;
    }

    /* got data? */
    if ((status == 1) && (pfd.revents & POLLIN)) 
      return 0;
  }  
}


#if 0
static
int fusd_status_check_aux(fusd_status_t *fs, int count, void *data)
{
  int i;
  char *name = (char*)data;
  for (i=0; i<count; i++) {
    if (strcmp(fs[i].name, name) == 0)
      return STATUS_FOUND;
  }
  return STATUS_OK;
}
#endif


/*
 *  FUSD registration helper functions
 */

int fusd_build_reg_msg(fusd_msg_t *message, const char *name, mode_t mode, void *device_info)
{
  int retval = 0;
  
  /*
   * convenience: if the first characters of the name you're trying
   * to register are SKIP_PREFIX (usually "/dev/"), skip over them.
   */
  if (dev_root != NULL && strlen(name) > strlen(dev_root) &&
      !strncmp(name, dev_root, strlen(dev_root))) {
    name += strlen(dev_root);
  }

  if (strlen(name) > FUSD_MAX_NAME_LENGTH) {
    fprintf(stderr, "name '%s' too long, sorry :(", name);
    retval = -EINVAL;
    goto done;
  }

  /* set up the message */
  memset(message, 0, sizeof(fusd_msg_t));
  message->magic = FUSD_MSG_MAGIC;
  message->cmd = FUSD_REGISTER_DEVICE;
  message->datalen = 0;
  strcpy(message->parm.register_msg.name, name);
  message->parm.register_msg.mode = mode;
  message->parm.register_msg.device_info = device_info;

 done:
  return retval;
}


int fusd_open_control()
{
  int fd = open(FUSD_CONTROL_DEVNAME, O_RDWR | O_NONBLOCK);
  
  if (fd < 0) {
    int retval = 0;
    
    if (errno == ENOPKG) {
      fprintf(stderr, "libfusd: ensure fusdd is running!\n");
      retval = -ENOPKG;
    } 
    
    /* if the problem is that /dev/fusd does not exist, return the
     * message "Package not installed", which is hopefully more
     * illuminating than "no such file or directory" */
    else if (errno == ENOENT) {
      fprintf(stderr, "libfusd: %s does not exist; ensure FUSD's kernel module is installed\n",
	      FUSD_CONTROL_DEVNAME);
      fprintf(stderr, "libfusd: if DevFS not in use, be sure that fusdd is running\n");
      retval = -ENOPKG;
    } else {
      perror("libfusd: trying to open FUSD control channel");
      retval = -errno;
    }
    return retval;
  }  

  return fd;  
}


int fusd_postreg_block(const char *name)
{
  int stat;
  int x;
    
  /* waitfor the device to be created, if we are NOT devfs enabled */
  if ((stat = open(FUSD_STATUS_DEVNAME, O_RDWR | O_NONBLOCK)) < 0) {
    fprintf(stderr, "libfusd: %s does not exist; ensure FUSD's kernel module is installed\n",
	    FUSD_STATUS_DEVNAME);
    return -1;
  }
      
  /* skip blocking waitfor if devfs */
  if (ioctl(stat, FUSD_STATUS_NO_DEVFS, &x) != 0)
    goto out;
    
  /* attempt to write request */
  if (write(stat, name, strlen(name)) >= 0) {
    fusd_status_context_t fsctx = {
      fd: stat
    };

    if (fusd_status_wait(&fsctx) < 0) {
      fprintf(stderr, "WARNING: Specific waitfor mode failed (%s): %m\n", name);
      goto out;
    }
    goto out;
  }
  else if (errno != ENOSYS)
    fprintf(stderr, "WARNING: Failed to enter specific waitfor mode (%s): %m\n", name);
    
 out:
  close(stat);
  return 0;
}


/*
 *  FUSD Registration function
 */

int fusd_register(const char *name, mode_t mode, void *device_info,
		  struct fusd_file_operations *fops)
{
  int fd = -1, retval = 0;
  fusd_msg_t message;

  /* need initialization? */
  fusd_init();

  /* make sure the name is valid and we have a valid set of fops... */
  if (name == NULL || fops == NULL) {
    fprintf(stderr, "fusd_register: invalid name or fops argument\n");
    retval = -EINVAL;
    goto done;
  }

  /* open the fusd control channel */
  fd = fusd_open_control();
  if (fd < 0) {
    retval = -errno;
    goto done;
  }

  /* fd in use? */
  if (FUSD_FD_VALID(fd)) {
    retval = -EBADF;
    goto done;
  }

  /* build the message */
  retval = fusd_build_reg_msg(&message, name, mode, device_info);
  if (retval == 0) name = message.parm.register_msg.name;
  else goto done;

  /* make the request */
  if (write(fd, &message, sizeof(fusd_msg_t)) < 0) {
    if (errno == EIO) {
      fprintf(stderr, "FUSD version mismatch?\n");
    }
    retval = -errno;
    goto done;
  }

  /* OK, store the new file state */
  FUSD_SET_FOPS(fd, fops);
  FD_SET(fd, &fusd_fds);

  /* success! */
 done:
  if (retval < 0) {
    if (fd >= 0)
      close(fd);
    errno = -retval;
    retval = -1;
  } 
  else if (fusd_postreg_block(name) == 0) {    
    /* report the fd */
    retval = fd;
    errno = 0;
  }
  
  return retval;
}


int fusd_unregister(int fd)
{
  if (FUSD_FD_VALID(fd)) {
    /* clear fd location */
    FUSD_SET_FOPS(fd, &null_fops);
    FD_CLR(fd, &fusd_fds);
    /* close */
    return close(fd);
  }
  
  else {
    errno = EBADF;
    return -1;
  }
}


/* 
 * fusd_run: a convenience function for automatically running a FUSD
 * driver, for drivers that don't want to manually select on file
 * descriptors and call fusd_dispatch.  This function will
 * automatically select on all devices the user has registered and
 * call fusd_dispatch on any one that becomes readable.
 */
void fusd_run(void)
{
  fd_set tfds;
  int status;
  int maxfd;
  int i;

  /* locate maxmimum fd in use */
  for (maxfd=0, i=0; i < FD_SETSIZE; i++) {
    if (FD_ISSET(i, &fusd_fds)) {
      maxfd = i;
    }
  }
  maxfd++;


  while (1) {
    /* select */
    memmove(&tfds, &fusd_fds, sizeof(fd_set));
    status = select(maxfd, &tfds, NULL, NULL, NULL);

    /* error? */
    if (status < 0) {
      perror("libfusd: fusd_run: error on select");
      continue;
    }

    /* readable? */
    for (i = 0; i < maxfd; i++)
      if (FD_ISSET(i, &tfds))
	fusd_dispatch(i);
  }
}


/************************************************************************/


/* reads a fusd kernel-to-userspace message from fd, and puts a
 * fusd_msg into the memory pointed to by msg (we assume we are passed
 * a buffer managed by the caller).  if there is a data portion to the
 * message (msg->datalen > 0), we allocate memory for it, set data to
 * point to that memory.  the returned data pointer must also be
 * managed by the caller. */
static
int fusd_get_message(int fd, fusd_msg_t *msg, void **local_client)
{
  /* read the header part into the kernel */
  if (read(fd, msg, sizeof(fusd_msg_t)) < 0) {
    if (errno != EAGAIN)
      perror("error talking to FUSD control channel on header read");
    return -errno;
  }

  /* if this is from the net, we return the local client demux token */
  if (local_client)
    *local_client = msg->data;
  
  /* clear the data pointer */
  msg->data = NULL; 

  if (msg->magic != FUSD_MSG_MAGIC) {
    fprintf(stderr, "libfusd magic number failure\n");
    return -EINVAL;
  }

  /* if there's a data part to the message, read it from the kernel. */
  if (msg->datalen) {
    if ((msg->data = malloc(msg->datalen + 1)) == NULL) {
      fprintf(stderr, "libfusd: can't allocate memory\n");
      return -ENOMEM;  /* this is bad, we are now unsynced */
    }

    if (read(fd, msg->data, msg->datalen) < 0) {
      perror("error talking to FUSD control channel on data read");
      free(msg->data);
      msg->data = NULL;
      return -EIO;
    }

    /* For convenience, we now ensure that the byte *after* the buffer
     * is set to 0.  (Note we malloc'd one extra byte above.) */
    msg->data[msg->datalen] = '\0';
  }

  return 0;
}


/*
 * fusd_fdset_add: given an FDSET and "max", add the currently valid
 * FUSD fds to the set and update max accordingly.
 */
void fusd_fdset_add(fd_set *set, int *max)
{
  int i;

  for (i = 0; i < FD_SETSIZE; i++) {
    if (FD_ISSET(i, &fusd_fds)) {
      FD_SET(i, set);
      if (i > *max) {
	*max = i;
      }
    }
  }
}



/*
 * fusd_dispatch_fdset: given an fd_set full of descriptors, call
 * fusd_dispatch on every descriptor in the set which is a valid FUSD
 * fd.
 */
void fusd_dispatch_fdset(fd_set *set)
{
  int i;

  for (i = 0; i < FD_SETSIZE; i++)
    if (FD_ISSET(i, set) && FD_ISSET(i, &fusd_fds))
      fusd_dispatch(i);
}


/* 
 * fusd_dispatch_one() -- read a single kernel-to-userspace message
 * from fd, then call the appropriate userspace callback function,
 * based on the message that was read.  finally, return the result
 * back to the kernel, IF the return value from the callback is not
 * FUSD_NOREPLY.
 *
 * On success, returns 0.
 * On failure, returns a negative number indicating the errno.
 */
int fusd_dispatch_one(int fd, fusd_file_operations_t *fops, 
		      int from_net, void **local_device_info)
{
  fusd_file_info_t *file = NULL;
  fusd_msg_t *msg = NULL;
  int driver_retval = 0; /* returned to the FUSD driver */
  int user_retval = 0;    /* returned to the user who made the syscall */
  void *local_client = NULL;

  /* check for valid, look up ops */
  if (fops == NULL) {
    fprintf(stderr, "fusd_dispatch: no fops provided!\n");
    driver_retval = -EBADF;
    goto out_noreply;
  }

  /* allocate memory for fusd_msg_t */
  if ((msg = malloc(sizeof(fusd_msg_t))) == NULL) {
    driver_retval = -ENOMEM;
    fprintf(stderr, "libfusd: can't allocate memory\n");
    goto out_noreply;
  }
  memset(msg, '\0', sizeof(fusd_msg_t));

  /* read header and data, if it's there */
  if ((driver_retval = fusd_get_message(fd, msg, &local_client)) < 0)
    goto out_noreply;

  /* allocate file info struct */
  file = malloc(sizeof(fusd_file_info_t));
  if (NULL == file) {
    fprintf(stderr, "libfusd: can't allocate memory\n");
    driver_retval = -ENOMEM;
    goto out_noreply;
  }

  /* fill the file info struct */
  memset(file, '\0', sizeof(fusd_file_info_t));

  /* if we're processing stuff from the net, we need to keep track
   * of demux info required to route back to the remote client */
  if (from_net) {
    file->remote_client = msg->parm.fops_msg.device_info;
    file->local_client = local_client;
    if (local_device_info == NULL)
      fprintf(stderr, "libfusd: net support requires local device info parameter!\n");
    else
      msg->parm.fops_msg.device_info = *local_device_info;
  }

  /* fill the rest of the fields */
  file->fd = fd;
  file->device_info = msg->parm.fops_msg.device_info;
  file->private_data = msg->parm.fops_msg.private_info;
  file->flags = msg->parm.fops_msg.flags;
  file->pid = msg->parm.fops_msg.pid;
  file->uid = msg->parm.fops_msg.uid;
  file->gid = msg->parm.fops_msg.gid;
  file->fusd_msg = msg;  

  /* net support must track its own local device info */
  file->local_device_info = local_device_info;
  
  /* right now we only handle fops requests */
  if (msg->cmd != FUSD_FOPS_CALL && msg->cmd != FUSD_FOPS_NONBLOCK &&
      msg->cmd != FUSD_FOPS_CALL_DROPREPLY) {
    fprintf(stderr, "libfusd: got unknown msg->cmd from kernel\n");
    user_retval = -EINVAL;
    goto send_reply;
  }
  
  /* dispatch on operation type */
  user_retval = -ENOSYS;
  switch (msg->subcmd) {
  case FUSD_OPEN:
    if (fops && fops->open)
      user_retval = fops->open(file);
    break;
  case FUSD_CLOSE:
    if (fops && fops->close)
      user_retval = fops->close(file);
    break;
  case FUSD_READ:
    /* allocate a buffer and make the call */
    if (fops && fops->read) {
      if ((msg->data = malloc(msg->parm.fops_msg.length)) == NULL) {
	user_retval = -ENOMEM;
	fprintf(stderr, "libfusd: can't allocate memory\n");
      } else {
	msg->datalen = msg->parm.fops_msg.length;
	memset(msg->data, 0, msg->datalen);
	user_retval = fops->read(file, msg->data, msg->datalen,
				 &msg->parm.fops_msg.offset);
      }
    }
    break;
  case FUSD_WRITE:
    if (fops && fops->write)
      user_retval = fops->write(file, msg->data, msg->datalen,
				&msg->parm.fops_msg.offset);
    break;
  case FUSD_IOCTL:
    if (fops && fops->ioctl) {
      /* in the case of an ioctl read, allocate a buffer for the
       * driver to write to, IF there isn't already a buffer.  (there
       * might already be a buffer if this is a read+write) */
      if ((_IOC_DIR(msg->parm.fops_msg.cmd) & _IOC_READ) &&
	  msg->data == NULL) {
	msg->datalen = _IOC_SIZE(msg->parm.fops_msg.cmd);
	if ((msg->data = malloc(msg->datalen)) == NULL) {
	  user_retval = -ENOMEM;
	  break;
	}
      }
      if (msg->data != NULL)
	user_retval = fops->ioctl(file, msg->parm.fops_msg.cmd, msg->data);
      else
	user_retval = fops->ioctl(file, msg->parm.fops_msg.cmd,
				  (void *) msg->parm.fops_msg.arg);
    }
    break;
    
  case FUSD_POLL_DIFF:
    /* This callback requests notification when an event occurs on a file,
     * e.g. becoming readable or writable */
    if (fops && fops->poll_diff)
      user_retval = fops->poll_diff(file, msg->parm.fops_msg.cmd);
    break;    
  
  case FUSD_UNBLOCK:
    /* This callback is called when a system call is interrupted */
    if (fops && fops->unblock)
      user_retval = fops->unblock(file);    
    break;
    
  default:
    fprintf(stderr, "libfusd: Got unsupported operation\n");
    user_retval = -ENOSYS;
    break;
  }
  
  goto send_reply;


  /* out_noreply is only used for handling errors */
 out_noreply:
  if (msg->data != NULL)
    free(msg->data);
  if (msg != NULL)
    free(msg);
  goto done;

  /* send_reply is only used for success */
 send_reply:
  if (-user_retval <= 0xff) {
    /* 0xff is the maximum legal return value (?) - return val to user */
    driver_retval = fusd_return(file, user_retval);
  } else {
    /* if we got a FUSD_NOREPLY, don't free the msg structure */
    driver_retval = 0;
  }

  /* this is common to both errors and success */
 done:
  if (driver_retval < 0) {
    errno = -driver_retval;
    driver_retval = -1;
  }
  return driver_retval;
}


/* fusd_dispatch is now a wrapper around fusd_dispatch_one that calls
 * it repeatedly, until it fails.  this helps a lot with bulk data
 * transfer since there is no intermediate select in between the
 * reads.  (the kernel module helps by running the user process in
 * between).
 *
 * This function now prints an error to stderr in case of error,
 * instead of returning a -1.
 */
void fusd_dispatch_aux(int fd, int from_net, fusd_file_operations_t *use_fops, void **local_device_info)
{
  int retval, num_dispatches = 0;
  fusd_file_operations_t *fops = use_fops;

  if (fops == NULL) {
    /* make sure we have a valid FD, and get its fops structure */
    if (!FUSD_FD_VALID(fd)) {
      errno = EBADF;
      retval = -1;
      goto out;
    }
    fops = FUSD_GET_FOPS(fd);
  }
  
  /* now keep dispatching until a dispatch returns an error */
  do {
    retval = fusd_dispatch_one(fd, fops, from_net, local_device_info);
    
    if (retval >= 0)
      num_dispatches++;
  } while (retval >= 0 && num_dispatches <= MAX_MESSAGES_PER_DISPATCH);

  /* if we've dispatched at least one message successfully, and then
   * stopped because of EAGAIN - do not report an error.  this is the
   * common case. */
  if (num_dispatches > 0 && errno == EAGAIN) {
    retval = 0;
    errno = 0;
  }

 out:
  if (retval < 0 && errno != EPIPE)
    fprintf(stderr, "libfusd: fusd_dispatch error on fd %d: %m\n", fd);
}


void fusd_dispatch(int fd)
{
  fusd_dispatch_aux(fd, 0, NULL, NULL);
}

/*
 * fusd_destroy destroys all state associated with a fusd_file_info
 * pointer.  (It is implicitly called by fusd_return.)  If a driver
 * saves a fusd_file_info pointer by calling -FUSD_NOREPLY in order to
 * block a read, but gets a "close" request on the file before the
 * pointer is returned with fusd_return, it should be thrown away
 * using fusd_destroy.  
 */
void fusd_destroy(struct fusd_file_info *file)
{
  if (file == NULL)
    return;

  if (file->fusd_msg->data != NULL)
    free(file->fusd_msg->data);
  free(file->fusd_msg);
  free(file);
}


/*
 * construct a user-to-kernel message in reply to a file function
 * call. 
 *
 * On success, returns 0.
 * On failure, returns a negative number indicating the errno.
 */
int fusd_return(fusd_file_info_t *file, ssize_t retval)
{
  fusd_msg_t *msg = NULL;
  int fd;
  int driver_retval = 0;
  struct iovec iov[2];
  char *msg_data = NULL;
  int fd_tracking = file->local_device_info == NULL;
  
  if (file == NULL) {
    fprintf(stderr, "fusd_return: NULL file\n");
    return -EINVAL;
  }

  fd = file->fd;
  if (fd_tracking && !FUSD_FD_VALID(fd)) {
    fprintf(stderr, "fusd_return: badfd (fd %d)\n", fd);
    return -EBADF;
  }
  
  if ((msg = file->fusd_msg) == NULL) {
    fprintf(stderr, "fusd_return: fusd_msg is gone\n");
    return -EINVAL;
  }

  /* if this was a "DONTREPLY" message, just free the struct */
  if (msg->cmd == FUSD_FOPS_CALL_DROPREPLY)
    goto free_memory;

  /* do we copy data back to kernel?  how much? */
  switch(msg->subcmd) {
  case FUSD_READ:
    /* these operations can return data to userspace */
    if (retval > 0) {
      msg->datalen = MIN(retval, msg->parm.fops_msg.length);
      retval = msg->datalen;
    } else {
      msg->datalen = 0;
    }
    break;
  case FUSD_IOCTL:
    /* ioctl CAN (in read mode) return data to userspace */
    if ((retval == 0) && 
	(_IOC_DIR(msg->parm.fops_msg.cmd) & _IOC_READ))
      msg->datalen = _IOC_SIZE(msg->parm.fops_msg.cmd);
    else
      msg->datalen = 0;
    break;
  default:
    /* open, close, write, etc. do not return data */
    msg->datalen = 0;
    break;
  }

  /* save the local device info if we're tracking it */
  if (file->local_device_info)
    *(file->local_device_info) = file->device_info;
  
  /* handle net case: if we have one, patch in the remote's device info */
  msg->parm.fops_msg.device_info = 
    file->remote_client ? file->remote_client : file->device_info;
  
  /* handle net case: patch in the local demux info */
  msg_data = msg->data;
  msg->data = file->local_client;
  
  /* fill the file info struct */
  msg->cmd++; /* change FOPS_CALL to FOPS_REPLY; NONBLOCK to NONBLOCK_REPLY */
  msg->parm.fops_msg.retval = retval;
  msg->parm.fops_msg.private_info = file->private_data;
  msg->parm.fops_msg.flags = file->flags;
  /* pid is NOT copied back. */

  /* send message to kernel */
  if (msg->datalen > 0 && msg_data != NULL) {
    iov[0].iov_base = msg;
    iov[0].iov_len = sizeof(fusd_msg_t);
    iov[1].iov_base = msg_data;
    iov[1].iov_len = msg->datalen;
    driver_retval = writev(fd, iov, 2);
  }
  else {
    driver_retval = write(fd, msg, sizeof(fusd_msg_t));
  }

  /* restore data pointer for free */
  msg->data = msg_data;

 free_memory:
  fusd_destroy(file);

  if (driver_retval < 0)
    return -errno;
  else
    return 0;
}


/* returns static string representing the flagset (e.g. RWE) */
#define RING 5
char *fusd_unparse_flags(int flags)
{
  static int i = 0;
  static char ringbuf[RING][5];
  char *s = ringbuf[i];
  i = (i + 1) % RING;
  
  sprintf(s, "%c%c%c", 
	  (flags & FUSD_NOTIFY_INPUT)?'R':'-',
	  (flags & FUSD_NOTIFY_OUTPUT)?'W':'-',
	  (flags & FUSD_NOTIFY_EXCEPT)?'E':'-');
  
  return s;
}
#undef RING