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NAME

       fanotify - monitoring filesystem events

DESCRIPTION

       The  fanotify  API provides notification and interception of filesystem events.  Use cases
       include virus scanning and hierarchical storage management.  Currently, only a limited set
       of  events  is supported.  In particular, there is no support for create, delete, and move
       events.  (See inotify(7) for details of an API that does notify those events.)

       Additional capabilities compared to the inotify(7) API include the ability to monitor  all
       of  the  objects in a mounted filesystem, the ability to make access permission decisions,
       and the possibility to read or modify files before access by other applications.

       The following system calls are used with  this  API:  fanotify_init(2),  fanotify_mark(2),
       read(2), write(2), and close(2).

   fanotify_init(), fanotify_mark(), and notification groups
       The  fanotify_init(2)  system  call creates and initializes an fanotify notification group
       and returns a file descriptor referring to it.

       An fanotify notification group is a kernel-internal object that holds  a  list  of  files,
       directories, and mount points for which events shall be created.

       For  each  entry in an fanotify notification group, two bit masks exist: the mark mask and
       the ignore mask.  The mark mask defines file  activities  for  which  an  event  shall  be
       created.   The  ignore  mask  defines  activities  for  which no event shall be generated.
       Having these two types of masks permits a mount  point  or  directory  to  be  marked  for
       receiving  events,  while at the same time ignoring events for specific objects under that
       mount point or directory.

       The fanotify_mark(2) system call adds a file, directory, or mount to a notification  group
       and  specifies which events shall be reported (or ignored), or removes or modifies such an
       entry.

       A possible usage of the ignore mask is for a file cache.  Events of interest  for  a  file
       cache  are modification of a file and closing of the same.  Hence, the cached directory or
       mount point is to be marked to receive these events.   After  receiving  the  first  event
       informing  that  a  file  has  been  modified,  the  corresponding  cache  entry  will  be
       invalidated.  No further modification events for this file are of interest until the  file
       is  closed.   Hence, the modify event can be added to the ignore mask.  Upon receiving the
       close event, the modify event can be removed from the ignore mask and the file cache entry
       can be updated.

       The  entries  in the fanotify notification groups refer to files and directories via their
       inode number and to mounts via their mount ID.  If files or  directories  are  renamed  or
       moved  within the same mount, the respective entries survive.  If files or directories are
       deleted or moved to another mount or if mounts are unmounted,  the  corresponding  entries
       are deleted.

   The event queue
       As  events occur on the filesystem objects monitored by a notification group, the fanotify
       system generates events that are collected in a queue.  These  events  can  then  be  read
       (using read(2) or similar) from the fanotify file descriptor returned by fanotify_init(2).

       Two   types   of   events  are  generated:  notification  events  and  permission  events.
       Notification events are merely informative and require  no  action  to  be  taken  by  the
       receiving  application  except  for  closing  the file descriptor passed in the event (see
       below).  Permission events are requests to the receiving  application  to  decide  whether
       permission for a file access shall be granted.  For these events, the recipient must write
       a response which decides whether access is granted or not.

       An event is removed from the event queue of the fanotify group  when  it  has  been  read.
       Permission  events  that have been read are kept in an internal list of the fanotify group
       until either a permission decision  has  been  taken  by  writing  to  the  fanotify  file
       descriptor or the fanotify file descriptor is closed.

   Reading fanotify events
       Calling  read(2)  for the file descriptor returned by fanotify_init(2) blocks (if the flag
       FAN_NONBLOCK is not specified in the call to fanotify_init(2)) until either a  file  event
       occurs or the call is interrupted by a signal (see signal(7)).

       After  a  successful  read(2),  the  read  buffer  contains  one  or more of the following
       structures:

           struct fanotify_event_metadata {
               __u32 event_len;
               __u8 vers;
               __u8 reserved;
               __u16 metadata_len;
               __aligned_u64 mask;
               __s32 fd;
               __s32 pid;
           };

       For performance reasons, it is recommended to use a large buffer size (for  example,  4096
       bytes), so that multiple events can be retrieved by a single read(2).

       The  return value of read(2) is the number of bytes placed in the buffer, or -1 in case of
       an error (but see BUGS).

       The fields of the fanotify_event_metadata structure are as follows:

       event_len
              This is the length of the data for the current event and the  offset  to  the  next
              event  in  the  buffer.   In  the current implementation, the value of event_len is
              always FAN_EVENT_METADATA_LEN.  However, the API is  designed  to  allow  variable-
              length structures to be returned in the future.

       vers   This  field  holds  a  version  number  for  the structure.  It must be compared to
              FANOTIFY_METADATA_VERSION to verify that the structures returned at  runtime  match
              the  structures  defined  at  compile time.  In case of a mismatch, the application
              should abandon trying to use the fanotify file descriptor.

       reserved
              This field is not used.

       metadata_len
              This is the length of the structure.  The field was introduced  to  facilitate  the
              implementation  of optional headers per event type.  No such optional headers exist
              in the current implementation.

       mask   This is a bit mask describing the event (see below).

       fd     This is an open file descriptor for the object being accessed,  or  FAN_NOFD  if  a
              queue overflow occurred.  The file descriptor can be used to access the contents of
              the monitored file or  directory.   The  reading  application  is  responsible  for
              closing this file descriptor.

              When  calling  fanotify_init(2),  the  caller  may  specify  (via the event_f_flags
              argument) various file status flags that are to be set on the open file description
              that  corresponds  to  this  file  descriptor.   In addition, the (kernel-internal)
              FMODE_NONOTIFY file status flag is set on the open  file  description.   This  flag
              suppresses  fanotify  event  generation.   Hence, when the receiver of the fanotify
              event accesses the notified file  or  directory  using  this  file  descriptor,  no
              additional events will be created.

       pid    This  is  the  ID  of  the  process  that caused the event.  A program listening to
              fanotify events can compare this PID to the PID returned by getpid(2), to determine
              whether  the  event is caused by the listener itself, or is due to a file access by
              another process.

       The bit mask in mask indicates which events have occurred for a single filesystem  object.
       Multiple  bits  may be set in this mask, if more than one event occurred for the monitored
       filesystem object.  In particular, consecutive events for the same filesystem  object  and
       originating  from  the  same process may be merged into a single event, with the exception
       that two permission events are never merged into one queue entry.

       The bits that may appear in mask are as follows:

       FAN_ACCESS
              A file or a directory (but see BUGS) was accessed (read).

       FAN_OPEN
              A file or a directory was opened.

       FAN_MODIFY
              A file was modified.

       FAN_CLOSE_WRITE
              A file that was opened for writing (O_WRONLY or O_RDWR) was closed.

       FAN_CLOSE_NOWRITE
              A file or directory that was opened read-only (O_RDONLY) was closed.

       FAN_Q_OVERFLOW
              The event queue exceeded the limit of 16384 entries.  This limit can be  overridden
              by specifying the FAN_UNLIMITED_QUEUE flag when calling fanotify_init(2).

       FAN_ACCESS_PERM
              An  application  wants  to  read  a file or directory, for example using read(2) or
              readdir(2).  The reader must write a response (as described below) that  determines
              whether the permission to access the filesystem object shall be granted.

       FAN_OPEN_PERM
              An application wants to open a file or directory.  The reader must write a response
              that determines whether the permission to  open  the  filesystem  object  shall  be
              granted.

       To check for any close event, the following bit mask may be used:

       FAN_CLOSE
              A file was closed.  This is a synonym for:

                  FAN_CLOSE_WRITE | FAN_CLOSE_NOWRITE

       The  following  macros  are  provided  to  iterate over a buffer containing fanotify event
       metadata returned by a read(2) from an fanotify file descriptor:

       FAN_EVENT_OK(meta, len)
              This macro checks the remaining length len of the buffer meta against the length of
              the  metadata  structure and the event_len field of the first metadata structure in
              the buffer.

       FAN_EVENT_NEXT(meta, len)
              This macro uses the length  indicated  in  the  event_len  field  of  the  metadata
              structure  pointed  to  by  meta  to  calculate  the  address  of the next metadata
              structure that follows meta.  len is the number of bytes of metadata that currently
              remain  in  the buffer.  The macro returns a pointer to the next metadata structure
              that follows meta, and reduces len by the number of bytes in the metadata structure
              that has been skipped over (i.e., it subtracts meta->event_len from len).

       In addition, there is:

       FAN_EVENT_METADATA_LEN
              This  macro  returns  the size (in bytes) of the structure fanotify_event_metadata.
              This is the minimum size (and currently the only size) of any event metadata.

   Monitoring an fanotify file descriptor for events
       When an fanotify event occurs, the fanotify file descriptor  indicates  as  readable  when
       passed to epoll(7), poll(2), or select(2).

   Dealing with permission events
       For  permission events, the application must write(2) a structure of the following form to
       the fanotify file descriptor:

           struct fanotify_response {
               __s32 fd;
               __u32 response;
           };

       The fields of this structure are as follows:

       fd     This is the file descriptor from the structure fanotify_event_metadata.

       response
              This field indicates whether or not the permission is to  be  granted.   Its  value
              must  be  either FAN_ALLOW to allow the file operation or FAN_DENY to deny the file
              operation.

       If access is denied, the requesting application call will receive an EPERM error.

   Closing the fanotify file descriptor
       When all file descriptors referring to the fanotify notification  group  are  closed,  the
       fanotify  group  is  released  and  its resources are freed for reuse by the kernel.  Upon
       close(2), outstanding permission events will be set to allowed.

   /proc/[pid]/fdinfo
       The file /proc/[pid]/fdinfo/[fd]  contains  information  about  fanotify  marks  for  file
       descriptor fd of process pid.  See proc(5) for details.

ERRORS

       In  addition  to the usual errors for read(2), the following errors can occur when reading
       from the fanotify file descriptor:

       EINVAL The buffer is too small to hold the event.

       EMFILE The per-process limit on the number of  open  files  has  been  reached.   See  the
              description of RLIMIT_NOFILE in getrlimit(2).

       ENFILE The  system-wide  limit  on  the  total number of open files has been reached.  See
              /proc/sys/fs/file-max in proc(5).

       ETXTBSY
              This error is returned by read(2) if  O_RDWR  or  O_WRONLY  was  specified  in  the
              event_f_flags  argument  when  calling fanotify_init(2) and an event occurred for a
              monitored file that is currently being executed.

       In addition to the usual errors for write(2), the following errors can occur when  writing
       to the fanotify file descriptor:

       EINVAL Fanotify  access  permissions  are  not  enabled in the kernel configuration or the
              value of response in the response structure is not valid.

       ENOENT The file descriptor fd in the response structure is not valid.  This may occur when
              a response for the permission event has already been written.

VERSIONS

       The  fanotify  API  was  introduced  in  version 2.6.36 of the Linux kernel and enabled in
       version 2.6.37.  Fdinfo support was added in version 3.8.

CONFORMING TO

       The fanotify API is Linux-specific.

NOTES

       The fanotify API is available only if  the  kernel  was  built  with  the  CONFIG_FANOTIFY
       configuration option enabled.  In addition, fanotify permission handling is available only
       if the CONFIG_FANOTIFY_ACCESS_PERMISSIONS configuration option is enabled.

   Limitations and caveats
       Fanotify reports only events that a user-space program  triggers  through  the  filesystem
       API.  As a result, it does not catch remote events that occur on network filesystems.

       The fanotify API does not report file accesses and modifications that may occur because of
       mmap(2), msync(2), and munmap(2).

       Events for directories are created only if the  directory  itself  is  opened,  read,  and
       closed.   Adding,  removing,  or  changing  children of a marked directory does not create
       events for the monitored directory itself.

       Fanotify monitoring of directories is not recursive: to  monitor  subdirectories  under  a
       directory,  additional marks must be created.  (But note that the fanotify API provides no
       way of detecting when a subdirectory has been created  under  a  marked  directory,  which
       makes recursive monitoring difficult.)  Monitoring mounts offers the capability to monitor
       a whole directory tree.

       The event queue can overflow.  In this case, events are lost.

BUGS

       Before Linux 3.19, fallocate(2) did not generate fanotify events.  Since Linux 3.19, calls
       to fallocate(2) generate FAN_MODIFY events.

       As of Linux 3.17, the following bugs exist:

       *  On  Linux, a filesystem object may be accessible through multiple paths, for example, a
          part of a filesystem may be remounted using the --bind option of mount(8).  A  listener
          that  marked  a  mount  will  be  notified  only  of  events  that were triggered for a
          filesystem object using the same mount.  Any other event will pass unnoticed.

       *  When an event is generated, no check is  made  to  see  whether  the  user  ID  of  the
          receiving  process  has  authorization  to read or write the file before passing a file
          descriptor for  that  file.   This  poses  a  security  risk,  when  the  CAP_SYS_ADMIN
          capability is set for programs executed by unprivileged users.

       *  If  a  call  to  read(2) processes multiple events from the fanotify queue and an error
          occurs, the return value will be the total length of the events successfully copied  to
          the  user-space buffer before the error occurred.  The return value will not be -1, and
          errno will not be set.  Thus, the reading application has no way to detect the error.

EXAMPLE

       The following program demonstrates the usage of the fanotify  API.   It  marks  the  mount
       point  passed  as  a  command-line argument and waits for events of type FAN_PERM_OPEN and
       FAN_CLOSE_WRITE.  When a permission event occurs, a FAN_ALLOW response is given.

       The following output was recorded while editing the  file  /home/user/temp/notes.   Before
       the  file  was  opened,  a  FAN_OPEN_PERM  event  occurred.   After the file was closed, a
       FAN_CLOSE_WRITE event occurred.  Execution of the program ends when the user  presses  the
       ENTER key.

   Example output
           # ./fanotify_example /home
           Press enter key to terminate.
           Listening for events.
           FAN_OPEN_PERM: File /home/user/temp/notes
           FAN_CLOSE_WRITE: File /home/user/temp/notes

           Listening for events stopped.

   Program source

       #define _GNU_SOURCE     /* Needed to get O_LARGEFILE definition */
       #include <errno.h>
       #include <fcntl.h>
       #include <limits.h>
       #include <poll.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/fanotify.h>
       #include <unistd.h>

       /* Read all available fanotify events from the file descriptor 'fd' */

       static void
       handle_events(int fd)
       {
           const struct fanotify_event_metadata *metadata;
           struct fanotify_event_metadata buf[200];
           ssize_t len;
           char path[PATH_MAX];
           ssize_t path_len;
           char procfd_path[PATH_MAX];
           struct fanotify_response response;

           /* Loop while events can be read from fanotify file descriptor */

           for(;;) {

               /* Read some events */

               len = read(fd, (void *) &buf, sizeof(buf));
               if (len == -1 && errno != EAGAIN) {
                   perror("read");
                   exit(EXIT_FAILURE);
               }

               /* Check if end of available data reached */

               if (len <= 0)
                   break;

               /* Point to the first event in the buffer */

               metadata = buf;

               /* Loop over all events in the buffer */

               while (FAN_EVENT_OK(metadata, len)) {

                   /* Check that run-time and compile-time structures match */

                   if (metadata->vers != FANOTIFY_METADATA_VERSION) {
                       fprintf(stderr,
                               "Mismatch of fanotify metadata version.\n");
                       exit(EXIT_FAILURE);
                   }

                   /* metadata->fd contains either FAN_NOFD, indicating a
                      queue overflow, or a file descriptor (a nonnegative
                      integer). Here, we simply ignore queue overflow. */

                   if (metadata->fd >= 0) {

                       /* Handle open permission event */

                       if (metadata->mask & FAN_OPEN_PERM) {
                           printf("FAN_OPEN_PERM: ");

                           /* Allow file to be opened */

                           response.fd = metadata->fd;
                           response.response = FAN_ALLOW;
                           write(fd, &response,
                                 sizeof(struct fanotify_response));
                       }

                       /* Handle closing of writable file event */

                       if (metadata->mask & FAN_CLOSE_WRITE)
                           printf("FAN_CLOSE_WRITE: ");

                       /* Retrieve and print pathname of the accessed file */

                       snprintf(procfd_path, sizeof(procfd_path),
                                "/proc/self/fd/%d", metadata->fd);
                       path_len = readlink(procfd_path, path,
                                           sizeof(path) - 1);
                       if (path_len == -1) {
                           perror("readlink");
                           exit(EXIT_FAILURE);
                       }

                       path[path_len] = '\0';
                       printf("File %s\n", path);

                       /* Close the file descriptor of the event */

                       close(metadata->fd);
                   }

                   /* Advance to next event */

                   metadata = FAN_EVENT_NEXT(metadata, len);
               }
           }
       }

       int
       main(int argc, char *argv[])
       {
           char buf;
           int fd, poll_num;
           nfds_t nfds;
           struct pollfd fds[2];

           /* Check mount point is supplied */

           if (argc != 2) {
               fprintf(stderr, "Usage: %s MOUNT\n", argv[0]);
               exit(EXIT_FAILURE);
           }

           printf("Press enter key to terminate.\n");

           /* Create the file descriptor for accessing the fanotify API */

           fd = fanotify_init(FAN_CLOEXEC | FAN_CLASS_CONTENT | FAN_NONBLOCK,
                              O_RDONLY | O_LARGEFILE);
           if (fd == -1) {
               perror("fanotify_init");
               exit(EXIT_FAILURE);
           }

           /* Mark the mount for:
              - permission events before opening files
              - notification events after closing a write-enabled
                file descriptor */

           if (fanotify_mark(fd, FAN_MARK_ADD | FAN_MARK_MOUNT,
                             FAN_OPEN_PERM | FAN_CLOSE_WRITE, AT_FDCWD,
                             argv[1]) == -1) {
               perror("fanotify_mark");
               exit(EXIT_FAILURE);
           }

           /* Prepare for polling */

           nfds = 2;

           /* Console input */

           fds[0].fd = STDIN_FILENO;
           fds[0].events = POLLIN;

           /* Fanotify input */

           fds[1].fd = fd;
           fds[1].events = POLLIN;

           /* This is the loop to wait for incoming events */

           printf("Listening for events.\n");

           while (1) {
               poll_num = poll(fds, nfds, -1);
               if (poll_num == -1) {
                   if (errno == EINTR)     /* Interrupted by a signal */
                       continue;           /* Restart poll() */

                   perror("poll");         /* Unexpected error */
                   exit(EXIT_FAILURE);
               }

               if (poll_num > 0) {
                   if (fds[0].revents & POLLIN) {

                       /* Console input is available: empty stdin and quit */

                       while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\n')
                           continue;
                       break;
                   }

                   if (fds[1].revents & POLLIN) {

                       /* Fanotify events are available */

                       handle_events(fd);
                   }
               }
           }

           printf("Listening for events stopped.\n");
           exit(EXIT_SUCCESS);
       }

SEE ALSO

       fanotify_init(2), fanotify_mark(2), inotify(7)

COLOPHON

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