fanotify

FANOTIFY(7)                Linux Programmer's Manual               FANOTIFY(7)

NAME
       fanotify - monitoring filesystem events

DESCRIPTION
       The  fanotify  API provides notification and interception of filesystem
       events.  Use cases include virus scanning and hierarchical storage man-
       agement.   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 activi-
       ties 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  invali-
       dated.   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 fan-
       otify 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 fan-
       otify_init(2)) until either a file event occurs or the call  is  inter-
       rupted 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  buf-
       fer, 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  imple-
              mentation,  the  value  of  event_len  is always FAN_EVENT_META-
              DATA_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  struc-
              tures  returned  at runtime match the structures defined at com-
              pile time.  In case of a mismatch, the application should  aban-
              don 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 implementa-
              tion.

       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 cre-
              ated.

       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 struc-
              ture  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 fan-
              otify_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  fan-
              otify_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  fan-
              otify_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  con-
              figuration 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_FAN-
       OTIFY_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 subdi-
       rectories under a directory, additional marks must  be  created.   (But
       note  that  the fanotify API provides no way of detecting when a subdi-
       rectory has been created under a marked directory, which  makes  recur-
       sive 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
       This page is part of release 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                             2017-09-15                       FANOTIFY(7)
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