Ubuntu Manpage: fanotify_init - create and initialize fanotify group

NAME

       fanotify_init - create and initialize fanotify group

SYNOPSIS

       #include <fcntl.h>
       #include <sys/fanotify.h>

       int fanotify_init(unsigned int flags, unsigned int event_f_flags);

DESCRIPTION

For an overview of the fanotify API, see fanotify(7).

fanotify_init() initializes a new fanotify group and returns a file descriptor for the event queue
associated with the group.

The file descriptor is used in calls to fanotify_mark(2) to specify the files, directories, mounts or
filesystems for which fanotify events shall be created. These events are received by reading from the
file descriptor. Some events are only informative, indicating that a file has been accessed. Other
events can be used to determine whether another application is permitted to access a file or directory.
Permission to access filesystem objects is granted by writing to the file descriptor.

Multiple programs may be using the fanotify interface at the same time to monitor the same files.

In the current implementation, the number of fanotify groups per user is limited to 128. This limit
cannot be overridden.

Calling fanotify_init() requires the CAP_SYS_ADMIN capability. This constraint might be relaxed in
future versions of the API. Therefore, certain additional capability checks have been implemented as
indicated below.

The flags argument contains a multi-bit field defining the notification class of the listening
application and further single bit fields specifying the behavior of the file descriptor.

If multiple listeners for permission events exist, the notification class is used to establish the
sequence in which the listeners receive the events.

Only one of the following notification classes may be specified in flags:

FAN_CLASS_PRE_CONTENT
This value allows the receipt of events notifying that a file has been accessed and events for
permission decisions if a file may be accessed. It is intended for event listeners that need to
access files before they contain their final data. This notification class might be used by
hierarchical storage managers, for example.

FAN_CLASS_CONTENT
This value allows the receipt of events notifying that a file has been accessed and events for
permission decisions if a file may be accessed. It is intended for event listeners that need to
access files when they already contain their final content. This notification class might be used
by malware detection programs, for example.

FAN_REPORT_FID (since Linux 5.1)
This value allows the receipt of events which contain additional information about the underlying
filesystem object correlated to an event. An additional structure encapsulates the information
about the object and is included alongside the generic event metadata structure. The file
descriptor that is used to represent the object correlated to an event is instead substituted with
a file handle. It is intended for applications that may find the use of a file handle to identify
an object more suitable than a file descriptor. Additionally, it may be used for applications
that are interested in directory entry events, such as FAN_CREATE, FAN_ATTRIB, FAN_MOVE, and
FAN_DELETE for example. Note that the use of directory modification events are not supported when
monitoring a mount point. The use of FAN_CLASS_CONTENT or FAN_CLASS_PRE_CONTENT is not permitted
with this flag and will result in the error EINVAL. See fanotify(7) for additional information.

FAN_CLASS_NOTIF
This is the default value. It does not need to be specified. This value only allows the receipt
of events notifying that a file has been accessed. Permission decisions before the file is
accessed are not possible.

Listeners with different notification classes will receive events in the order FAN_CLASS_PRE_CONTENT,
FAN_CLASS_CONTENT, FAN_CLASS_NOTIF. The order of notification for listeners in the same notification
class is undefined.

The following bits can additionally be set in flags:

FAN_CLOEXEC
Set the close-on-exec flag (FD_CLOEXEC) on the new file descriptor. See the description of the
O_CLOEXEC flag in open(2).

FAN_NONBLOCK
Enable the nonblocking flag (O_NONBLOCK) for the file descriptor. Reading from the file
descriptor will not block. Instead, if no data is available, read(2) fails with the error EAGAIN.

FAN_UNLIMITED_QUEUE
Remove the limit of 16384 events for the event queue. Use of this flag requires the CAP_SYS_ADMIN
capability.

FAN_UNLIMITED_MARKS
Remove the limit of 8192 marks. Use of this flag requires the CAP_SYS_ADMIN capability.

FAN_REPORT_TID (since Linux 4.20)
Report thread ID (TID) instead of process ID (PID) in the pid field of the struct
fanotify_event_metadata supplied to read(2) (see fanotify(7)).

The event_f_flags argument defines the file status flags that will be set on the open file descriptions
that are created for fanotify events. For details of these flags, see the description of the flags
values in open(2). event_f_flags includes a multi-bit field for the access mode. This field can take
the following values:

O_RDONLY
This value allows only read access.

O_WRONLY
This value allows only write access.

O_RDWR This value allows read and write access.

Additional bits can be set in event_f_flags. The most useful values are:

O_LARGEFILE
Enable support for files exceeding 2 GB. Failing to set this flag will result in an EOVERFLOW
error when trying to open a large file which is monitored by an fanotify group on a 32-bit system.

O_CLOEXEC (since Linux 3.18)
Enable the close-on-exec flag for the file descriptor. See the description of the O_CLOEXEC flag
in open(2) for reasons why this may be useful.

The following are also allowable: O_APPEND, O_DSYNC, O_NOATIME, O_NONBLOCK, and O_SYNC. Specifying any
other flag in event_f_flags yields the error EINVAL (but see BUGS).

RETURN VALUE

       On success, fanotify_init() returns a new file descriptor.  On error, -1 is returned, and errno is set to
       indicate the error.

ERRORS

       EINVAL An invalid value was passed in flags or event_f_flags.  FAN_ALL_INIT_FLAGS (deprecated since Linux
              kernel version 4.20) defines all allowable bits for flags.

       EMFILE The number of fanotify groups for this user exceeds 128.

       EMFILE The per-process limit on the number of open file descriptors has been reached.

       ENOMEM The allocation of memory for the notification group failed.

       ENOSYS This kernel does not implement fanotify_init().  The fanotify API is available only if the  kernel
              was configured with CONFIG_FANOTIFY.

       EPERM  The operation is not permitted because the caller lacks the CAP_SYS_ADMIN capability.

VERSIONS

       fanotify_init() was introduced in version 2.6.36 of the Linux kernel and enabled in version 2.6.37.

CONFORMING TO

       This system call is Linux-specific.

BUGS

       The following bug was present in Linux kernels before version 3.18:

       *  The O_CLOEXEC is ignored when passed in event_f_flags.

       The following bug was present in Linux kernels before version 3.14:

       *  The  event_f_flags  argument  is  not  checked  for  invalid  flags.  Flags that are intended only for
          internal use, such as FMODE_EXEC, can be set, and will consequently be set for  the  file  descriptors
          returned when reading from the fanotify file descriptor.

COLOPHON

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