Ubuntu Manpage: fanotify_init - create and initialize fanotify group

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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_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)).

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 record of type FAN_EVENT_INFO_TYPE_FID
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 monitoring a directory or a filesystem that are interested in the directory
entry modification events FAN_CREATE, FAN_DELETE, and FAN_MOVE, or in events such as FAN_ATTRIB,
FAN_DELETE_SELF, and FAN_MOVE_SELF. All the events above require an fanotify group that
identifies filesystem objects by file handles. Note that for the directory entry modification
events the reported file handle identifies the modified directory and not the
created/deleted/moved child object. 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
details.

FAN_REPORT_DIR_FID (since Linux 5.9)
Events for fanotify groups initialized with this flag will contain (see exceptions below)
additional information about a directory object correlated to an event. An additional record of
type FAN_EVENT_INFO_TYPE_DFID encapsulates the information about the directory object and is
included alongside the generic event metadata structure. For events that occur on a non-directory
object, the additional structure includes a file handle that identifies the parent directory
filesystem object. Note that there is no guarantee that the directory filesystem object will be
found at the location described by the file handle information at the time the event is received.
When combined with the flag FAN_REPORT_FID, two records may be reported with events that occur on
a non-directory object, one to identify the non-directory object itself and one to identify the
parent directory object. Note that in some cases, a filesystem object does not have a parent, for
example, when an event occurs on an unlinked but open file. In that case, with the FAN_REPORT_FID
flag, the event will be reported with only one record to identify the non-directory object itself,
because there is no directory associated with the event. Without the FAN_REPORT_FID flag, no
event will be reported. See fanotify(7) for additional details.

FAN_REPORT_NAME (since Linux 5.9)
Events for fanotify groups initialized with this flag will contain additional information about
the name of the directory entry correlated to an event. This flag must be provided in conjunction
with the flag FAN_REPORT_DIR_FID. Providing this flag value without FAN_REPORT_DIR_FID will
result in the error EINVAL. This flag may be combined with the flag FAN_REPORT_FID. An
additional record of type FAN_EVENT_INFO_TYPE_DFID_NAME, which encapsulates the information about
the directory entry, is included alongside the generic event metadata structure and substitutes
the additional information record of type FAN_EVENT_INFO_TYPE_DFID. The additional record
includes a file handle that identifies a directory filesystem object followed by a name that
identifies an entry in that directory. For the directory entry modification events FAN_CREATE,
FAN_DELETE, and FAN_MOVE, the reported name is that of the created/deleted/moved directory entry.
For other events that occur on a directory object, the reported file handle is that of the
directory object itself and the reported name is '.'. For other events that occur on a non-
directory object, the reported file handle is that of the parent directory object and the reported
name is the name of a directory entry where the object was located at the time of the event. The
rationale behind this logic is that the reported directory file handle can be passed to
open_by_handle_at(2) to get an open directory file descriptor and that file descriptor along with
the reported name can be used to call fstatat(2). The same rule that applies to record type
FAN_EVENT_INFO_TYPE_DFID also applies to record type FAN_EVENT_INFO_TYPE_DFID_NAME: if a non-
directory object has no parent, either the event will not be reported or it will be reported
without the directory entry information. Note that there is no guarantee that the filesystem
object will be found at the location described by the directory entry information at the time the
event is received. See fanotify(7) for additional details.

FAN_REPORT_DFID_NAME
This is a synonym for (FAN_REPORT_DIR_FID|FAN_REPORT_NAME).

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

       This  page  is  part  of  release  5.10  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                                              2020-11-01                                   FANOTIFY_INIT(2)