man-pages/man2/fanotify_init.2

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.TH FANOTIFY_INIT 2 2019-03-06 "Linux" "Linux Programmer's Manual"
.SH NAME
fanotify_init \- create and initialize fanotify group
.SH SYNOPSIS
.B #include <fcntl.h>
.br
.B #include <sys/fanotify.h>
.PP
.BI "int fanotify_init(unsigned int " flags ", unsigned int " event_f_flags );
.SH DESCRIPTION
For an overview of the fanotify API, see
.BR fanotify (7).
.PP
.BR fanotify_init ()
initializes a new fanotify group and returns a file descriptor for the event
queue associated with the group.
.PP
The file descriptor is used in calls to
.BR fanotify_mark (2)
to specify the files, directories, and mounts 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.
.PP
Multiple programs may be using the fanotify interface at the same time to
monitor the same files.
.PP
In the current implementation, the number of fanotify groups per user is
limited to 128.
This limit cannot be overridden.
.PP
Calling
.BR fanotify_init ()
requires the
.B 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.
.PP
The
.I 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.
.PP
If multiple listeners for permission events exist,
the notification class is used to establish the sequence
in which the listeners receive the events.
.PP
Only one of the following notification classes may be specified in
.IR flags :
.TP
.B 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.
.TP
.B 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.
.TP
.B 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.
.PP
Listeners with different notification classes will receive events in the
order
.BR FAN_CLASS_PRE_CONTENT ,
.BR FAN_CLASS_CONTENT ,
.BR FAN_CLASS_NOTIF .
The order of notification for listeners in the same notification class
is undefined.
.PP
The following bits can additionally be set in
.IR flags :
.TP
.B FAN_CLOEXEC
Set the close-on-exec flag
.RB ( FD_CLOEXEC )
on the new file descriptor.
See the description of the
.B O_CLOEXEC
flag in
.BR open (2).
.TP
.B FAN_NONBLOCK
Enable the nonblocking flag
.RB ( O_NONBLOCK )
for the file descriptor.
Reading from the file descriptor will not block.
Instead, if no data is available,
.BR read (2)
fails with the error
.BR EAGAIN .
.TP
.B FAN_UNLIMITED_QUEUE
Remove the limit of 16384 events for the event queue.
Use of this flag requires the
.B CAP_SYS_ADMIN
capability.
.TP
.B FAN_UNLIMITED_MARKS
Remove the limit of 8192 marks.
Use of this flag requires the
.B CAP_SYS_ADMIN
capability.
.TP
.BR FAN_REPORT_TID " (since Linux 4.20)"
.\" commit d0a6a87e40da49cfc7954c491d3065a25a641b29
Report thread ID (TID) instead of process ID (PID)
in the
.I pid
field of the
.I "struct fanotify_event_metadata"
supplied to
.BR read (2)
(see
.BR fanotify (7)).
.PP
The
.I 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
.I flags
values in
.BR open (2).
.I event_f_flags
includes a multi-bit field for the access mode.
This field can take the following values:
.TP
.B O_RDONLY
This value allows only read access.
.TP
.B O_WRONLY
This value allows only write access.
.TP
.B O_RDWR
This value allows read and write access.
.PP
Additional bits can be set in
.IR event_f_flags .
The most useful values are:
.TP
.B O_LARGEFILE
Enable support for files exceeding 2\ GB.
Failing to set this flag will result in an
.B EOVERFLOW
error when trying to open a large file which is monitored by
an fanotify group on a 32-bit system.
.TP
.BR O_CLOEXEC " (since Linux 3.18)"
.\" commit 0b37e097a648aa71d4db1ad108001e95b69a2da4
Enable the close-on-exec flag for the file descriptor.
See the description of the
.B O_CLOEXEC
flag in
.BR open (2)
for reasons why this may be useful.
.PP
The following are also allowable:
.BR O_APPEND ,
.BR O_DSYNC ,
.BR O_NOATIME ,
.BR O_NONBLOCK ,
and
.BR O_SYNC .
Specifying any other flag in
.I event_f_flags
yields the error
.B EINVAL
(but see BUGS).
.SH RETURN VALUE
On success,
.BR fanotify_init ()
returns a new file descriptor.
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EINVAL
An invalid value was passed in
.I flags
or
.IR event_f_flags .
.B FAN_ALL_INIT_FLAGS
defines all allowable bits for
.IR flags .
.TP
.B EMFILE
The number of fanotify groups for this user exceeds 128.
.TP
.B EMFILE
The per-process limit on the number of open file descriptors has been reached.
.TP
.B ENOMEM
The allocation of memory for the notification group failed.
.TP
.B ENOSYS
This kernel does not implement
.BR fanotify_init ().
The fanotify API is available only if the kernel was configured with
.BR CONFIG_FANOTIFY .
.TP
.B EPERM
The operation is not permitted because the caller lacks the
.B CAP_SYS_ADMIN
capability.
.SH VERSIONS
.BR fanotify_init ()
was introduced in version 2.6.36 of the Linux kernel and enabled in version
2.6.37.
.SH CONFORMING TO
This system call is Linux-specific.
.SH BUGS
The following bug was present in Linux kernels before version 3.18:
.IP * 3
.\" Fixed by commit 0b37e097a648aa71d4db1ad108001e95b69a2da4
The
.B O_CLOEXEC
is ignored when passed in
.IR event_f_flags .
.PP
The following bug was present in Linux kernels before version 3.14:
.IP * 3
.\" Fixed by commit 48149e9d3a7e924010a0daab30a6197b7d7b6580
The
.I event_f_flags
argument is not checked for invalid flags.
Flags that are intended only for internal use,
such as
.BR FMODE_EXEC ,
can be set, and will consequently be set for the file descriptors
returned when reading from the fanotify file descriptor.
.SH SEE ALSO
.BR fanotify_mark (2),
.BR fanotify (7)