The fanotify API provides notification and interception of
filesystem events. Use cases include virus scanning and
hierarchical storage management. In the original fanotify API,
only a limited set of events was supported. In particular, there
was no support for create, delete, and move events. The support
for those events was added in Linux 5.1. (See inotify(7) for
details of an API that did notify those events pre Linux 5.1.)
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, filesystems, and mounts 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 filesystem, mount, or
directory to be marked for receiving events, while at the same
time ignoring events for specific objects under a mount or
directory.
The fanotify_mark(2) system call adds a file, directory,
filesystem, 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 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
filesystems or 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
with one exception: if a valid file descriptor is provided within
a generic event, the file descriptor must be closed. 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)).
The use of one of the flags FAN_REPORT_FID, FAN_REPORT_DIR_FID in
fanotify_init(2) influences what data structures are returned to
the event listener for each event. Events reported to a group
initialized with one of these flags will use file handles to
identify filesystem objects instead of file descriptors.
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;
};
In case of an fanotify group that identifies filesystem objects
by file handles, you should also expect to receive one or more
additional information records of the structure detailed below
following the generic fanotify_event_metadata structure within
the read buffer:
struct fanotify_event_info_header {
__u8 info_type;
__u8 pad;
__u16 len;
};
struct fanotify_event_info_fid {
struct fanotify_event_info_header hdr;
__kernel_fsid_t fsid;
unsigned char file_handle[0];
};
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. Unless the
group identifies filesystem objects by file handles, the
value of event_len is always FAN_EVENT_METADATA_LEN. For
a group that identifies filesystem objects by file
handles, event_len also includes the variable length file
identifier records.
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 run time 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. With
an fanotify group that identifies filesystem objects by
file handles, applications should expect this value to be
set to FAN_NOFD for each event that is received. 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 If flag FAN_REPORT_TID was set in fanotify_init(2), this
is the TID of the thread that caused the event.
Otherwise, this the PID 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_OPEN_EXEC
A file was opened with the intent to be executed. See
NOTES in fanotify_mark(2) for additional details.
FAN_ATTRIB
A file or directory metadata was changed.
FAN_CREATE
A child file or directory was created in a watched parent.
FAN_DELETE
A child file or directory was deleted in a watched parent.
FAN_DELETE_SELF
A watched file or directory was deleted.
FAN_MOVED_FROM
A file or directory has been moved from a watched parent
directory.
FAN_MOVED_TO
A file or directory has been moved to a watched parent
directory.
FAN_MOVE_SELF
A watched file or directory was moved.
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.
FAN_OPEN_EXEC_PERM
An application wants to open a file for execution. The
reader must write a response that determines whether the
permission to open the filesystem object for execution
shall be granted. See NOTES in fanotify_mark(2) for
additional details.
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
To check for any move event, the following bit mask may be used:
FAN_MOVE
A file or directory was moved. This is a synonym for:
FAN_MOVED_FROM | FAN_MOVED_TO
The following bits may appear in mask only in conjunction with
other event type bits:
FAN_ONDIR
The events described in the mask have occurred on a
directory object. Reporting events on directories
requires setting this flag in the mark mask. See
fanotify_mark(2) for additional details. The FAN_ONDIR
flag is reported in an event mask only if the fanotify
group identifies filesystem objects by file handles.
The fields of the fanotify_event_info_fid structure are as
follows:
hdr This is a structure of type fanotify_event_info_header.
It is a generic header that contains information used to
describe an additional information record attached to the
event. For example, when an fanotify file descriptor is
created using FAN_REPORT_FID, a single information record
is expected to be attached to the event with info_type
field value of FAN_EVENT_INFO_TYPE_FID. When an fanotify
file descriptor is created using the combination of
FAN_REPORT_FID and FAN_REPORT_DIR_FID, there may be two
information records attached to the event: one with
info_type field value of FAN_EVENT_INFO_TYPE_DFID,
identifying a parent directory object, and one with
info_type field value of FAN_EVENT_INFO_TYPE_FID,
identifying a non-directory object. The
fanotify_event_info_header contains a len field. The
value of len is the size of the additional information
record including the fanotify_event_info_header itself.
The total size of all additional information records is
not expected to be bigger than ( event_len - metadata_len
).
fsid This is a unique identifier of the filesystem containing
the object associated with the event. It is a structure
of type __kernel_fsid_t and contains the same value as
f_fsid when calling statfs(2).
file_handle
This is a variable length structure of type struct
file_handle. It is an opaque handle that corresponds to a
specified object on a filesystem as returned by
name_to_handle_at(2). It can be used to uniquely identify
a file on a filesystem and can be passed as an argument to
open_by_handle_at(2). Note that for the directory entry
modification events FAN_CREATE, FAN_DELETE, and FAN_MOVE,
the file_handle identifies the modified directory and not
the created/deleted/moved child object. If the value of
info_type field is FAN_EVENT_INFO_TYPE_DFID_NAME, the file
handle is followed by a null terminated string that
identifies the created/deleted/moved directory entry name.
For other events such as FAN_OPEN, FAN_ATTRIB,
FAN_DELETE_SELF, and FAN_MOVE_SELF, if the value of
info_type field is FAN_EVENT_INFO_TYPE_FID, the
file_handle identifies the object correlated to the event.
If the value of info_type field is
FAN_EVENT_INFO_TYPE_DFID, the file_handle identifies the
directory object correlated to the event or the parent
directory of a non-directory object correlated to the
event. If the value of info_type field is
FAN_EVENT_INFO_TYPE_DFID_NAME, the file_handle identifies
the same directory object that would be reported with
FAN_EVENT_INFO_TYPE_DFID and the file handle is followed
by a null terminated string that identifies the name of a
directory entry in that directory, or '.' to identify the
directory object itself.
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. Additionally, if the notification group has been
created with the FAN_ENABLE_AUDIT flag, then the FAN_AUDIT flag
can be set in the response field. In that case, the audit
subsystem will log information about the access decision to the
audit logs.
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.
