fanotify.7: BUGS: error events can be lost when reading from fanotify FD

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.TH FANOTIFY 7 2014-04-24 "Linux" "Linux Programmer's Manual"
.SH NAME
fanotify \- monitoring filesystem events
.SH DESCRIPTION
The fanotify API provides notification and interception of
filesystem events.
Use cases include virus scanning and hierarchical storage management.
Currently, only a limited set of events is supported.
In particular, there is no support for create, delete, and move events.
(See
.BR inotify (7)
for details of an API that does notify those events.)

Additional capabilities compared to the
.BR 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:
.BR fanotify_init (2),
.BR fanotify_mark (2),
.BR read (2),
.BR write (2),
and
.BR close (2).
.SS fanotify_init(), fanotify_mark(), and notification groups
The
.BR fanotify_init (2)
system call creates and initializes an fanotify notification group
and returns a file descriptor referring to it.
.PP
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.
.PP
For each entry in an fanotify notification group, two bit masks exist: the
.I mark
mask and the
.I 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 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.
.PP
The
.BR 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.
.PP
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 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.
.PP
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,
the respective entries survive.
If files or directories are deleted or mounts are unmounted,
the corresponding entries are deleted.
.SS 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
.BR read (2)
or similar)
from the fanotify file descriptor
returned by
.BR fanotify_init (2).

Two types of events are generated:
.I notification
events and
.I 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 fanotify file descriptor
is closed.
.SS Reading fanotify events
Calling
.BR read (2)
for the file descriptor returned by
.BR fanotify_init (2)
blocks (if the flag
.B FAN_NONBLOCK
is not specified in the call to
.BR fanotify_init (2))
until either a file event occurs or the call is interrupted by a signal
(see
.BR signal (7)).

After a successful
.BR read (2),
the read buffer contains one or more of the following structures:

.in +4n
.nf
struct fanotify_event_metadata {
    __u32 event_len;
    __u8 vers;
    __u8 reserved;
    __u16 metadata_len;
    __aligned_u64 mask;
    __s32 fd;
    __s32 pid;
};
.fi
.in
.PP
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
.BR read (2).

The return value of
.BR 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
.I fanotify_event_metadata
structure are as follows:
.TP
.I 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 implementation, the value of
.I event_len
is always
.BR FAN_EVENT_METADATA_LEN .
However, the API is designed to allow
variable-length structures to be returned in the future.
.TP
.I vers
This field holds a version number for the structure.
It must be compared to
.B FANOTIFY_METADATA_VERSION
to verify that the structures returned at runtime match
the structures defined at compile time.
In case of a mismatch, the application should abandon trying to use the
fanotify file descriptor.
.TP
.I reserved
This field is not used.
.TP
.I 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.
.TP
.I mask
This is a bit mask describing the event (see below).
.TP
.I fd
This is an open file descriptor for the object being accessed, or
.B 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.
.IP
When calling
.BR fanotify_init (2),
the caller may specify (via the
.I 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)
.B 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.
.TP
.I 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
.BR getpid (2),
to determine whether the event is caused by the listener itself,
or is due to a file access by another process.
.PP
The bit mask in
.I 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.
.PP
The bits that may appear in
.I mask
are as follows:
.TP
.B FAN_ACCESS
A file or a directory (but see BUGS) was accessed (read).
.TP
.B FAN_OPEN
A file or a directory was opened.
.TP
.B FAN_MODIFY
A file was modified.
.TP
.B FAN_CLOSE_WRITE
A file that was opened for writing
.RB ( O_WRONLY
or
.BR O_RDWR )
was closed.
.TP
.B FAN_CLOSE_NOWRITE
A file or directory that was opened read-only
.RB ( O_RDONLY )
was closed.
.TP
.B FAN_Q_OVERFLOW
The event queue exceeded the limit of 16384 entries.
This limit can be overridden by specifying the
.BR FAN_UNLIMITED_QUEUE
flag when calling
.BR fanotify_init (2).
.TP
.B FAN_ACCESS_PERM
An application wants to read a file or directory, for example using
.BR read (2)
or
.BR readdir (2).
The reader must write a response (as described below)
that determines whether the permission to
access the filesystem object shall be granted.
.TP
.B 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.
.PP
To check for any close event, the following bit mask may be used:
.TP
.B FAN_CLOSE
A file was closed.
This is a synonym for:

    FAN_CLOSE_WRITE | FAN_CLOSE_NOWRITE
.PP
The following macros are provided to iterate over a buffer containing
fanotify event metadata returned by a
.BR read (2)
from an fanotify file descriptor:
.TP
.B FAN_EVENT_OK(meta, len)
This macro checks the remaining length
.I len
of the buffer
.I meta
against the length of the metadata structure and the
.I event_len
field of the first metadata structure in the buffer.
.TP
.B FAN_EVENT_NEXT(meta, len)
This macro sets the pointer
.I meta
to the next metadata structure using the length indicated in the
.I event_len
field of the metadata structure and reduces the remaining length of the
buffer
.IR len .
.SS Monitoring an fanotify file descriptor for events
When an fanotify event occurs, the fanotify file descriptor indicates as
readable when passed to
.BR epoll (7),
.BR poll (2),
or
.BR select (2).
.SS Dealing with permission events
For permission events, the application must
.BR write (2)
a structure of the following form to the
fanotify file descriptor:

.in +4n
.nf
struct fanotify_response {
    __s32 fd;
    __u32 response;
};
.fi
.in
.PP
The fields of this structure are as follows:
.TP
.I fd
This is the file descriptor from the structure
.IR fanotify_event_metadata .
.TP
.I response
This field indicates whether or not the permission is to be granted.
Its value must be either
.B FAN_ALLOW
to allow the file operation or
.B FAN_DENY
to deny the file operation.
.PP
If access is denied, the requesting application call will receive an
.BR EPERM
error.
.SS Closing the fanotify file descriptor
.PP
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
.BR close (2),
outstanding permission events will be set to allowed.
.SS /proc/[pid]/fdinfo
The file
.I /proc/[pid]/fdinfo/[fd]
contains information about fanotify marks for file descriptor
.I fd
of process
.IR pid .
See the kernel source file
.I Documentation/filesystems/proc.txt
for details.
.SH ERRORS
In addition to the usual errors for
.BR read (2),
the following errors can occur when reading from the
fanotify file descriptor:
.TP
.B EINVAL
The buffer is too small to hold the event.
.TP
.B EMFILE
The per-process limit on the number of open files has been reached.
See the description of
.B RLIMIT_NOFILE
in
.BR getrlimit (2).
.TP
.B ENFILE
The system-wide limit on the number of open files has been reached.
See
.I /proc/sys/fs/file-max
in
.BR proc (5).
.TP
.B ETXTBSY
This error is returned by
.BR read (2)
if
.B O_RDWR
or
.B O_WRONLY
was specified in the
.I event_f_flags
argument when calling
.BR fanotify_init (2)
and an event occurred for a monitored file that is currently being executed.
.PP
In addition to the usual errors for
.BR write (2),
the following errors can occur when writing to the fanotify file descriptor:
.TP
.B EINVAL
Fanotify access permissions are not enabled in the kernel configuration
or the value of
.I response
in the response structure is not valid.
.TP
.B ENOENT
The file descriptor
.I fd
in the response structure is not valid.
This may occur when a response for the permission event has already been
written.
.SH 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.
.SH "CONFORMING TO"
The fanotify API is Linux-specific.
.SH NOTES
The fanotify API is available only if the kernel was built with the
.B CONFIG_FANOTIFY
configuration option enabled.
In addition, fanotify permission handling is available only if the
.B CONFIG_FANOTIFY_ACCESS_PERMISSIONS
configuration option is enabled.
.SS 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.
.PP
The fanotify API does not report file accesses and modifications that
may occur because of
.BR mmap (2),
.BR msync (2),
and
.BR munmap (2).
.PP
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.
.PP
Fanotify monitoring of directories is not recursive:
to monitor subdirectories under a directory,
additional marks must be created.
(But note that the fanotify API provides no way of detecting when a
subdirectory has been created under a marked directory,
which makes recursive monitoring difficult.)
Monitoring mounts offers the capability to monitor a whole directory tree.
.PP
The event queue can overflow.
In this case, events are lost.
.SH BUGS
As of Linux 3.15,
the following bugs exists:
.IP * 3
.\" FIXME: A patch was proposed.
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
.B CAP_SYS_ADMIN
capability is set for programs executed by unprivileged users.
.IP *
If a call to
.BR read( 2 )
processes multiple events from the fanotify queue and an error occurs, the
return value will be the cumulated length of the events successfully copied to
the user space buffer before the error occured.
The return value will not be \-1.
.I errno
will not be set.
Thus, the user program has no way to detect the error.
.SH 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
.B FAN_PERM_OPEN
and
.BR FAN_CLOSE_WRITE .
When a permission event occurs, a
.B FAN_ALLOW
response is given.
.PP
The following output was recorded while editing the file
.IR /home/user/temp/notes .
Before the file was opened, a
.B FAN_OPEN_PERM
event occurred.
After the file was closed, a
.B FAN_CLOSE_WRITE
event occurred.
Execution of the program ends when the user presses the ENTER key.
.SS Example output
.in +4n
.nf
# ./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.
.fi
.in
.SS Program source
.nf
#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;
    char buf[4096];
    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 = (struct fanotify_event_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, \-1,
                      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);
}
.fi
.SH "SEE ALSO"
.ad l
.BR fanotify_init (2),
.BR fanotify_mark (2),
.BR inotify (7)