Re: fanotify man pages for review

From: Michael Kerrisk (man-pages)
Date: Wed May 07 2014 - 15:16:15 EST

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In reply to: Michael Kerrisk (man-pages): "Re: fanotify man pages for review"
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Hello Eric,

Ping for this request. Could you please take a look at these pages?

Thanks,

Michael

On Tue, Apr 29, 2014 at 3:51 PM, Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> wrote:
> Hello Eric (and all),
>
> Heinrich Schuchardt has made a magnificent effort writing some man
> pages that extensively document the fanotify API that you added in
> Linux 2.6.36/37. Could I ask you (and anyone else who is interested)
> to review them please for completeness and accuracy. I would
> really like to get such a review before publishing the pages, in
> order to minimize the chance of publishing errors
>
> The pages are:
>
> fanotify.7:
> An overview of the fanotify API complete with an
> example program, n
>
> fanotify_init.2
> Description of the fanotify_init() system call
>
> fanotify_mark.2
> Description of the fanotify_mark() system call
>
> Cheers,
>
> Michael
>
>
>
> diff --git a/man2/fanotify_init.2 b/man2/fanotify_init.2
> new file mode 100644
> index 0000000..e54fe7e
> --- /dev/null
> +++ b/man2/fanotify_init.2
> @@ -0,0 +1,206 @@
> +.\" Copyright (C) 2013, Heinrich Schuchardt <xypron.glpk@xxxxxx>
> +.\"
> +.\" %%%LICENSE_START(VERBATIM)
> +.\" Permission is granted to make and distribute verbatim copies of this
> +.\" manual provided the copyright notice and this permission notice are
> +.\" preserved on all copies.
> +.\"
> +.\" Permission is granted to copy and distribute modified versions of
> +.\" this manual under the conditions for verbatim copying, provided that
> +.\" the entire resulting derived work is distributed under the terms of
> +.\" a permission notice identical to this one.
> +.\"
> +.\" Since the Linux kernel and libraries are constantly changing, this
> +.\" manual page may be incorrect or out-of-date. The author(s) assume.
> +.\" no responsibility for errors or omissions, or for damages resulting.
> +.\" from the use of the information contained herein. The author(s) may.
> +.\" not have taken the same level of care in the production of this.
> +.\" manual, which is licensed free of charge, as they might when working.
> +.\" professionally.
> +.\"
> +.\" Formatted or processed versions of this manual, if unaccompanied by
> +.\" the source, must acknowledge the copyright and authors of this work.
> +.\" %%%LICENSE_END
> +.TH FANOTIFY_INIT 2 2014-04-24 "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>
> +.sp
> +.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 of the same value is undefined.
> +.PP
> +The following bit mask values can be set additionally in
> +.IR flags :
> +.TP
> +.B FAN_CLOEXEC
> +This flag sets 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
> +This flag enables 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)
> +will fail with the error
> +.BR EAGAIN .
> +.TP
> +.B FAN_UNLIMITED_QUEUE
> +This flag removes the limit of 16384 events for the event queue.
> +It requires the
> +.B CAP_SYS_ADMIN
> +capability.
> +.TP
> +.B FAN_UNLIMITED_MARKS
> +This flag removes the limit of 8192 marks.
> +It requires the
> +.B CAP_SYS_ADMIN
> +capability.
> +.PP
> +The argument
> +.I event_f_flags
> +defines the file flags with which file descriptors for fanotify events shall
> +be created.
> +For explanations of possible values, see the argument
> +.I flags
> +of the
> +.BR open (2)
> +system call.
> +Useful values are:
> +.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.
> +.TP
> +.B O_CLOEXEC
> +This flag enables the close-on-exec flag for the file descriptor.
> +.TP
> +.B O_LARGEFILE
> +This flag enables 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.
> +.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
> +.IR flags .
> +.B FAN_ALL_INIT_FLAGS
> +defines all allowable bits.
> +.TP
> +.B EMFILE
> +The number of fanotify groups of the user exceeds 128.
> +.TP
> +.B ENOMEM
> +The allocation of memory for the notification group failed.
> +.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 "SEE ALSO"
> +.BR fanotify_mark (2),
> +.BR fanotify (7)
> diff --git a/man2/fanotify_mark.2 b/man2/fanotify_mark.2
> new file mode 100644
> index 0000000..693eff8
> --- /dev/null
> +++ b/man2/fanotify_mark.2
> @@ -0,0 +1,327 @@
> +.\" Copyright (C) 2013, Heinrich Schuchardt <xypron.glpk@xxxxxx>
> +.\"
> +.\" %%%LICENSE_START(VERBATIM)
> +.\" Permission is granted to make and distribute verbatim copies of this
> +.\" manual provided the copyright notice and this permission notice are
> +.\" preserved on all copies.
> +.\"
> +.\" Permission is granted to copy and distribute modified versions of
> +.\" this manual under the conditions for verbatim copying, provided that
> +.\" the entire resulting derived work is distributed under the terms of
> +.\" a permission notice identical to this one.
> +.\"
> +.\" Since the Linux kernel and libraries are constantly changing, this
> +.\" manual page may be incorrect or out-of-date. The author(s) assume.
> +.\" no responsibility for errors or omissions, or for damages resulting.
> +.\" from the use of the information contained herein. The author(s) may.
> +.\" not have taken the same level of care in the production of this.
> +.\" manual, which is licensed free of charge, as they might when working.
> +.\" professionally.
> +.\"
> +.\" Formatted or processed versions of this manual, if unaccompanied by
> +.\" the source, must acknowledge the copyright and authors of this work.
> +.\" %%%LICENSE_END
> +.TH FANOTIFY_MARK 2 2014-04-24 "Linux" "Linux Programmer's Manual"
> +.SH NAME
> +fanotify_mark \- add, remove, or modify an fanotify mark on a filesystem
> +object
> +.SH SYNOPSIS
> +.nf
> +.B #include <sys/fanotify.h>
> +.sp
> +.BI "int fanotify_mark(int " fanotify_fd ", unsigned int " flags ,
> +.BI " uint64_t " mask ", int " dirfd \
> +", const char *" pathname );
> +.fi
> +.SH DESCRIPTION
> +For an overview of the fanotify API, see
> +.BR fanotify (7).
> +.PP
> +.BR fanotify_mark (2)
> +adds, removes, or modifies an fanotify mark on a filesystem object.
> +The caller must have read permission on the filesystem object that is to be
> +marked.
> +.PP
> +The
> +.I fanotify_fd
> +argument is a file descriptor returned by
> +.BR fanotify_init (2).
> +.PP
> +.I flags
> +is a bit mask describing the modification to perform.
> +It must include exactly one of the following values:
> +.TP
> +.B FAN_MARK_ADD
> +The events in
> +.I mask
> +will be added to the mark mask (or to the ignore mask).
> +.I mask
> +must be nonempty or the error
> +.B EINVAL
> +will occur.
> +.TP
> +.B FAN_MARK_REMOVE
> +The events in argument
> +.I mask
> +will be removed from the mark mask (or from the ignore mask).
> +.I mask
> +must be nonempty or the error
> +.B EINVAL
> +will occur.
> +.TP
> +.B FAN_MARK_FLUSH
> +Remove either all mount or non-mount marks from the fanotify group.
> +If
> +.I flag
> +contains
> +.BR FAN_MARK_MOUNT ,
> +all marks for mounts are removed from the group.
> +Otherwise, all marks for directories and files are removed.
> +No flag other than
> +.B FAN_MARK_MOUNT
> +can be used in conjunction with
> +.BR FAN_MARK_FLUSH .
> +.I mask
> +is ignored.
> +.PP
> +If none of the values above is specified, or more than one is specified, the
> +call fails with the error
> +.BR EINVAL .
> +.PP
> +In addition,
> +.I flags
> +may contain zero or more of the following:
> +.TP
> +.B FAN_MARK_DONT_FOLLOW
> +If
> +.I pathname
> +is a symbolic link, mark the link itself, rather than the file to which it
> +refers.
> +(By default,
> +.BR fanotify_mark ()
> +dereferences
> +.I pathname
> +if it is a symbolic link.)
> +.TP
> +.B FAN_MARK_ONLYDIR
> +If the filesystem object to be marked is not a directory, the error
> +.B ENOTDIR
> +shall be raised.
> +.TP
> +.B FAN_MARK_MOUNT
> +Mark the mount point specified by
> +.IR pathname .
> +If
> +.I pathname
> +is not itself a mount point, the mount point containing
> +.I pathname
> +will be marked.
> +All directories, subdirectories, and the contained files of the mount point
> +will be monitored.
> +.TP
> +.B FAN_MARK_IGNORED_MASK
> +The events in
> +.I mask
> +shall be added to or removed from the ignore mask.
> +.TP
> +.B FAN_MARK_IGNORED_SURV_MODIFY
> +The ignore mask shall survive modify events.
> +If this flag is not set, the ignore mask is cleared when a modify event occurs
> +for the ignored file or directory.
> +.PP
> +.I mask
> +defines which events shall be listened to (or which shall be ignored).
> +It is a bit mask composed of the following values:
> +.TP
> +.B FAN_ACCESS
> +Create an event when a file or directory (but see BUGS) is accessed (read).
> +.TP
> +.B FAN_MODIFY
> +Create an event when a file is modified (write).
> +.TP
> +.B FAN_CLOSE_WRITE
> +Create an event when a writable file is closed.
> +.TP
> +.B FAN_CLOSE_NOWRITE
> +Create an event when a read-only file or directory is closed.
> +.TP
> +.B FAN_OPEN
> +Create an event when a file or directory is opened.
> +.TP
> +.B FAN_OPEN_PERM
> +Create an event when a permission to open a file or directory is requested.
> +An fanotify file descriptor created with
> +.B FAN_CLASS_PRE_CONTENT
> +or
> +.B FAN_CLASS_CONTENT
> +is required.
> +.TP
> +.B FAN_ACCESS_PERM
> +Create an event when a permission to read a file or directory is requested.
> +An fanotify file descriptor created with
> +.B FAN_CLASS_PRE_CONTENT
> +or
> +.B FAN_CLASS_CONTENT
> +is required.
> +.TP
> +.B FAN_ONDIR
> +Events for directories shall be created, for example when
> +.BR opendir (2),
> +.BR readdir (2)
> +(but see BUGS), and
> +.BR closedir (2)
> +are called.
> +Without this flag, only events for files are created.
> +.TP
> +.B FAN_EVENT_ON_CHILD
> +Events for the immediate children of marked directories shall be created.
> +The flag has no effect when marking mounts.
> +Note that events are not generated for children of the subdirectories
> +of marked directories.
> +To monitor complete directory trees it is necessary to mark the relevant
> +mount.
> +.PP
> +The following composed value is defined:
> +.TP
> +.B FAN_CLOSE
> +A file is closed
> +.RB ( FAN_CLOSE_WRITE | FAN_CLOSE_NOWRITE ).
> +.PP
> +The filesystem object to be marked is determined by the file descriptor
> +.I dirfd
> +and the pathname specified in
> +.IR pathname :
> +.IP * 3
> +If
> +.I pathname
> +is NULL,
> +.I dirfd
> +defines the filesystem object to be marked.
> +.IP *
> +If
> +.I pathname
> +is NULL, and
> +.I dirfd
> +takes the special value
> +.BR AT_FDCWD ,
> +the current working directory is to be marked.
> +.IP *
> +If
> +.I pathname
> +is absolute, it defines the filesystem object to be marked, and
> +.I dirfd
> +is ignored.
> +.IP *
> +If
> +.I pathname
> +is relative, and
> +.I dirfd
> +does not have the value
> +.BR AT_FDCWD ,
> +then the filesystem object to be marked is determined by interpreting
> +.I pathname
> +relative the directory referred to by
> +.IR dirfd .
> +.IP *
> +If
> +.I pathname
> +is relative, and
> +.I dirfd
> +has the value
> +.BR AT_FDCWD,
> +then the filesystem object to be marked is determined by interpreting
> +.I pathname
> +relative the current working directory.
> +.SH RETURN VALUE
> +On success,
> +.BR fanotify_mark ()
> +returns 0.
> +On error, \-1 is returned, and
> +.I errno
> +is set to indicate the error.
> +.SH ERRORS
> +.TP
> +.B EBADF
> +An invalid file descriptor was passed in
> +.IR fanotify_fd .
> +.TP
> +.B EINVAL
> +An invalid value was passed in
> +.IR flags
> +or
> +.IR mask ,
> +or
> +.I fanotify_fd
> +was not an fanotify file descriptor.
> +.TP
> +.B EINVAL
> +The fanotify file descriptor was opened with
> +.B FAN_CLASS_NOTIF
> +and mask contains a flag for permission events
> +.RB ( FAN_OPEN_PERM
> +or
> +.BR FAN_ACCESS_PERM ).
> +.TP
> +.B ENOENT
> +The filesystem object indicated by
> +.IR dirfd
> +and
> +.IR pathname
> +does not exist.
> +This error also occurs when trying to remove a mark from an object which is not
> +marked.
> +.TP
> +.B ENOMEM
> +The necessary memory could not be allocated.
> +.TP
> +.B ENOSPC
> +The number of marks exceeds the limit of 8192 and
> +.B FAN_UNLIMITED_MARKS
> +was not specified in the call to
> +.BR fanotify_init (2).
> +.TP
> +.B ENOTDIR
> +.I flags
> +contains
> +.BR FAN_MARK_ONLYDIR ,
> +and
> +.I dirfd
> +and
> +.I pathname
> +do not specify a directory.
> +.SH VERSIONS
> +.BR fanotify_mark ()
> +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
> +As of Linux 3.15,
> +the following bugs exist:
> +.IP * 3
> +.\" FIXME: Patch is in next-20140424.
> +If
> +.I flags
> +contains
> +.BR FAN_MARK_FLUSH ,
> +.I dfd
> +and
> +.I pathname
> +must indicate a filesystem object, even though this object is not used.
> +.IP *
> +.\" FIXME: Patch is in next-20140424.
> +.BR readdir (2)
> +does not result in a
> +.B FAN_ACCESS
> +event.
> +.IP *
> +.\" FIXME: Patch proposed.
> +If
> +.BR fanotify_mark (2)
> +is called with
> +.B FAN_MARK_FLUSH,
> +.I flags
> +is not checked for invalid values.
> +.SH SEE ALSO
> +.BR fanotify_init (2),
> +.BR fanotify (7)
> diff --git a/man7/fanotify.7 b/man7/fanotify.7
> new file mode 100644
> index 0000000..083244f
> --- /dev/null
> +++ b/man7/fanotify.7
> @@ -0,0 +1,684 @@
> +.\" Copyright (C) 2013, Heinrich Schuchardt <xypron.glpk@xxxxxx>
> +.\"
> +.\" %%%LICENSE_START(VERBATIM)
> +.\" Permission is granted to make and distribute verbatim copies of this
> +.\" manual provided the copyright notice and this permission notice are
> +.\" preserved on all copies.
> +.\"
> +.\" Permission is granted to copy and distribute modified versions of
> +.\" this manual under the conditions for verbatim copying, provided that
> +.\" the entire resulting derived work is distributed under the terms of
> +.\" a permission notice identical to this one.
> +.\"
> +.\" Since the Linux kernel and libraries are constantly changing, this
> +.\" manual page may be incorrect or out-of-date. The author(s) assume.
> +.\" no responsibility for errors or omissions, or for damages resulting.
> +.\" from the use of the information contained herein. The author(s) may.
> +.\" not have taken the same level of care in the production of this.
> +.\" manual, which is licensed free of charge, as they might when working.
> +.\" professionally.
> +.\"
> +.\" Formatted or processed versions of this manual, if unaccompanied by
> +.\" the source, must acknowledge the copyright and authors of this work.
> +.\" %%%LICENSE_END
> +.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 are monitoring of complete mounts, 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 closed 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 monitired 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:
> +notification events and 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.
> +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.
> +
> +Queue entries for notification events are removed when the event has been
> +read.
> +Queue entries for permission events are removed when the permission
> +decision has been taken by writing to the fanotify file descriptor.
> +.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)).
> +
> +The return value of
> +.BR read (2)
> +is the length of the filled buffer, or \-1 in case of an error.
> +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
> +The fields of this structure 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 .
> +In principle, the API design would allow to return variable-length structures.
> +Therefore, and for performance reasons, it is recommended to use a larger
> +buffer size when reading, for example 4096 bytes.
> +.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.
> +.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
> +.B FMODE_NONOTIFY
> +file status flag is set on the corresponding 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.
> +The reading application is responsible for closing the file descriptor.
> +.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 program.
> +.PP
> +The bit mask in
> +.I mask
> +signals 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 obect.
> +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 in the call to
> +.BR fanotify_init (2)
> +by setting the flag
> +.BR FAN_UNLIMITED_QUEUE .
> +.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 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 short 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 might occur because the file was already deleted by another thread or
> +process.
> +.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 bug 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.
> +.SH EXAMPLE
> +The following program demonstrates the usage of the fanotify API.
> +It marks the mount point passed as 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)
>
> --
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/

--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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