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22 KiB
Groff
763 lines
22 KiB
Groff
.\" Copyright (C) 2013, Heinrich Schuchardt <xypron.glpk@gmx.de>
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.\" and Copyright (C) 2014, Michael Kerrisk <mtk.manpages@gmail.com>
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.\"
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.\" %%%LICENSE_START(VERBATIM)
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.\" Permission is granted to make and distribute verbatim copies of this
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.\" manual provided the copyright notice and this permission notice are
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.\" preserved on all copies.
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.\"
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.\" Permission is granted to copy and distribute modified versions of
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.\" this manual under the conditions for verbatim copying, provided that
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.\" the entire resulting derived work is distributed under the terms of
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.\" a permission notice identical to this one.
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.\"
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.\" Since the Linux kernel and libraries are constantly changing, this
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.\" manual page may be incorrect or out-of-date. The author(s) assume.
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.\" no responsibility for errors or omissions, or for damages resulting.
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.\" from the use of the information contained herein. The author(s) may.
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.\" not have taken the same level of care in the production of this.
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.\" manual, which is licensed free of charge, as they might when working.
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.\" professionally.
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.\"
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.\" Formatted or processed versions of this manual, if unaccompanied by
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.\" the source, must acknowledge the copyright and authors of this work.
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.\" %%%LICENSE_END
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.TH FANOTIFY 7 2015-08-08 "Linux" "Linux Programmer's Manual"
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.SH NAME
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fanotify \- monitoring filesystem events
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.SH DESCRIPTION
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The fanotify API provides notification and interception of
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filesystem events.
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Use cases include virus scanning and hierarchical storage management.
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Currently, only a limited set of events is supported.
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In particular, there is no support for create, delete, and move events.
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(See
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.BR inotify (7)
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for details of an API that does notify those events.)
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Additional capabilities compared to the
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.BR inotify (7)
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API include the ability to monitor all of the objects
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in a mounted filesystem,
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the ability to make access permission decisions, and the
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possibility to read or modify files before access by other applications.
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The following system calls are used with this API:
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.BR fanotify_init (2),
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.BR fanotify_mark (2),
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.BR read (2),
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.BR write (2),
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and
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.BR close (2).
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.SS fanotify_init(), fanotify_mark(), and notification groups
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The
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.BR fanotify_init (2)
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system call creates and initializes an fanotify notification group
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and returns a file descriptor referring to it.
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.PP
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An fanotify notification group is a kernel-internal object that holds
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a list of files, directories, and mount points for which events shall be
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created.
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.PP
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For each entry in an fanotify notification group, two bit masks exist: the
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.I mark
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mask and the
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.I ignore
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mask.
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The mark mask defines file activities for which an event shall be created.
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The ignore mask defines activities for which no event shall be generated.
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Having these two types of masks permits a mount point or directory to be
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marked for receiving events, while at the same time ignoring events for
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specific objects under that mount point or directory.
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.PP
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The
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.BR fanotify_mark (2)
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system call adds a file, directory, or mount to a notification group
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and specifies which events
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shall be reported (or ignored), or removes or modifies such an entry.
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.PP
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A possible usage of the ignore mask is for a file cache.
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Events of interest for a file cache are modification of a file and closing
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of the same.
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Hence, the cached directory or mount point is to be marked to receive these
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events.
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After receiving the first event informing that a file has been modified,
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the corresponding cache entry will be invalidated.
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No further modification events for this file are of interest until the file
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is closed.
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Hence, the modify event can be added to the ignore mask.
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Upon receiving the close event, the modify event can be removed from the
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ignore mask and the file cache entry can be updated.
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.PP
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The entries in the fanotify notification groups refer to files and
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directories via their inode number and to mounts via their mount ID.
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If files or directories are renamed or moved within the same mount,
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the respective entries survive.
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If files or directories are deleted or moved to another mount or if mounts are
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unmounted, the corresponding entries are deleted.
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.SS The event queue
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As events occur on the filesystem objects monitored by a notification group,
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the fanotify system generates events that are collected in a queue.
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These events can then be read (using
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.BR read (2)
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or similar)
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from the fanotify file descriptor
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returned by
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.BR fanotify_init (2).
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Two types of events are generated:
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.I notification
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events and
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.I permission
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events.
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Notification events are merely informative
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and require no action to be taken by
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the receiving application except for closing the file descriptor passed
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in the event (see below).
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Permission events are requests to the receiving application to decide
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whether permission for a file access shall be granted.
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For these events, the recipient must write a response which decides whether
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access is granted or not.
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An event is removed from the event queue of the fanotify group
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when it has been read.
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Permission events that have been read are kept in an internal list of the
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fanotify group until either a permission decision has been taken by
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writing to the fanotify file descriptor or the fanotify file descriptor
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is closed.
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.SS Reading fanotify events
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Calling
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.BR read (2)
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for the file descriptor returned by
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.BR fanotify_init (2)
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blocks (if the flag
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.B FAN_NONBLOCK
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is not specified in the call to
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.BR fanotify_init (2))
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until either a file event occurs or the call is interrupted by a signal
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(see
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.BR signal (7)).
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After a successful
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.BR read (2),
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the read buffer contains one or more of the following structures:
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.in +4n
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.nf
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struct fanotify_event_metadata {
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__u32 event_len;
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__u8 vers;
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__u8 reserved;
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__u16 metadata_len;
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__aligned_u64 mask;
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__s32 fd;
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__s32 pid;
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};
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.fi
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.in
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.PP
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For performance reasons, it is recommended to use a large
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buffer size (for example, 4096 bytes),
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so that multiple events can be retrieved by a single
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.BR read (2).
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The return value of
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.BR read (2)
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is the number of bytes placed in the buffer,
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or \-1 in case of an error (but see BUGS).
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The fields of the
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.I fanotify_event_metadata
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structure are as follows:
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.TP
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.I event_len
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This is the length of the data for the current event and the offset
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to the next event in the buffer.
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In the current implementation, the value of
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.I event_len
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is always
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.BR FAN_EVENT_METADATA_LEN .
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However, the API is designed to allow
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variable-length structures to be returned in the future.
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.TP
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.I vers
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This field holds a version number for the structure.
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It must be compared to
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.B FANOTIFY_METADATA_VERSION
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to verify that the structures returned at runtime match
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the structures defined at compile time.
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In case of a mismatch, the application should abandon trying to use the
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fanotify file descriptor.
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.TP
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.I reserved
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This field is not used.
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.TP
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.I metadata_len
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This is the length of the structure.
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The field was introduced to facilitate the implementation of
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optional headers per event type.
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No such optional headers exist in the current implementation.
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.TP
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.I mask
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This is a bit mask describing the event (see below).
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.TP
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.I fd
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This is an open file descriptor for the object being accessed, or
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.B FAN_NOFD
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if a queue overflow occurred.
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The file descriptor can be used to access the contents
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of the monitored file or directory.
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The reading application is responsible for closing this file descriptor.
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.IP
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When calling
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.BR fanotify_init (2),
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the caller may specify (via the
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.I event_f_flags
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argument) various file status flags that are to be set
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on the open file description that corresponds to this file descriptor.
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In addition, the (kernel-internal)
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.B FMODE_NONOTIFY
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file status flag is set on the open file description.
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This flag suppresses fanotify event generation.
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Hence, when the receiver of the fanotify event accesses the notified file or
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directory using this file descriptor, no additional events will be created.
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.TP
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.I pid
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This is the ID of the process that caused the event.
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A program listening to fanotify events can compare this PID
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to the PID returned by
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.BR getpid (2),
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to determine whether the event is caused by the listener itself,
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or is due to a file access by another process.
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.PP
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The bit mask in
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.I mask
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indicates which events have occurred for a single filesystem object.
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Multiple bits may be set in this mask,
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if more than one event occurred for the monitored filesystem object.
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In particular,
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consecutive events for the same filesystem object and originating from the
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same process may be merged into a single event, with the exception that two
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permission events are never merged into one queue entry.
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.PP
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The bits that may appear in
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.I mask
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are as follows:
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.TP
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.B FAN_ACCESS
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A file or a directory (but see BUGS) was accessed (read).
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.TP
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.B FAN_OPEN
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A file or a directory was opened.
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.TP
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.B FAN_MODIFY
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A file was modified.
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.TP
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.B FAN_CLOSE_WRITE
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A file that was opened for writing
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.RB ( O_WRONLY
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or
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.BR O_RDWR )
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was closed.
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.TP
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.B FAN_CLOSE_NOWRITE
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A file or directory that was opened read-only
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.RB ( O_RDONLY )
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was closed.
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.TP
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.B FAN_Q_OVERFLOW
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The event queue exceeded the limit of 16384 entries.
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This limit can be overridden by specifying the
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.BR FAN_UNLIMITED_QUEUE
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flag when calling
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.BR fanotify_init (2).
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.TP
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.B FAN_ACCESS_PERM
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An application wants to read a file or directory, for example using
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.BR read (2)
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or
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.BR readdir (2).
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The reader must write a response (as described below)
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that determines whether the permission to
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access the filesystem object shall be granted.
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.TP
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.B FAN_OPEN_PERM
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An application wants to open a file or directory.
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The reader must write a response that determines whether the permission to
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open the filesystem object shall be granted.
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.PP
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To check for any close event, the following bit mask may be used:
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.TP
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.B FAN_CLOSE
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A file was closed.
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This is a synonym for:
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FAN_CLOSE_WRITE | FAN_CLOSE_NOWRITE
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.PP
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The following macros are provided to iterate over a buffer containing
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fanotify event metadata returned by a
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.BR read (2)
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from an fanotify file descriptor:
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.TP
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.B FAN_EVENT_OK(meta, len)
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This macro checks the remaining length
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.I len
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of the buffer
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.I meta
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against the length of the metadata structure and the
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.I event_len
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field of the first metadata structure in the buffer.
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.TP
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.B FAN_EVENT_NEXT(meta, len)
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This macro uses the length indicated in the
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.I event_len
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field of the metadata structure pointed to by
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.IR meta
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to calculate the address of the next metadata structure that follows
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.IR meta .
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.I len
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is the number of bytes of metadata that currently remain in the buffer.
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The macro returns a pointer to the next metadata structure that follows
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.IR meta ,
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and reduces
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.I len
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by the number of bytes in the metadata structure that
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has been skipped over (i.e., it subtracts
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.IR meta\->event_len
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from
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.IR len ).
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.PP
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In addition, there is:
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.TP
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.B FAN_EVENT_METADATA_LEN
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This macro returns the size (in bytes) of the structure
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.IR fanotify_event_metadata .
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This is the minimum size (and currently the only size) of any event metadata.
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.\"
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.SS Monitoring an fanotify file descriptor for events
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When an fanotify event occurs, the fanotify file descriptor indicates as
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readable when passed to
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.BR epoll (7),
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.BR poll (2),
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or
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.BR select (2).
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.SS Dealing with permission events
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For permission events, the application must
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.BR write (2)
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a structure of the following form to the
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fanotify file descriptor:
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.in +4n
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.nf
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struct fanotify_response {
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__s32 fd;
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__u32 response;
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};
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.fi
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.in
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.PP
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The fields of this structure are as follows:
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.TP
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.I fd
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This is the file descriptor from the structure
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.IR fanotify_event_metadata .
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.TP
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.I response
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This field indicates whether or not the permission is to be granted.
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Its value must be either
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.B FAN_ALLOW
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to allow the file operation or
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.B FAN_DENY
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to deny the file operation.
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.PP
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If access is denied, the requesting application call will receive an
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.BR EPERM
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error.
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.SS Closing the fanotify file descriptor
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.PP
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When all file descriptors referring to the fanotify notification group are
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closed, the fanotify group is released and its resources
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are freed for reuse by the kernel.
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Upon
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.BR close (2),
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outstanding permission events will be set to allowed.
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.SS /proc/[pid]/fdinfo
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The file
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.I /proc/[pid]/fdinfo/[fd]
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contains information about fanotify marks for file descriptor
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.I fd
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of process
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.IR pid .
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See the kernel source file
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.I Documentation/filesystems/proc.txt
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for details.
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.SH ERRORS
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In addition to the usual errors for
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.BR read (2),
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the following errors can occur when reading from the
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fanotify file descriptor:
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.TP
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.B EINVAL
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The buffer is too small to hold the event.
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.TP
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.B EMFILE
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The per-process limit on the number of open files has been reached.
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See the description of
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.B RLIMIT_NOFILE
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in
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.BR getrlimit (2).
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.TP
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.B ENFILE
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The system-wide limit on the number of open files has been reached.
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See
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.I /proc/sys/fs/file-max
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in
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.BR proc (5).
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.TP
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.B ETXTBSY
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This error is returned by
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.BR read (2)
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if
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.B O_RDWR
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or
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.B O_WRONLY
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was specified in the
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.I event_f_flags
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argument when calling
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.BR fanotify_init (2)
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and an event occurred for a monitored file that is currently being executed.
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.PP
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In addition to the usual errors for
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.BR write (2),
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the following errors can occur when writing to the fanotify file descriptor:
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.TP
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.B EINVAL
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Fanotify access permissions are not enabled in the kernel configuration
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or the value of
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.I response
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in the response structure is not valid.
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.TP
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.B ENOENT
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The file descriptor
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.I fd
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in the response structure is not valid.
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This may occur when a response for the permission event has already been
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written.
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.SH VERSIONS
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The fanotify API was introduced in version 2.6.36 of the Linux kernel and
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enabled in version 2.6.37.
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Fdinfo support was added in version 3.8.
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.SH CONFORMING TO
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The fanotify API is Linux-specific.
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.SH NOTES
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The fanotify API is available only if the kernel was built with the
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.B CONFIG_FANOTIFY
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configuration option enabled.
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In addition, fanotify permission handling is available only if the
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.B CONFIG_FANOTIFY_ACCESS_PERMISSIONS
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configuration option is enabled.
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.SS Limitations and caveats
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Fanotify reports only events that a user-space program triggers through the
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filesystem API.
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As a result,
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it does not catch remote events that occur on network filesystems.
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.PP
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The fanotify API does not report file accesses and modifications that
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may occur because of
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.BR mmap (2),
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.BR msync (2),
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and
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.BR munmap (2).
|
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.PP
|
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Events for directories are created only if the directory itself is opened,
|
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read, and closed.
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Adding, removing, or changing children of a marked directory does not create
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events for the monitored directory itself.
|
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.PP
|
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Fanotify monitoring of directories is not recursive:
|
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to monitor subdirectories under a directory,
|
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additional marks must be created.
|
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(But note that the fanotify API provides no way of detecting when a
|
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subdirectory has been created under a marked directory,
|
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which makes recursive monitoring difficult.)
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Monitoring mounts offers the capability to monitor a whole directory tree.
|
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.PP
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The event queue can overflow.
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In this case, events are lost.
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.SH BUGS
|
|
Before Linux 3.19,
|
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.BR fallocate (2)
|
|
did not generate fanotify events.
|
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Since Linux 3.19,
|
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.\" commit 820c12d5d6c0890bc93dd63893924a13041fdc35
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calls to
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.BR fallocate (2)
|
|
generate
|
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.B FAN_MODIFY
|
|
events.
|
|
|
|
As of Linux 3.17,
|
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the following bugs exist:
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.IP * 3
|
|
On Linux, a filesystem object may be accessible through multiple paths,
|
|
for example, a part of a filesystem may be remounted using the
|
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.IR \-\-bind
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option of
|
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.BR mount (8).
|
|
A listener that marked a mount will be notified only of events that were
|
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triggered for a filesystem object using the same mount.
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Any other event will pass unnoticed.
|
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.IP *
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|
.\" FIXME . A patch was proposed.
|
|
When an event is generated,
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no check is made to see whether the user ID of the
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receiving process has authorization to read or write the file
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before passing a file descriptor for that file.
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|
This poses a security risk, when the
|
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.B CAP_SYS_ADMIN
|
|
capability is set for programs executed by unprivileged users.
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.IP *
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If a call to
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.BR read (2)
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processes multiple events from the fanotify queue and an error occurs,
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the return value will be the total length of the events successfully
|
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copied to the user-space buffer before the error occurred.
|
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The return value will not be \-1, and
|
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.I errno
|
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will not be set.
|
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Thus, the reading application has no way to detect the error.
|
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.SH EXAMPLE
|
|
The following program demonstrates the usage of the fanotify API.
|
|
It marks the mount point passed as a command-line argument
|
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and waits for events of type
|
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.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;
|
|
struct fanotify_event_metadata buf[200];
|
|
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 = 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, AT_FDCWD,
|
|
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)
|