man2/fanotify_init.2

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  24 .TH FANOTIFY_INIT 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  25 .SH NAME
  26 fanotify_init \- create and initialize fanotify group
  27 .SH SYNOPSIS
  28 .B #include <fcntl.h>
  29 .br
  30 .B #include <sys/fanotify.h>
  31 .PP
  32 .BI "int fanotify_init(unsigned int " flags ", unsigned int " event_f_flags );
  33 .SH DESCRIPTION
  34 For an overview of the fanotify API, see
  35 .BR fanotify (7).
  36 .PP
  37 .BR fanotify_init ()
  38 initializes a new fanotify group and returns a file descriptor for the event
  39 queue associated with the group.
  40 .PP
  41 The file descriptor is used in calls to
  42 .BR fanotify_mark (2)
  43 to specify the files, directories, and mounts for which fanotify events
  44 shall be created.
  45 These events are received by reading from the file descriptor.
  46 Some events are only informative, indicating that a file has been accessed.
  47 Other events can be used to determine whether
  48 another application is permitted to access a file or directory.
  49 Permission to access filesystem objects is granted by writing to the file
  50 descriptor.
  51 .PP
  52 Multiple programs may be using the fanotify interface at the same time to
  53 monitor the same files.
  54 .PP
  55 In the current implementation, the number of fanotify groups per user is
  56 limited to 128.
  57 This limit cannot be overridden.
  58 .PP
  59 Calling
  60 .BR fanotify_init ()
  61 requires the
  62 .B CAP_SYS_ADMIN
  63 capability.
  64 This constraint might be relaxed in future versions of the API.
  65 Therefore, certain additional capability checks have been implemented as
  66 indicated below.
  67 .PP
  68 The
  69 .I flags
  70 argument contains a multi-bit field defining the notification class of the
  71 listening application and further single bit fields specifying the behavior
  72 of the file descriptor.
  73 .PP
  74 If multiple listeners for permission events exist,
  75 the notification class is used to establish the sequence
  76 in which the listeners receive the events.
  77 .PP
  78 Only one of the following notification classes may be specified in
  79 .IR flags :
  80 .TP
  81 .B FAN_CLASS_PRE_CONTENT
  82 This value allows the receipt of events notifying that a file has been
  83 accessed and events for permission decisions if a file may be accessed.
  84 It is intended for event listeners that need to access files before they
  85 contain their final data.
  86 This notification class might be used by hierarchical storage managers,
  87 for example.
  88 .TP
  89 .B FAN_CLASS_CONTENT
  90 This value allows the receipt of events notifying that a file has been
  91 accessed and events for permission decisions if a file may be accessed.
  92 It is intended for event listeners that need to access files when they
  93 already contain their final content.
  94 This notification class might be used by malware detection programs, for
  95 example.
  96 .TP
  97 .B FAN_CLASS_NOTIF
  98 This is the default value.
  99 It does not need to be specified.
 100 This value only allows the receipt of events notifying that a file has been
 101 accessed.
 102 Permission decisions before the file is accessed are not possible.
 103 .PP
 104 Listeners with different notification classes will receive events in the
 105 order
 106 .BR FAN_CLASS_PRE_CONTENT ,
 107 .BR FAN_CLASS_CONTENT ,
 108 .BR FAN_CLASS_NOTIF .
 109 The order of notification for listeners in the same notification class
 110 is undefined.
 111 .PP
 112 The following bits can additionally be set in
 113 .IR flags :
 114 .TP
 115 .B FAN_CLOEXEC
 116 Set the close-on-exec flag
 117 .RB ( FD_CLOEXEC )
 118 on the new file descriptor.
 119 See the description of the
 120 .B O_CLOEXEC
 121 flag in
 122 .BR open (2).
 123 .TP
 124 .B FAN_NONBLOCK
 125 Enable the nonblocking flag
 126 .RB ( O_NONBLOCK )
 127 for the file descriptor.
 128 Reading from the file descriptor will not block.
 129 Instead, if no data is available,
 130 .BR read (2)
 131 fails with the error
 132 .BR EAGAIN .
 133 .TP
 134 .B FAN_UNLIMITED_QUEUE
 135 Remove the limit of 16384 events for the event queue.
 136 Use of this flag requires the
 137 .B CAP_SYS_ADMIN
 138 capability.
 139 .TP
 140 .B FAN_UNLIMITED_MARKS
 141 Remove the limit of 8192 marks.
 142 Use of this flag requires the
 143 .B CAP_SYS_ADMIN
 144 capability.
 145 .TP
 146 .BR FAN_REPORT_TID " (since Linux 4.20)"
 147 .\" commit d0a6a87e40da49cfc7954c491d3065a25a641b29
 148 Report thread id (TID) instead of process id (PID).
 149 .PP
 150 The
 151 .I event_f_flags
 152 argument
 153 defines the file status flags that will be set on the open file descriptions
 154 that are created for fanotify events.
 155 For details of these flags, see the description of the
 156 .I flags
 157 values in
 158 .BR open (2).
 159 .I event_f_flags
 160 includes a multi-bit field for the access mode.
 161 This field can take the following values:
 162 .TP
 163 .B O_RDONLY
 164 This value allows only read access.
 165 .TP
 166 .B O_WRONLY
 167 This value allows only write access.
 168 .TP
 169 .B O_RDWR
 170 This value allows read and write access.
 171 .PP
 172 Additional bits can be set in
 173 .IR event_f_flags .
 174 The most useful values are:
 175 .TP
 176 .B O_LARGEFILE
 177 Enable support for files exceeding 2\ GB.
 178 Failing to set this flag will result in an
 179 .B EOVERFLOW
 180 error when trying to open a large file which is monitored by
 181 an fanotify group on a 32-bit system.
 182 .TP
 183 .BR O_CLOEXEC " (since Linux 3.18)"
 184 .\" commit 0b37e097a648aa71d4db1ad108001e95b69a2da4
 185 Enable the close-on-exec flag for the file descriptor.
 186 See the description of the
 187 .B O_CLOEXEC
 188 flag in
 189 .BR open (2)
 190 for reasons why this may be useful.
 191 .PP
 192 The following are also allowable:
 193 .BR O_APPEND ,
 194 .BR O_DSYNC ,
 195 .BR O_NOATIME ,
 196 .BR O_NONBLOCK ,
 197 and
 198 .BR O_SYNC .
 199 Specifying any other flag in
 200 .I event_f_flags
 201 yields the error
 202 .B EINVAL
 203 (but see BUGS).
 204 .SH RETURN VALUE
 205 On success,
 206 .BR fanotify_init ()
 207 returns a new file descriptor.
 208 On error, \-1 is returned, and
 209 .I errno
 210 is set to indicate the error.
 211 .SH ERRORS
 212 .TP
 213 .B EINVAL
 214 An invalid value was passed in
 215 .I flags
 216 or
 217 .IR event_f_flags .
 218 .B FAN_ALL_INIT_FLAGS
 219 defines all allowable bits for
 220 .IR flags .
 221 .TP
 222 .B EMFILE
 223 The number of fanotify groups for this user exceeds 128.
 224 .TP
 225 .B EMFILE
 226 The per-process limit on the number of open file descriptors has been reached.
 227 .TP
 228 .B ENOMEM
 229 The allocation of memory for the notification group failed.
 230 .TP
 231 .B ENOSYS
 232 This kernel does not implement
 233 .BR fanotify_init ().
 234 The fanotify API is available only if the kernel was configured with
 235 .BR CONFIG_FANOTIFY .
 236 .TP
 237 .B EPERM
 238 The operation is not permitted because the caller lacks the
 239 .B CAP_SYS_ADMIN
 240 capability.
 241 .SH VERSIONS
 242 .BR fanotify_init ()
 243 was introduced in version 2.6.36 of the Linux kernel and enabled in version
 244 2.6.37.
 245 .SH CONFORMING TO
 246 This system call is Linux-specific.
 247 .SH BUGS
 248 The following bug was present in Linux kernels before version 3.18:
 249 .IP * 3
 250 .\" Fixed by commit 0b37e097a648aa71d4db1ad108001e95b69a2da4
 251 The
 252 .B O_CLOEXEC
 253 is ignored when passed in
 254 .IR event_f_flags .
 255 .PP
 256 The following bug was present in Linux kernels before version 3.14:
 257 .IP * 3
 258 .\" Fixed by commit 48149e9d3a7e924010a0daab30a6197b7d7b6580
 259 The
 260 .I event_f_flags
 261 argument is not checked for invalid flags.
 262 Flags that are intended only for internal use,
 263 such as
 264 .BR FMODE_EXEC ,
 265 can be set, and will consequently be set for the file descriptors
 266 returned when reading from the fanotify file descriptor.
 267 .SH SEE ALSO
 268 .BR fanotify_mark (2),
 269 .BR fanotify (7)