man/dracut.modules.7.asc

   1 DRACUT.MODULES(7)
   2 =================
   3 :doctype: manpage
   4 :man source:   dracut
   5 :man manual:   dracut
   6 :man version:  {version}
   7
   8 NAME
   9 ----
  10 dracut.modules - dracut modules
  11
  12 DESCRIPTION
  13 -----------
  14
  15 dracut uses a modular system to build and extend the initramfs image. All
  16 modules are located in _/usr/lib/dracut/modules.d_ or in _<git-src>/modules.d_.
  17 The most basic dracut module is _99base_. In _99base_ the initial shell script
  18 init is defined, which gets run by the kernel after initramfs loading. Although
  19 you can replace init with your own version of _99base_, this is not encouraged.
  20 Instead you should use, if possible, the hooks of dracut. All hooks, and the
  21 point of time in which they are executed, are described in <<stages>>.
  22
  23 The main script, which creates the initramfs is dracut itself. It parses all
  24 arguments and sets up the directory, in which everything is installed. It then
  25 executes all check, install, installkernel scripts found in the modules, which
  26 are to be processed. After everything is installed, the install directory is
  27 archived and compressed to the final initramfs image. All helper functions used
  28 by check, install and installkernel are found in in the file _dracut-functions_.
  29 These shell functions are available to all module installer (install,
  30 installkernel) scripts, without the need to source _dracut-functions_.
  31
  32 A module can check the preconditions for install and installkernel with the
  33 check script. Also dependencies can be expressed with check. If a module passed
  34 check, install and installkernel will be called to install all of the necessary
  35 files for the module. To split between kernel and non-kernel parts of the
  36 installation, all kernel module related parts have to be in installkernel. All
  37 other files found in a module directory are module specific and mostly are hook
  38 scripts and udev rules.
  39
  40
  41 [[stages]]
  42 == Boot Process Stages
  43
  44 dracut modules can insert custom script at various points, to control the boot
  45 process.
  46 These hooks are plain directories containing shell scripts ending with ".sh",
  47 which are sourced by init.
  48 Common used functions are in _dracut-lib.sh_, which can be sourced by any script.
  49
  50 === Hook: cmdline
  51
  52 The _cmdline_ hook is a place to insert scripts to parse the kernel command line
  53 and prepare the later actions, like setting up udev rules and configuration
  54 files.
  55
  56 In this hook the most important environment variable is defined: root. The
  57 second one is rootok, which indicates, that a module claimed to be able to parse
  58 the root defined. So for example, **root=**__iscsi:....__ will be claimed by the
  59 iscsi dracut module, which then sets rootok.
  60
  61 === Hook: pre-udev
  62
  63 This hook is executed right after the cmdline hook and a check if root and
  64 rootok were set. Here modules can take action with the final root, and before
  65 udev has been run.
  66
  67 === Start Udev
  68
  69 Now udev is started and the logging for udev is setup.
  70
  71 === Hook: pre-trigger
  72
  73 In this hook, you can set udev environment variables with **udevadm control
  74 --property=KEY=_value_** or control the further execution of udev with
  75 udevadm.
  76
  77 === Trigger Udev
  78
  79 udev is triggered by calling udevadm trigger, which sends add events for all
  80 devices and subsystems.
  81
  82 === Main Loop
  83
  84 In the main loop of dracut loops until udev has settled and
  85 all scripts in _initqueue/finished_ returned true.
  86 In this loop there are three hooks, where scripts can be inserted
  87 by calling /sbin/initqueue.
  88
  89 ==== Initqueue
  90
  91 This hook gets executed every time a script is inserted here, regardless of the
  92 udev state.
  93
  94 ==== Initqueue settled
  95
  96 This hook (initqueue/settled) gets executed every time udev has settled.
  97
  98 ==== Initqueue timeout
  99
 100 This hook (initqueue/timeout) gets executed, when the main loop counter becomes
 101 half of the rd.retry counter.
 102
 103 ==== Initqueue online
 104
 105 This hook (initqueue/online) gets executed whenever a network interface comes online
 106 (that is, once it is up and configured by the configured network module).
 107
 108 ==== Initqueue finished
 109
 110 This hook (initqueue/finished) is called after udev has settled and
 111 if all scripts herein return 0 the main loop will be ended.
 112 Arbitrary scripts can be added here, to loop in the
 113 initqueue until something happens, which a dracut module wants to wait for.
 114
 115 === Hook: pre-mount
 116
 117 Before the root device is mounted all scripts in the hook pre-mount are
 118 executed. In some cases (e.g. NFS) the real root device is already mounted,
 119 though.
 120
 121 === Hook: mount
 122
 123 This hook is mainly to mount the real root device.
 124
 125 === Hook: pre-pivot
 126
 127 This hook is called before cleanup hook, This is a good place for
 128 actions other than cleanups which need to be called before pivot.
 129
 130 === Hook: cleanup
 131
 132 This hook is the last hook and is called before init finally switches root to
 133 the real root device. This is a good place to clean up and kill processes not
 134 needed anymore.
 135
 136
 137 === Cleanup and switch_root
 138
 139 Init (or systemd) kills all udev processes, cleans up the environment,
 140 sets up the arguments for the real init process and finally calls switch_root.
 141 switch_root removes the whole filesystem hierarchy of the initramfs,
 142 chroot()s to the real root device and calls /sbin/init with the specified
 143 arguments.
 144
 145 To ensure all files in the initramfs hierarchy can be removed, all processes
 146 still running from the initramfs should not have any open file descriptors left.
 147
 148 == Network Infrastructure
 149
 150 FIXME
 151
 152 == Writing a Module
 153
 154 A simple example module is _90kernel-modules_, which modprobes a kernel module
 155 after udev has settled and the basic device drivers have been loaded.
 156
 157 All module installation information is in the file module-setup.sh.
 158
 159 First we create a check() function, which just exits with 0 indicating that this
 160 module should be included by default.
 161
 162 check():
 163 ----
 164 return 0
 165 ----
 166
 167 Then we create the install() function, which installs a cmdline hook with
 168 priority number 20 called _parse-insmodpost.sh_. It also installs the
 169 _insmodpost.sh_ script in _/sbin_.
 170
 171 install():
 172 ----
 173 inst_hook cmdline 20 "$moddir/parse-insmodpost.sh"
 174 inst_simple "$moddir/insmodpost.sh" /sbin/insmodpost.sh
 175 ----
 176
 177 The _parse-instmodpost.sh_ parses the kernel command line for a argument
 178 rd.driver.post, blacklists the module from being autoloaded and installs the
 179 hook _insmodpost.sh_ in the _initqueue/settled_.
 180
 181 _parse-insmodpost.sh_:
 182 ----
 183 for p in $(getargs rd.driver.post=); do
 184     echo "blacklist $p" >> /etc/modprobe.d/initramfsblacklist.conf
 185     _do_insmodpost=1
 186 done
 187
 188 [ -n "$_do_insmodpost" ] && /sbin/initqueue --settled --unique --onetime /sbin/insmodpost.sh
 189 unset _do_insmodpost
 190
 191 ----
 192
 193 _insmodpost.sh_, which is called in the _initqueue/settled_ hook will just
 194 modprobe the kernel modules specified in all rd.driver.post kernel command line
 195 parameters. It runs after udev has settled and is only called once (--onetime).
 196
 197 _insmodpost.sh_:
 198 ----
 199 . /lib/dracut-lib.sh
 200
 201 for p in $(getargs rd.driver.post=); do
 202     modprobe $p
 203 done
 204
 205 ----
 206
 207
 208 === module-setup.sh: check()
 209
 210 _check()_ is called by dracut to evaluate the inclusion of a dracut module in
 211 the initramfs.
 212
 213 $hostonly:: If the $hostonly variable is set, then the module check() function
 214 should be in "hostonly" mode, which means, that the check() should only return
 215 0, if the module is really needed to boot this specific host.
 216
 217 check() should return with:
 218
 219 0:: Include the dracut module in the initramfs.
 220
 221 1:: Do not include the dracut module. The requirements are not fulfilled
 222 (missing tools, etc.)
 223
 224 255:: Only include the dracut module, if another module requires it or if
 225 explicitly specified in the config file or on the argument list.
 226
 227
 228 === module-setup.sh: depends()
 229
 230 The function depends() should echo all other dracut module names the module
 231 depends on.
 232
 233 === module-setup.sh: cmdline()
 234
 235 This function should print the kernel command line options needed to boot the
 236 current machine setup. It should start with a space and should not print a
 237 newline.
 238
 239 === module-setup.sh: install()
 240
 241 The install() function is called to install everything non-kernel related.
 242 To install binaries, scripts, and other files, you can use the functions
 243 mentioned in <<creation>>.
 244
 245 To address a file in the current module directory, use the variable "$moddir".
 246
 247 === module-setup.sh: installkernel()
 248
 249 In installkernel() all kernel related files should be installed. You can use all
 250 of the functions mentioned in <<creation>> to install files.
 251
 252 === [[creation]]Creation Functions
 253
 254 ==== inst_multiple [-o] <file> [ <file> ...]
 255
 256 installs multiple binaries and files. If executables are specified without a
 257 path, dracut will search the path PATH=/usr/sbin:/sbin:/usr/bin:/bin for the
 258 binary. If the option "-o" is given as the first parameter, a missing file does
 259 not lead to an error.
 260
 261 ==== inst <src> [<dst>]
 262
 263 installs _one_ file <src> either to the same place in the initramfs or to an
 264 optional <dst>. inst with more than two arguments is treated the same as
 265 inst_multiple, all arguments are treated as files to install and none as
 266 install destinations.
 267
 268 ==== inst_hook <hookdir> <prio> <src>
 269
 270 installs an executable/script <src> in the dracut hook <hookdir> with priority
 271 <prio>.
 272
 273 ==== inst_rules <udevrule> [ <udevrule> ...]
 274
 275 installs one or more udev rules. Non-existant udev rules are reported, but do
 276 not let dracut fail.
 277
 278 ==== instmods <kernelmodule> [ <kernelmodule> ... ]
 279
 280 instmods should be used only in the installkernel() function.
 281
 282 instmods installs one or more kernel modules in the initramfs. <kernelmodule>
 283 can also be a whole subsystem, if prefixed with a "=", like "=drivers/net/team".
 284
 285 instmods will not install the kernel module, if $hostonly is set and the kernel
 286 module is not currently needed by any /sys/*...*/uevent MODALIAS.
 287 To install a kernel module regardless of the hostonly mode use the form:
 288 ----
 289 hostonly='' instmods <kernelmodule>
 290 ----
 291
 292 === Initramfs Functions
 293
 294 FIXME
 295
 296
 297 === Network Modules
 298
 299 FIXME
 300
 301 AUTHOR
 302 ------
 303 Harald Hoyer
 304
 305 SEE ALSO
 306 --------
 307 *dracut*(8)