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 hooks (initqueue/settled) gets executed every time udev has settled.
  97
  98 ==== Initqueue timeout
  99
 100 This hooks (initqueue/timeout) gets executed, when the main loop counter becomes
 101 half of the rd.retry counter.
 102
 103 ==== Initqueue finished
 104
 105 This hook (initqueue/finished) is called after udev has settled and
 106 if all scripts herein return 0 the main loop will be ended.
 107 Arbitrary scripts can be added here, to loop in the
 108 initqueue until something happens, which a dracut module wants to wait for.
 109
 110 === Hook: pre-mount
 111
 112 Before the root device is mounted all scripts in the hook pre-mount are
 113 executed. In some cases (e.g. NFS) the real root device is already mounted,
 114 though.
 115
 116 === Hook: mount
 117
 118 This hook is mainly to mount the real root device.
 119
 120 === Hook: pre-pivot
 121
 122 This hook is called before cleanup hook, This is a good place for
 123 actions other than cleanups which need to be called before pivot.
 124
 125 === Hook: cleanup
 126
 127 This hook is the last hook and is called before init finally switches root to
 128 the real root device. This is a good place to clean up and kill processes not
 129 needed anymore.
 130
 131
 132 === Cleanup and switch_root
 133
 134 Init (or systemd) kills all udev processes, cleans up the environment,
 135 sets up the arguments for the real init process and finally calls switch_root.
 136 switch_root removes the whole filesystem hierarchy of the initramfs,
 137 chroot()s to the real root device and calls /sbin/init with the specified
 138 arguments.
 139
 140 To ensure all files in the initramfs hierarchy can be removed, all processes
 141 still running from the initramfs should not have any open file descriptors left.
 142
 143 == Network Infrastructure
 144
 145 FIXME
 146
 147 == Writing a Module
 148
 149 A simple example module is _96insmodpost_, which modprobes a kernel module after
 150 udev has settled and the basic device drivers have been loaded.
 151
 152 All module installation information is in the file module-setup.sh.
 153
 154 First we create a check() function, which just exits with 0 indicating that this
 155 module should be included by default.
 156
 157 check():
 158 ----
 159 return 0
 160 ----
 161
 162 The we create the install() function, which installs a cmdline hook with
 163 priority number 20 called _parse-insmodpost.sh_. It also installs the
 164 _insmodpost.sh_ script in _/sbin_.
 165
 166 install():
 167 ----
 168 inst_hook cmdline 20 "$moddir/parse-insmodpost.sh"
 169 inst_simple "$moddir/insmodpost.sh" /sbin/insmodpost.sh
 170 ----
 171
 172 The _parse-instmodpost.sh_ parses the kernel command line for a argument
 173 rd.driver.post, blacklists the module from being autoloaded and installs the
 174 hook _insmodpost.sh_ in the _initqueue/settled_.
 175
 176 _parse-insmodpost.sh_:
 177 ----
 178 for p in $(getargs rd.driver.post=); do
 179     echo "blacklist $p" >> /etc/modprobe.d/initramfsblacklist.conf
 180     _do_insmodpost=1
 181 done
 182
 183 [ -n "$_do_insmodpost" ] && /sbin/initqueue --settled --unique --onetime /sbin/insmodpost.sh
 184 unset _do_insmodpost
 185
 186 ----
 187
 188 _insmodpost.sh_, which is called in the _initqueue/settled_ hook will just
 189 modprobe the kernel modules specified in all rd.driver.post kernel command line
 190 parameters. It runs after udev has settled and is only called once (--onetime).
 191
 192 _insmodpost.sh_:
 193 ----
 194 . /lib/dracut-lib.sh
 195
 196 for p in $(getargs rd.driver.post=); do
 197     modprobe $p
 198 done
 199
 200 ----
 201
 202
 203 === module-setup.sh: check()
 204
 205 _check()_ is called by dracut to evaluate the inclusion of a dracut module in
 206 the initramfs.
 207
 208 $hostonly:: If the $hostonly variable is set, then the module check() function
 209 should be in "hostonly" mode, which means, that the check() should only return
 210 0, if the module is really needed to boot this specific host.
 211
 212 check() should return with:
 213
 214 0:: Include the dracut module in the initramfs.
 215
 216 1:: Do not include the dracut module. The requirements are not fulfilled
 217 (missing tools, etc.)
 218
 219 255:: Only include the dracut module, if another module requires it or if
 220 explicitly specified in the config file or on the argument list.
 221
 222
 223 === module-setup.sh: depends()
 224
 225 The function depends() should echo all other dracut module names the module
 226 depends on.
 227
 228 === module-setup.sh: cmdline()
 229
 230 This function should print the kernel command line options needed to boot the
 231 current machine setup. It should start with a space and should not print a
 232 newline.
 233
 234 === module-setup.sh: install()
 235
 236 The install() function is called to install everything non-kernel related.
 237 To install binaries, scripts, and other files, you can use the functions
 238 mentioned in <<creation>>.
 239
 240 To address a file in the current module directory, use the variable "$moddir".
 241
 242 === module-setup.sh: installkernel()
 243
 244 In installkernel() all kernel related files should be installed. You can use all
 245 of the functions mentioned in <<creation>> to install files.
 246
 247 === [[creation]]Creation Functions
 248
 249 ==== inst_multiple [-o] <file> [ <file> ...]
 250
 251 installs multiple binaries and files. If executables are specified without a
 252 path, dracut will search the path PATH=/usr/sbin:/sbin:/usr/bin:/bin for the
 253 binary. If the option "-o" is given as the first parameter, a missing file does
 254 not lead to an error.
 255
 256 ==== inst <src> [<dst>]
 257
 258 installs _one_ file <src> either to the same place in the initramfs or to an
 259 optional <dst>. inst with more than two arguments is treated the same as
 260 inst_multiple, all arguments are treated as files to install and none as
 261 install destinations.
 262
 263 ==== inst_hook <hookdir> <prio> <src>
 264
 265 installs an executable/script <src> in the dracut hook <hookdir> with priority
 266 <prio>.
 267
 268 ==== inst_rules <udevrule> [ <udevrule> ...]
 269
 270 installs one or more udev rules. Non-existant udev rules are reported, but do
 271 not let dracut fail.
 272
 273 ==== instmods <kernelmodule> [ <kernelmodule> ... ]
 274
 275 instmods should be used only in the installkernel() function.
 276
 277 instmods installs one or more kernel modules in the initramfs. <kernelmodule>
 278 can also be a whole subsystem, if prefixed with a "=", like "=drivers/net/team".
 279
 280 instmods will not install the kernel module, if $hostonly is set and the kernel
 281 module is not currently needed by any /sys/*...*/uevent MODALIAS.
 282 To install a kernel module regardless of the hostonly mode use the form:
 283 ----
 284 hostonly='' instmods <kernelmodule>
 285 ----
 286
 287 === Initramfs Functions
 288
 289 FIXME
 290
 291
 292 === Network Modules
 293
 294 FIXME
 295
 296 AUTHOR
 297 ------
 298 Harald Hoyer
 299
 300 SEE ALSO
 301 --------
 302 *dracut*(8)