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88 lines
2.8 KiB
Plaintext
88 lines
2.8 KiB
Plaintext
[[init]]
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= init
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[[what-is-the-init-executable]]
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== What is the init executable?
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When the Linux kernel finishes booting, it runs an executable as the
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first and only userland process.
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The default path is `/init`, but we an set a custom one with the `init=`
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kernel command line argument.
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This process is then responsible for setting up the entire userland (or
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destroying everything when you want to have fun).
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This typically means reading some configuration files (e.g.
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`/etc/initrc`) and forking a bunch of userland executables based on
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those files.
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systemd is a "popular" `/init` implementation for desktop distros as of
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2017.
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BusyBox provides its own minimalistic init implementation which
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Buildroot uses by default.
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[[custom-init]]
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== Custom init
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Is the default BusyBox `/init` too bloated for you, minimalism freak?
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No problem, just use the `init` kernel boot parameter:
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....
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./run -e 'init=/sleep_forever.out'
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....
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Remember that shell scripts can also be used for `init`
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https://unix.stackexchange.com/questions/174062/init-as-a-shell-script/395375#395375:
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....
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./run -e 'init=/count.sh'
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....
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Also remember that if your init returns, the kernel will panic, there
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are just two non-panic possibilities:
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* run forever in a loop or long sleep
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* `poweroff` the machine
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[[disable-networking]]
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== Disable networking
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The default BusyBox init scripts enable networking, and there is a 15
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second timeout in case your network is down or if your kernel / emulator
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setup does not support it.
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To disable networking, use:
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....
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./build -p -n
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....
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To restore it, run:
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....
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./build -t initscripts-reconfigure
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....
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[[the-init-environment]]
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== The init environment
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The docs make it clear
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https://www.kernel.org/doc/html/v4.14/admin-guide/kernel-parameters.html
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_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
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The kernel parses parameters from the kernel command line up to “–”; if
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it doesn’t recognize a parameter and it doesn’t contain a ‘.’, the
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parameter gets passed to init: parameters with ‘=’ go into init’s
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environment, others are passed as command line arguments to init.
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Everything after “–” is passed as an argument to init.
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_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
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And you can try it out with:
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....
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./run -e 'init=/init_env_poweroff.sh - asdf=qwer zxcv' -n
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....
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