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stage-2: parameterized systemd executable #93110

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merged 1 commit into from Jul 14, 2020
Merged

stage-2: parameterized systemd executable #93110

merged 1 commit into from Jul 14, 2020

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grahamc
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@grahamc grahamc commented Jul 14, 2020 •
edited

This lets users do sneaky things before systemd starts, and
permanently affect the environment in which systemd runs. For example,
we could start systemd in a non-default network namespace by setting
the systemdExecutable to a wrapper script containing:

#!/bin/sh
ip netns add virtual
touch /var/run/netns/physical
mount -o bind /proc/self/ns/net /var/run/netns/physical
exec ip netns exec virtual systemd

note: the above example does literally work, but there are unresolved problems with udev and dhcp.

Motivation for this change
Things done
  • Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
  • Built on platform(s)
    • NixOS
    • macOS
    • other Linux distributions
  • Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
  • Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
  • Tested execution of all binary files (usually in ./result/bin/)
  • Determined the impact on package closure size (by running nix path-info -S before and after)
  • Ensured that relevant documentation is up to date
  • Fits CONTRIBUTING.md.

@edolstra
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The option name and description are misleading, since this is not a program but a shell fragment that gets concatenated after exec. It might be better to name it startSystemd or something like that (with default value exec systemd). PATH=... LOCALE_ARCHIVE=... could be split from the exec so it's easier to inject arbitrary shell code to run before systemd.

A more general solution would be to modularize the stage 2 script similar to the activation script, such that the script fragment that starts systemd gets ordered after all other fragments. But that's probably a bit overkill.

@grahamc
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grahamc commented Jul 14, 2020

Ah, in this case, the fragment would be written to a wrapper script which would still be exec'd the same way. Not a shell fragment. It does look like a shell fragment, but only because Git strips lines that start with a # and my shebang was eaten.

@grahamc
stage-2: parameterized systemd executable
64dd9c1 
This lets users do sneaky things before systemd starts, and
permanently affect the environment in which systemd runs. For example,
we could start systemd in a non-default network namespace by setting
the systemdExecutable to a wrapper script containing:

    #!/bin/sh
    ip netns add virtual
    touch /var/run/netns/physical
    mount -o bind /proc/self/ns/net /var/run/netns/physical
    exec ip netns exec virtual systemd

_note: the above example does literally work, but there are unresolved
problems with udev and dhcp._
@grahamc
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grahamc commented Jul 14, 2020

I updated the description and type to show that it is a single value, not lines.

@grahamc grahamc merged commit 0c10b75 into NixOS:master Jul 14, 2020
@grahamc grahamc deleted the systemd-executable branch July 14, 2020 19:45
@dasJ dasJ mentioned this pull request Mar 28, 2022
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@cleverca22 cleverca22 cleverca22 approved these changes

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6.topic: nixos 8.has: module (update) 10.rebuild-darwin: 0 10.rebuild-linux: 1-10
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