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I have an interesting use case. I'm running systemd in a container to simulate a VM for testing purposes. I'm able to run systemd just fine, but I need my default user to not be root so that anything execing into the container will get the right user and only then become root via sudo.

My ENTRYPOINT is sudo /usr/lib/systemd/systemd --system --unit=multi-user.target, and it works just fine for most things. However, due to the sudo invocation, systemd does not get PID 1, which some downstream systemd units depend on.

According to the docs:

Process model

When sudo runs a command, it calls fork(2), sets up the execution environment as described above, and calls the execve system call in the child process. The main sudo process waits until the command has completed, then passes the command's exit status to the security policy's close function and exits. If an I/O logging plugin is configured or if the security policy explicitly requests it, a new pseudo-terminal (“pty”) is created and a second sudo process is used to relay job control signals between the user's existing pty and the new pty the command is being run in. This extra process makes it possible to, for example, suspend and resume the command. Without it, the command would be in what POSIX terms an “orphaned process group” and it would not receive any job control signals. As a special case, if the policy plugin does not define a close function and no pty is required, sudo will execute the command directly instead of calling fork(2) first. The sudoers policy plugin will only define a close function when I/O logging is enabled, a pty is required, or the pam_session or pam_setcred options are enabled. Note that pam_session and pam_setcred are enabled by default on systems using PAM.

Is there a way to tell sudo to replace itself with the child process that it runs, similar to exec in Bash? Is there another better way to do this? It seems that I'd need to change "the policy plugin" potentially for just this one command, but I'm not sure how to do this.

  • sounds like you want a small setuid exec wrapper, security concerns of which being lessened if it is only used on a test virt – thrig Feb 14 at 20:57
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    Are you trying to start a Docker container with a user that is not root? – ctrl-alt-delor Feb 14 at 22:55
  • The use case is that I need to mimic a VM as closely as possible from within Docker (so I can run "VMs" on Travis CI). Thus, I want to run the actual init system as PID 1. However, I want it so that docker exec gets an unprivileged user instead. It seems that I need to write some C or rust to setuid/setgid to root and then execve, replacing the current program with systemd as true PID 1. – Naftuli Kay Feb 14 at 23:08
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If you have control of the docker run command line, then you could consider passing it a --user option (such as --user=0:0) instead of using sudo in the ENTRYPOINT command.

In that case you can use ENTRYPOINT set to /usr/lib/systemd/systemd --system --unit=multi-user.target, which will run systemd as PID 1, while still keeping a USER setting to take effect in your docker exec commands.

See overriding USER in the docker run reference manual.

Conversely, if you don't have control over the docker run command line, you could try to pass --user to the docker exec commands, keeping USER as root in the container definition. That has some disadvantages, such as the fact you have to pass that option to all invocations and you need to figure out what user:group (or uid:gid) to pass it (root is easy, always 0:0.)

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I solved this with a small Rust application called escalator. It uses the suid executable bit to execute as root, then setuid and setgid to truly become root before executing what is passed to it, replacing itself with the child process.

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