On portability of init systems

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There is one thing in the current init system debate that irritates me. It is about the portability of init systems. The new init systems are evaluated by how portable they are. This is one of the arguments that is most often brought against systemd, which is understandable given the polarizing attitude of one of systemd's authors. In this context, the currently used sysvinit is considered portable. But in my book, it is not portable at all.

  • There is no standard for the proc filesystem.
  • Any program using the proc filesystem is therefore not portable.
  • sysvinit uses the proc filesystem (in src/{bootlog,hddown,killall5}.c).
  • Therefore, sysvinit is not portable.

But why can we use sysvinit to boot Debian/{kFreeBSD,Hurd}?

Debian/kFreeBSD uses linprocfs for /proc to provide a familiar Linux-like environment for the userspace tools available in Debian (say, pgrep). linprocfs was originally written by the FreeBSD folks to support running Linux binaries using the Linux Binary Compatibility layer.

Debian/Hurd uses procfs to provide /poc. This procfs translator is written mainly to provide a Linux-compatible /proc filesystem for the same reason linprocfs is used by Debian/kFreeBSD.

So sysvinit works on those system not because it is portable, but because the environment has been made Linux-like enough for sysvinit. We (most likely anyone not using Linux) often do this, because it is the easiest way to run popular software developed (mainly) for Linux. This is often the path of the least resistance, as opposed to getting the upstream project to support the native way of doing things on platform X.

During gsoc last year I had to patch our procfs to finally be able to safely shut down Debian/Hurd systems using sysvinit. The problem was, that sysvinit at certain runlevel transitions (like shutting down, or I guess, switching to single user mode), sysvinit assumes that it is okay to stop and kill (almost) all processes on the system (that's what killall5 does). This might be okay on monolithic systems, but on (multiserver) microkernel systems like the Hurd, where your root filesystem and your network driver and stack are running as userspace processes, it is clearly not. I wonder how Linux systems using a FUSE-based root filesystem get away with this.

This highlights that not only sysvinit depends on a Linux-specific kernel interface (/proc), but it also hard-codes assumptions about the system architecture.

Amusingly, systemd get's this right (ok, I'm not sure if it does, but it could get this right...). systemd organizes processes in cgroups, one for each service it starts and one for each login session or something like that. It can (could?) kill only those processes in it was responsible for, leaving all essential translators (system servers) alone. In fact, even my tiny cgroupfs prototype can keep track of translators that are started by the root filesystem translator.

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