Lennart Poettering
4️⃣2️⃣ Here's the 42nd post highlighting key new features of the upcoming v258 release of systemd. #systemd258
Part of the protocol spoken between service processes and the service manager (if it is systemd, that is) are a number of environment variables. Specifically, $MAINPID and $MANAGERPID are two variables that have been part of the protocol for a long time: they contain the PID numbers of the main service process and of the service manager itself.
Lennart Poettering
The former is useful in auxiliary processes associated with a service (for example ExecStartPost=, ExecStop= or so) for interacting with the main process of the service, for example to send a UNIX signal to it. The latter may be useful for service code to auto-detect if it is invoked by a service manager (in which case getppid() must match $MANAGERPID) or from a shell.
But we live in 2025, and POSIX PIDs are quite broken: they are recycled too frequently to be useful as stable identifiers…
Lennart Poettering
…of processes, which creates a number of security and robustness issues. (These issues are not just theoretic, the PID space is so small that PID recycling is trivially easy to trigger and even happens by accident quite often).
New Linux kernels provide a way out: there are now "pidfds", i.e. fds that reference a specific processes. Which is a stable handle to processes, even beyond their lifetime. And their inode numbers are 64bit integers that are never recycled during…
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