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Monday, 31 August 2015 10:51

Systemd's latest conquest: the 'su' command

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In what appears to be a relentless march to absorb more and more of a Linux/UNIX system, systemd, touted as an init system, has now incorporated the su command.

This command su is used within a shell to obtain root status for performing administrative tasks on a Linux/UNIX system. It is not a full root login.

But to Red Hat Linux employee Lennart Poettering, the main developer of systemd, su is not good enough.

He writes: "Well, there have been long discussions about this, but the problem is that what 'su' is supposed to do is very unclear. On one hand it's supposed to open a new session and change a number of execution context parameters (uid, gid, env, ...), and on the other it's supposed to inherit a lot concepts from the originating session (tty, cgroup, audit, ...).

"Since this is so weakly defined it's a really weird mix&match of old and new paramters (sic). To keep this somewhat managable (sic) we decided to only switch the absolute minimum over, and that excludes XDG_RUNTIME_DIR, specifically because XDG_RUNTIME_DIR is actually bound to the session/audit runtime and those we do not transition. Instead we simply unset it.

"Long story short: "su" is really a broken concept. It will given (sic) you kind of a shell, and it's fine to use it for that, but it's not a full login, and shouldn't be mistaken for one."

Poettering's substitute command is machinectl shell; it also enables one to establish an su-like session on a different container or virtual machine.

At least one drawback could be present: many servers disable direct root logins, except from the system console itself, in order to track who is logging in and what they are doing. Poettering's new command appears to break this audit trail.

As a senior Debian developer, Craig Sanders, put it some time ago, systemd is "absorbing way too many low-level system functions into itself – udev has been merged; it does logging; has half-arsed substitutes for ntpd, cron, automount, inetd, and network configuration. This feature-creep is on-going, with more being absorbed into systemd all the time... and announced just a few days ago, a console daemon to replace the kernel's virtual terminals.

"Apart from the inevitable problems associated with being a jack-of-all-trades (and) master-of-none, the result will be the death of innovation for all functions absorbed into systemd as it is impossible to replace any one of them without replacing systemd entirely... which makes the job of developing improvements just too big a job.

"Right now, we have several alternatives to choose between for cron, ntp, logging, etc – each of them with different advantages and disadvantages. With systemd, it becomes a one-size-fits-all-or-else situation. If what it does doesn't suit you then tough luck, because you can't replace it without breaking your system.

"The second major problem with systemd is that it is becoming (or has become) mandatory - unnecessary dependencies on logind or systemd itself make it nearly impossible to avoid having systemd installed."

It remains to be seen which other functions systemd will seek to take over. As one wag put it, it might come to the point where one has just systemd and the kernel making up a LInux distribution.


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Sam Varghese

Sam Varghese has been writing for iTWire since 2006, a year after the site came into existence. For nearly a decade thereafter, he wrote mostly about free and open source software, based on his own use of this genre of software. Since May 2016, he has been writing across many areas of technology. He has been a journalist for nearly 40 years in India (Indian Express and Deccan Herald), the UAE (Khaleej Times) and Australia (Daily Commercial News (now defunct) and The Age). His personal blog is titled Irregular Expression.

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