In Fedora, your initramfs contains a copy of your sysctl settings

March 9, 2018

It all started when I discovered that my office workstation had wound up with its maximum PID value set to a very large number (as mentioned in passing in this entry). I managed to track this down to a sysctl.d file from Fedora's ceph-osd RPM package, which I had installed for reasons that are not entirely clear to me. That was straightforward. So I removed the package, along with all of the other ceph packages, and rebooted for other reasons. To my surprise, this didn't change the setting; I still had a kernel.pid_max value of 4194304. A bunch of head scratching ensued, including extreme measures like downloading and checking the Fedora systemd source. In the end, the culprit turned out to be my initramfs.

In Fedora, dracut copies sysctl.d files into your initramfs when it builds one (generally when you install a kernel update), and there's nothing that forces an update or rebuild of your initramfs when something modifies what sysctl.d files the system has or what they contain. Normally this is relatively harmless; you will have sysctl settings applied in the initramfs and then reapplied when sysctl runs a second time as the system is booting from your root filesystem. If you added new sysctl.d files or settings, they won't be in the initramfs but they'll get set the second time around. If you changed sysctl settings, the initramfs versions of the sysctl.d files will set the old values but then your updated settings will get set the second time around. But if you removed settings, nothing can fix that up; the old initramfs version of your sysctl.d file will apply the setting, and nothing will override it later.

(In Fedora 27's Dracut, this is done by a core systemd related Dracut module in /usr/lib/dracut/modules.d, 00systemd/

It's my view that this behavior is dangerous. As this incident and others have demonstrated, any time that normal system files get copied into initramfs, you have the chance that the live versions will get out of sync with the versions in initramfs and then you can have explosions. The direct consequence of this is that you should strive to put as little in initramfs as possible, in order to minimize the chances of problems and confusion. Putting a frozen copy of sysctl.d files into the initramfs is not doing this. If there are sysctl settings that have to be applied in order to boot the system, they should be in a separate, clearly marked area and only that area should go in the initramfs.

(However, our Ubuntu 16.04 machines don't have sysctl.d files in their initramfs, so this behavior isn't universal and probably isn't required by either systemd or booting in general.)

Since that's not likely to happen any time soon, I guess I'm just going to have to remember to rebuild my initramfs any time I remove a sysctl setting. More broadly, I should probably adopt a habit of preemptively rebuilding my initramfs any time something inexplicable is going on, because that might be where the problem is. Or at least I should check what the initramfs contains, just in case Fedora's dracut setup has decided to captured something.

(It's my opinion that another sign that this is a bad idea in general is there's no obvious package to file a bug against. Who is at fault? As far as I know there's no mechanism in RPM to trigger an action when files in a certain path are added, removed, or modified, and anyway you don't necessarily want to rebuild an initramfs by surprise.)

PS: For extra fun you actually have multiple initramfses; you have one per installed kernel. Normally this doesn't matter because you're only using the latest kernel and thus the latest initramfs, but if you have to boot an earlier kernel for some reason the files captured in its initramfs may be even more out of date than you expect.

Written on 09 March 2018.
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Last modified: Fri Mar 9 23:00:24 2018
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