Entries tagged udev

Related tags: evil, jquery, kernels, kvm, linux kernel, stealing.

We have to be ready to do anything. Do you hear me?

Friday, 22 January 2010

Good people steal ideas, right? On that basis I setup a static domain to host the javascript and icons I use upon a few different sites & projects. This was preempted by the release of a new version of the excellent jQuery library.

I also managed to put together a tremendous hack to solve a pretty annoying problem running multiple distributions from a single external kernel under KVM.

Ubuntu users, in particular, will be well aware of dmesg SPAM coming from the use of CONFIG_SYSFS_DEPRECATED.

In short the way that the kernel presents information beneath the /sys tree has changed over the life of the kernel - and this has a knock-on effect to the userspace supplied by different distributions and releases of GNU/Linux.

Some distributions need an "old" kernel and an "old" udev with "old" udev rules in order to create the appropriate device nodes such that the kernel will boot & mount its filesystems. (i.e. These need CONFIG_SYSFS_DEPRECATED to be set.)

Conversely some distributions mandate a "new" minimum kernel version, and supply a "new" version of udev with "new" udev rules and they absolutely will not function when presented with an "old" kernel. (i.e. They must have kernels without CONFIG_SYSFS_DEPRECATED set.)

I've solved this problem via a kernel patch which is both evil and genius. The details are a little me-specific, but in short:

  • devtmpfs is used to setup and mount an initial /dev tree before /sbin/init is launched..
  • udev launches later and mounts a tmpfs over /dev such that it can start creating its own nodes.
  • At this point evil begins: I've patched the kernel such that any attempt to mount a tmpfs filesystem at /dev is silently changed to mount a devtmpfss filesystem instead.
    • The alternative is that udev creates many nodes, but manages to fail to create the root & swap nodes such that the KVM guests fail to boot.

Ultimately udev doesn't get an empty /dev tree to play with, instead it finds one already pre-populated, such that any devices it cannot create are there regardless - because the devtmpfs implementation has already created them.

Genius. And evil. So very evil.

Meh.

Steal that idea. I dare you .. (I'm impressed at how well devtmpfs works, and how easy I was able to make my "patch of evil"tm. Just a few lines in fs/namespace.c.)

ObSubject: The Last House On The Left

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