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Shadow casting

For those of you who’d like to follow Google Summer of Code progress (I’d very much like to have the blogs of the student available on Universe by the way), today (or rather yesterday) can be counted in as a very important date 🙂

Seraphim prepared a patch for shadow (the package containing all the basic utilities for users management in Linux) to work with OpenPAM, rather than depending strictly on Linux-PAM. I committed it in tree now for and upstream applied it to trunk already, so now it can be built even when using OpenPAM rather than Linux-PAM. Cool, eh?

This patch (and the related bug) are important not only because they fix a very important part of the functionality of a Linux system with OpenPAM, but also because it will act as a base reference to fix other software to use OpenPAM too.

Indeed, one of the most obnoxious problems with OpenPAM is that a lot of packages instead of writing their own conversation functions rely on misc_conv. Seraphim prepared a patch that can be applied to almost any other package relying on that to make it OpenPAM-compatible. It’s very good.

Unfortunately, as Seraphim also blogged there is one catch with being able to provide OpenPAM support, especially for the future. The problem is that although mostly API compatible, OpenPAM and Linux-PAM are not ABI compatible. Although in a very subtle way, because, as Seraphim learnt, you can have a system built against Linux-PAM run against OpenPAM just fine, up to a point.

The problem is that ABI does not only refer to the name of the functions, or the type of their parameters, but also to the meaning of flag values. In this case, Linux-PAM and OpenPAM give different flags different meanings, so modules built against OpenPAM will not work properly with software built against Linux-PAM.

This is going to be tricky, especially once we’ll allow users to switch from one to the other and vice-versa, because it means all the software will have to be rebuilt to continue functioning as it’s supposed to. And no preserved-rebuild will help us there.

Oh well, there’s time to think of that!

Comments 1
  1. At least to prevent things from actively breaking, you can use versioned symbols for the functions involved (or just the whole lib). That will at least fail in the right way (at dynamic link time) instead of getting into all kinds of unexpected behaviour.

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