Local device access — from plugdev to logind

One of the more curious problems of running Linux on desktops is handling the local device access. The idea is usually quite simple: local users should have access to devices such as removable media (floppies, pendrives), scanners, speakers, webcams and ability to power off or reboot the computer. At the same time, remote users should have that access restricted.

Why? I think the main rationale behind this is that those users have physical access to those functions (yes, you could say that they have physical hard drive access too). They can insert the floppies, plug the pendrive, press the power button or just pull out the plug. They usually suffer the speaker noises and scare in front of the webcam.

At the same time, remote (or inactive users) shouldn’t be given the right to shut down the system unexpectedly, shout into the speakers, stream the user’s webcam or install Windows to his pendrive. I think that doesn’t need explaining.

I would like to shortly describe a few attempts to solve the problem and the issues with them.

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The pointless art of subslots

The sub-slots feature of EAPI 5 was announced as if it was the ultimate solution to the problem of SONAME changes on library upgrades. However, the longer I see it, the more I believe that it is not really a good solution, and that it misses the actual issue targeting somewhere nearby.

The issue is likely well-known by most of the Gentoo users. Every time a library changes its ABI, it changes the SONAME (the filename programs link to) to avoid breaking existing programs. When the package is upgraded, the new version is installed under the new name, and the old one is removed. As a direct result, all applications linking to the old version become broken and need to be rebuilt.

The classic way of handling this is to run the revdep-rebuild tool. It takes a while to scan the system with it but it supposedly finds all broken executables and initiates a rebuild of them. Of course, the system is in broken state until all relevant packages are rebuilt, and sometimes they just fail to build…

As you can guess, this is far from being perfect. That’s why people tried to find a better solution, and a few solutions were actually implemented. I’d like to describe them in a quasi-chronological order.

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A few words on the topic of optimization

Optimization is a very broad topic when referring to compiled languages like C or C++. There are many good guides on that topic. However, I see that people nevertheless forget about a few basic principles; thus I’d like to write a few short words myself, explaining how to avoid common pitfalls when optimizing or at least trying to.

First of all, I’d like to notice that I will be considering optimization as targeted towards making the program execution time shorter through use of faster code (algorithms, methods). I will not cover the area in detail but just give a few tips or remainders which should be taken into account when optimizing.

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The impact of C++ templates on library ABI

Author: Michał Górny
Date: 20 Aug 2012
Copyright: http://creativecommons.org/licenses/by/3.0/
Source: http://dev.gentoo.org/~mgorny/articles/the-impact-of-cxx-templates-on-library-abi.rst

Preamble

The general aspect of maintaining binary compatibility of C++ library interfaces has been already covered thoroughly multiple times. A good reference of articles on the topic can be found on wiki page of ABI compliance checker tool [1]. Sadly, those articles usually consider the topic of C++ templates only briefly, if at all.

While in fact the topic is fairly complex, and I believe that considering the overall usefulness and popularity of the templates, it should be considered more thoroughly. Thus, in this article I will try to address the issues arising from use of templates, methods of dealing with them and trying to prevent them.

Both the overall topic of templates in respect to the programming techniques, and the wide topic of ABI are already explained in detail in many other articles and guides. Moreover, I believe that myself I am not fluent enough to be able to cover those topics in detail here. Thus, I will assume that a reader of this article is already familiar with both the general topic of templates in C++, and the basic aspects of an ABI and its compatibility.

Moreover, in the solutions and problems listed here I will assume that a particular toolchain in question does conform to the C++98 standard, and is able to properly support templates with regard to multiple instantiations.

[1] http://ispras.linuxbase.org/index.php/ABI_compliance_checker#Articles

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Research question: integral type sizes on various platforms

I’m a bit curious about sizes of various integral types on different platforms, and I’d really appreciate a little help from people running various non-common architectures/toolchains. I’ve prepared a little package which just tries to get various type sizes using the C++ compiler, and I’d really appreciate if you could run it and paste the results in a comment.

To run it:

wget http://dev.gentoo.org/~mgorny/cxx-type-sizes-0.tar.bz2
tar -xf cxx-type-sizes-0.tar.bz2
cd cxx-type-sizes-0/
./configure
make
cat output/_all

It will try to compile a few programs, and then run them. Then it concatenates the results into output/_all and that’s the file I’d like to get, along with your platform, toolchain, CHOST and ARCH, ABI and everything else you consider relevant.

I’d really like to get a single output for each architecture, and possibly additional outputs if some toolchain/other magic resulted in different results than the previous one. I’ll put the results then into a nice table. Thanks in advance.

Current results.