Portability of tar features

The tar format is one of the oldest archive formats in use. It comes as no surprise that it is ugly — built as layers of hacks on the older format versions to overcome their limitations. However, given the POSIX standarization in late 80s and the popularity of GNU tar, you would expect the interoperability problems to be mostly resolved nowadays.

This article is directly inspired by my proof-of-concept work on new binary package format for Gentoo. My original proposal used volume label to provide user- and file(1)-friendly way of distinguish our binary packages. While it is a GNU tar extension, it falls within POSIX ustar implementation-defined file format and you would expect that non-compliant implementations would extract it as regular files. What I did not anticipate is that some implementation reject the whole archive instead.

This naturally raised more questions on how portable various tar formats actually are. To verify that, I have decided to analyze the standards for possible incompatibility dangers and build a suite of test inputs that could be used to check how various implementations cope with that. This article describes those points and provides test results for a number of implementations.

Please note that this article is focused merely on read-wise format compatibility. In other words, it establishes how tar files should be written in order to achieve best probability that it will be read correctly afterwards. It does not investigate what formats the listed tools can write and whether they can correctly create archives using specific features.

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New copyright policy explained

On 2018-09-15 meeting, the Trustees have given the final stamp of approval to the new Gentoo copyright policy outlined in GLEP 76. This policy is the result of work that has been slowly progressing since 2005, and that has taken considerable speed by the end of 2017. It is a major step forward from the status quo that has been used since the forming of Gentoo Foundation, and that mostly has been inherited from earlier Gentoo Technologies.

The policy aims to cover all copyright-related aspects, bringing Gentoo in line with the practices used in many other large open source projects. Most notably, it introduces a concept of Gentoo Certificate of Origin that requires all contributors to confirm that they are entitled to submit their contributions to Gentoo, and corrects the copyright attribution policy to be viable under more jurisdictions.

This article aims to shortly reiterate over the most important points in the new copyright policy, and provide a detailed guide on following it in Q&A form.

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Overriding misreported screen dimensions with KMS-backed drivers

With Qt5 gaining support for high-DPI displays, and applications starting to exercise that support, it’s easy for applications to suddenly become unusable with some screens. For example, my old Samsung TV reported itself as 7″ screen. While this used not to really matter with websites forcing you to force the resolution of 96 DPI, the high-DPI applications started scaling themselves to occupy most of my screen, with elements becoming really huge (and ugly, apparently due to some poor scaling).

It turns out that it is really hard to find a solution for this. Most of the guides and tips are focused either on proprietary drivers or on getting custom resolutions. The DisplaySize specification in xorg.conf apparently did not change anything either. Finally, I was able to resolve the issue by overriding the EDID data for my screen. This guide explains how I did it.

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Securing google-authenticator-libpam against reading secrets

I have recently worked on enabling 2-step authentication via SSH on the Gentoo developer machine. I have selected google-authenticator-libpam amongst different available implementations as it seemed the best maintained and having all the necessary features, including a friendly tool for users to configure it. However, its design has a weakness: it stores the secret unprotected in user’s home directory.

This means that if an attacker manages to gain at least temporary access to the filesystem with user’s privileges — through a malicious process, vulnerability or simply because someone left the computer unattended for a minute — he can trivially read the secret and therefore clone the token source without leaving a trace. It would completely defeat the purpose of the second step, and the user may not even notice until the attacker makes real use of the stolen secret.

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new* helpers can read from stdin

Did you know that new* helpers can read from stdin? Well, now you know! So instead of writing to a temporary file you can install your inline text straight to the destination:

src_install() {
  # old code
  cat <<-EOF >"${T}"/mywrapper || die
    exec do-something --with-some-argument
  dobin "${T}"/mywrapper

  # replacement
  newbin - mywrapper <<-EOF
    exec do-something --with-some-argument