hisham hm

Fixing permission errors for scanning on Linux: running XSane as a regular user

I decided to stop running XSane as root. Here’s what I had to do:

* Restored the ownership of ~/.sane back to my user: sudo chown -R hisham ~/.sane
* Made /var/lock/sane writable by a regular user: sudo chmod a+rwxt /var/lock/sane
* As a quick-and-dirty fix, as suggested here, I gave permissions to my USB devices: sudo chmod a+rw -R /dev/bus/usb/*
* For a more permanent (and still untested) fix, I added a 40-libsane.rules file (found here) in /etc/udev/rules.d. This file already includes the USB id for my scanner. You may have to add yours.

You can’t automate SemVer, or: There is no way around Rice’s Theorem

Rice’s Theorem, proved in 1951, states that it is impossible to write a program that performs precisely any non-trivial analysis of the execution of other programs. More precisely, that’s impossible to code an analyzer for some non-trivial property that is able to decide whether an any given analyzed program has that property or not. And by “trivial property” we mean a property that either _all_ algorithms in the world have or _none_ has. So, yeah, “non-trivial property” is basically any property you can think of: “does it ever calculate 5 + 2”, “does it always use less than 10MB of memory”, “does it ever print something to the screen”, “does it ever access the network”?

At this point you might say “wait! I can write a program that checks if programs access the network or not! We can parse the code and if there are no calls whatsoever in it to any networking code such as connect(), then it doesn’t access the network!”. Sure, you can do that: but if the code has calls to connect(), you can’t decide for sure that it will access the network when it’s executed.

In 1936 Alan Turing proved that it is impossible to write a program that solves the Halting Problem, that is, to write an analyzer that checks programs and tells if it always terminates (”halts”) or it might enter an endless loop given some specific input. Okay, that’s a classic result, but that’s one property, how can Rice’s Theorem say we can’t make an analyzer for any property at all, even the silliest ones?

The proof for this amazingly powerful theorem is surprisingly simple. Turns out that if we had an analyzer for any silly property, we could use it to make a Halting Problem analyzer (which Turing proved to be impossible). Like this:

bool my_halting_problem_analyzer(Code analyzedProgram) {
   Code modifiedProgram = analyzedProgram + "; someCodeWithSillyProperty();"
   return my_silly_property_analyzer(modifiedProgram);

If the code in analyzedProgram always terminates, then the code in modifiedProgram will always reach the part that has the silly property, so my_silly_property_analyzer will return true, and my_halting_problem_analyzer returns true as well. If there is some input that makes the analyzedProgram hang in a loop, that means there’s some input that makes the silly property fail, resulting in false. Yay, we solved the Halting Problem using the silly property analyzer! Not.

Of course, this explanation is quite simplified1, so head to Wikipedia and your favorite formal languages book for the precise details. But the point stands that general semantic analysis of programs is impossible.

In particular, you can’t write a program that takes versions 1.0 and 1.1 of any program X and answer the question: “do they behave the same?”. In other words, it’s impossible to write an analyzer that looks at your master branch before you make a release and answers the question “should your new release tag be a major, minor or tiny release” according to the rules of SemVer (or any other API-compatibility-bound set of rules, for that matter).

This is because API compatibility is not only based on syntactically-expressible issues (that is, type signatures for functions and data structures). Any semantic changes to the code also break compatibility. A function may change its behavior but not its type signature (it still returns a string, but it used to be lower-case and it’s now upper-case), a struct can change they way it is used but the fields remain the same (field foo returned numbers from 0 to 10 and -1 when executed on Sundays, now it returns -1 on Saturdays as well). An automated tool won’t catch all this.

So, it is possible only to write a “pessimistic” tool, that may detect lots of situations syntactically and give the bad news: “hey, you must increment the major version here!”. But you can’t write a tool that is always able to look at code semantically and say the good news: “I assure you that no API behaviors have changed, you can safely name this a tiny version increase.”2

Yes, you can use test suites as an approximation for detecting semantic changes in API behaviors beyond type signatures and data structures. That would certainly improve your pessimistic analyzer — you’d be able to detect more situations where “you must increment major”. But even then it can only go so far, because in practice one can’t test for every possible input/output combination, so you still can’t be 100% sure. fuzz testing has uncovered bugs and unexpected behaviors even in programs with extensive test suites; as Dijkstra famously said, “Testing shows the presence, not the absence of bugs.” — likewise, test suites can show inconsistencies to the API specification, but not their adherence. So they can’t be taken to represent the semantics of a program entirely.

Anyway, in the end of the day, Rice’s Theorem shows us that general bullet-proof analysis of program behavior is not attainable, so no tool will ever be able to compare codebases and always tell us precisely that a new release is really “tiny-safe”. Semantic versioning just can’t be automated.

First time playing with OpenBSD

A list of notes on my first experience with OpenBSD:

* It wasn’t obvious at first which file to download, navigating their FTP site. I ended up with cd58.iso, which was the right choice. A tiny 7.8MB ISO!
* As soon as I typed “OpenBSD” as the VM name in VirtualBox, it gave me OpenBSD defaults. It defaulted to 64MB of RAM (only!), but I chose 256M, and the default 2G disk image.
* The first boot gave me a few options, including “Install” and “Autoinstall”. I chose “Autoinstall” since I thought that would install with the defaults, but it looked for an installation script in the local network (and obviously didn’t find it). Reboot, “Install” and off we go.
* No Dvorak options in the list of keyboards in the installer.
* I went with the default options and installed all offered packages (it offered me a list of coarse-grained bundles). It asked me about creating users, partitions, configuring network and if I wanted to run sshd by default.
* To enable Dvorak, the internet told me to do kbd us.dvorak nice.
* To make it permanent, I just grepped /etc for kbd and found out that cat us.dvorak > /etc/kbdtype would suffice.
* To install things, become root and then use pkg_add. For example: pkg_add wget
* By default it includes df but not free.
* To get color in the terminal, set TERM=wsvt25. htop showed in its usual colors!
* The default PATH includes the current (.) directory! Take that, Linux status quo!
* iconv is installed under /usr/local, and its library exports symbols with names such as libiconv_open instead of the usual iconv_open, which fools the typical AC_CHECK_LIB test in Autoconf. (In the source code, iconv_open works, so I guess iconv.h uses #define to translate the name. Added a hack to Dit to make it build cleanly out-of-the-box.

htop 2.0 released!

This week I finally released htop 2.0.0!

What’s new in htop 2.0

Since version 2.0, htop is now cross-platform!
Check out the video and slides of my presentation at FOSDEM 2016
about how this came to be. This release includes code supporting Linux, FreeBSD, OpenBSD and Mac OS X.

There are also, of course, some new features:

…And of course, lots of other tweaks and fixes!


The changelog with the main new changes follows below. Special thanks
to everyone who contributed for this release, through bug reports, bug
fixes, new features and financial support for the platform abstraction
layer project!

David Bowie in 1999 on the cultural impact of the internet

It’s impressive to see David Bowie’s foresight on the cultural impact of the internet back in 1999, and how the interviewer was completely oblivious to it:

« Bowie: [When I was really young,] it still produced sighs of horror from people when you said “I’m in rock’n'roll”. Now it’s a career opportunity. And the internet now carries that flag, from the subversive and possibly rebellious, and chaotic, and nihilistic… Forget about the Microsoft element: the monopolies do not have a monopoly — maybe on programs.

Interviewer: What you like about it is that anyone can say anything, or do anything?

Bowie: From where I am, by virtue of the fact that I am a pop singer and writer, I really embrace the idea that there’s a new demystification process going on between the artist and the audience. If you look back at this last decade, there hasn’t been a single entity, artist or group that personified or became the brand of the 90s. It started to fade in the 80s… in the 70s there were definitely such artists, in the 60s… the Beatles, and Hendrix… in the 50s there was Presley.

Now it’s sub-groups, it’s genres: it’s hip-hop, it’s girl-power, it’s a communal kind of thing. It’s about a community, it’s becoming more and more about the audience. Because the point of having someone who led the forces has dissapeared, because the vocabulary of rock is too well-known. It’s a currency that is not devoid of meaning anymore, but it’s become only conveyor of information, not a conveyor of rebellion, and the internet has taken on that, as I said. So I find that a terribly exciting era.

So, from my standpoint, being an artist, I’d like to see what the new construction is between artist and audience. There is a breakdown, personified I think about rave culture, where the audience is at least as important as the person who is playing at the rave. It’s almost like the artist is to accompany the audience and what the audience is doing. And that feeling is very much permeating music. And permeating the internet. »