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Fri, 6 Dec 2013

Another SELinux Lesson

— SjG @ 6:17 pm

So there’s this project that requires a ridiculously complicated communication protocol involving lots of byte-wrangling and formatting and weird transports. For the sake of brevity, I’ll only mention one of the endpoints, which requires decoding an email attachment.

Of course, this means a procmail script sending the email to PHP for processing and much ceremonious mucking about. On first go, it was failing. In debug mode, procmail was telling me that permission was denied. But it wasn’t user permissions: the file was owned by the same user as receiving the email and running the procmail script.

Naturally, when faced with cryptic permission failures, the first thing I did was look at /var/log/audit/audit.log and /var/log/messages for SELinux denials. There I found nothing at all. No errors, no warnings, no ugly “avc: denied” splatters.

Finally, this page here explained it to me. Rebuilding the policies with semodule -DB quickly revealed that my problem was, in fact, SELinux (as it all so often is). Once I could see the policies that were marked “dontaudit,” it was just an hour of building more and more complicated policies for procmail before stuff started working.

Once everything was good, happy, and shiny, a simple semodule -B returned the SELinux logging to the previous state, and I could once again spend my time fighting the convoluted bit-twiddling of the communication protocol.

Sat, 23 Nov 2013

Tic-Toc Bird

— SjG @ 2:38 pm

With no further commentary, I hereby offer you a Tic-Toc Bird of my own discoverytictoc-bird.

Sun, 10 Nov 2013

Diffraction Test

— SjG @ 8:28 pm

Reading different articles on macro photography, I see different opinions expressed on what the minimum aperture is that you can safely shoot at before you start losing out to diffraction. In many cases you’d like to shoot at the smallest possible aperture so as to eke out another few microns of field depth. The popular wisdom is that these two requirements meet somewhere between f/11 and f/22 (depending on who you’re reading).

For this crude test, I bolted the D90 to the desk, stuck on the Nikkor 105mm f/2.8 macro lens, put a ruler and a circuit board a short way away, focused manually, set up strobes (commanded via Nikon TTL) to light it, and then went through various apertures.

The Depth of Field range turned out as follows:

Full scene, f/4

Full scene, f/4

Full scene, f/45

Full scene, f/45

Nikon computes the effective aperture for the lens, so even though I had the lens wide open (and at f/2.8 according to the lens), the camera reports the corrected f/4.

So how about detail? Below are a collection of details at 100% crop from the same scene.

Just for kicks, I did a similar test with an image that was sort of kind of close to parallel to the focal plane.

My conclusions:
1. Pixel-peeping at 100% shows me that my focus isn’t great at any aperture.
2. While there’s a definite degradation at f/45, f/32 is barely worse than f/22. For absolute sharpest, it looks like f/11 is indeed the best.
3. Wide open, there are other aberrations at work.
4. Color and contrast shifts continue beyond the wide-open, although it gets more subtle.

Sat, 2 Nov 2013


— SjG @ 10:43 am

We had a Hack o’ Lantern this year, who responded to tweets, tweeted jokes, and “accidentally” revealed it’s plans for world domination.

Pure pumpkin cyber evil

Pure pumpkin cyber evil

For the World Domination Plan, I needed some artwork, which I hereby inflict upon the world.

They’re remixes. Tux is courtesy of Larry Ewing, Simon Budig, and Anja Gerwinski.



Mon, 7 Oct 2013

Dear Mr. Speaker

— SjG @ 8:02 pm

Dear Speaker Boerner,

I had thought that the current shutdown was simply obstructionism and petulance over the recent elections, but now I’m beginning to understand. You have realized that the hijacking of the Republican party by mad ideologues is untenable, and, for the good of the country, are euthanizing the party. It’s a good plan. At least you will leave a legacy of having taken decisive action, and putting the interests of the American People above petty politics. I congratulate you on your resolve.


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