Category Archives: Programming

FEniCS differences between Function, TrialFunction and TestFunction

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The FEniCS project allows for simple solution of partial differential equations. However, getting started from examples is so quick, it is easy to miss how the inner-workings of it behave. This should not happen (especially if it is a PhD project). There are three vital pieces of the puzzle that it might not be clear what they are for, and the documentation does not help here: the functions TrialFunction, TestFunction, and Function.

The thing is that, in the usual Finite Element Methods, we only have the distinction between the trial function u \in V_h and test function v \in V, and even then the distinction is often blurry. In fact, technically, the spaces V_h and V can be different. However, most of the time we take them to be the same (for example, the same discrete space of piecewise linear functions). The names we give them is just to have clearer in our heads what role they play in the game, but we could as well say “two functions in V“. Then why would FEniCS have even a three-fold difference? You can quickly check for yourself that, for example, if you swap the TrialFunction for a Function, FEniCS will complain.

There is no documentation on the topic as far as I have found, so these are my (maybe mistaken) deductions.

The meaning of the TrialFunction is to let FEniCS know what function we are solving for. In fact, when we call

FEniCS expects to find a TrialFunction in A, which is supposed to be the function we are looking for. On the other hand, u_ is the symbol in which the result will be stored, and this needs to be a Function (not Trial, not Test, just a regular Function).

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Brute force a crackme file password with Python

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I was to reverse a file for a challenge, MD5 hash 85c9feed0cb0f240a62b1e50d1ab0419.

The challenge was called mio cuggino, purposefully misspelled with two g letters. It asks for three numbers. The challenge led me to a brute force of the password with a Python script, learning how to interact with a subprocess stdin and stdout (SKIP to next section if you don’t care about context but only want the code).

Looking at the assembly with Radare, the first thing it does is to check that the numbers are non-negative and in increasing order. In details, it checks that:

  1. exactly three inputs have been provided;
  2. the first two are non-negative;
  3. the third is bigger than the second;
  4. the second is bigger than the first;
  5. the third is non-negative.

Very good, so the input pattern is three non-negative integers in increasing order. Fine. No clue about what those numbers should be though, yet.

Scroll the assembly just enough to unravel the magic.

A (pointer to) string is loaded into ebx, which contains the following Italian sentence:

Mi ha detto mio cuggino che una volta e’ stato co’ una che poi gli ha scritto sullo specchio benvenuto nell’AIDS, mio cuggino mio cuggino

The assembly basically takes the characters in the string that correspond to the first and second input (for ex, 0 as first input would map to the first char, M) and checks whether they are equal. If this is not satisfied, a Nope message is shown and the binary returns.

If this is satisfied, the same check is repeated with the third input (with the first one, although this doesn’t matter). If this is satisfied as well, a tricky sub.puts_640 function is called (with 5 inputs), and a Uhm message is shown.

Going to looking into that routine is absolutely useless as it’s completely unreadable, and even makes a bunch of additional calls that are further jumbled.

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Does C++ delete operator really free memory?

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Well, I have been wondering about this for quite a while now, and I have tried to run some tests to better understand what’s going on under the hood. The standard answer is that after you call delete you should not expect anything good from accessing that memory spot. However, this did not seem enough to me. What is it really happening when calling delete(ptr)? Even though there no standard behavior, what could happen, anyway? Here’s what I’ve found. I’m using g++ on Ubuntu 16.04, so this may play a role in the results.

What I first expected when using the delete operator was that the freed memory would be handed back to the system for usage in other processes. Let me say this does not happen under any of the circumstances I have tried.

Memory released with delete still seem to be allocated to the program it first allocated it with new. I have tried, and there is no memory usage decrease after calling delete. I had a software which allocated around 30MB of lists through new calls, and then released them with subsequent delete calls. What happened is that, looking at the System monitor while the program was running, even a long sleep after the delete calls, memory consumption my the program was the same. No decrease! This means that delete does not release memory to the system.

In fact, it looks like memory allocated by a program is his forever! However, the point is that, if deallocated, memory can be used again by the same program without having to allocate any more. I tried to allocate 15MB, freeing them, and then allocating another 15MB of data after, and the program never used 30MB. System monitor always showed it around 15MB. What I did, in respect to the previous test, was just to change the order in which things happened: half allocation, half deallocation, other half of allocation.

So, apparently memory used by a program can increase, but never shrink. Continue reading →

WordPress function url_to_postid not working with Anspress

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With the Anspress theme/plugin, using the WordPress native function url_to_postid() on a question permalink returns the page ID of the base page. For example,

would return the page ID of the /questions page.

To get the WP question ID of this-is-the-question, use the following:

(using get_post_types() instead of ‘questions’ allows the call to work even with permalinks of non-question contents.)

A method for calculating pattern relevance in a text

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This article aims at presenting a method for computing the relevance of a given string (pattern) in a text. This algorithm is at the core of my WordPress plugin Smart Tag Insert.

First of all, there is a difference between a simple pattern matching and computing text pattern relevance. The question we are trying to address here is the following: I have a string, and I would like to know how much that string is relevant for a specific text. For example, let’s say we have “download music” as the string of which relevance we are interested into. How can we determine how much relevant it is for a specific article?

The simple approach

The easy thing one could try is run a pattern match of “download music” in the article text. That is okay, but suppose the article contained strings like “download the music”, or “download some music”, or “downloading music”, or “download good quality music”. It is clear that, to a human, all these strings are equivalent when trying to understand what the article is about: it is about downloading music, regardless of whether it is good, bad, a lot or little.

A simple pattern match would fall short, because it would exclude all those other strings and make it look like the content is not very much about downloading music, just because “download music” was never found exactly that way.

So the first point we need to acknowledge if we want to try to teach a machine to compute text pattern relevance, is that we need to find a way, at least a rough way, to teach it to grasp the meaning of the content.

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Tutorial: Use PayPal Adaptive Payments API (with Embedded Lightbox)

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There are two things I am currently working on:

  1. School
  2. Integrating my Post Pay Counter WordPress plugin with PayPal, so that site administrators can pay their writers directly from their blog pages, without having to head to the PayPal website

Apparently, it turns out that you do not get to choose how much time you want to dedicate to the first point. That, matched with the fact that PayPal APIs are horrible and that the documentation is often useless, is making the whole process much longer than I thought. Meanwhile, I am sharing what I have come up with until now (which works pretty well, actually), scheduling a second part of the tutorial for when the job will be completed.

PayPal Adaptive Payments: what it is for

Adaptive payments handles payments between a sender of a payment and one or more receivers of the payment. You are an application owner, such as a merchant that owns a website, the owner of a widget on a social networking site, the provider of a payment application on mobile phones, and so on. Your application is the caller of Adaptive Payments API operations.

Standing to what I have been able to discover about PayPal’s different payment mechanisms over summer, Adaptive Payments represents the most flexible way to transfer funds from one account to another. Also, it seems the only method you can effectively integrate PayPal in your application. Adaptive Payments is in fact for those applications in which your account, as application developer, is not the one you are drawing funds from. Shortly, you need to move money on behalf of someone, and your application is the intermediary.

In my specific case, I needed a way to let administrators put their credentials into my plugin and have PayPal let me get money from their accounts and transfer it to their writers’ ones. Adaptive Payments method was really suitable because it allows six transactions per each request, so that it is possible to send different amounts to several people with only one API request. As bottom line (which I did not need and did not care to dive into), it also allows Chained Payments, in which the primary receiver passes part of the payment to other receivers, splitting the original amount.

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