Further Down Code2vec

Vector representations of code

Blog Further Down Code2vec

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Let us continue with our series on representing objects as vectors: natural language, code, let us take a deeper look at how code2vec works. Even personality profiles can be represented as vectors for data processing, and this was done in 1934, by hand!. So we certainly should be able to represent pieces of code faithfully as vectors. Let us see how that goes.

Recall from our last article on code embeddings that code2vec is, like word2vec and autoencoders, neural networks for an unrelated task which produce miraculous vectors representations of objects in their middle layers. This is pretty much all the theory one needs to know about this useful network as far as embedding is concerned.

In this article, we would like to take code2vec for a spin and produce some embeddings ourselves. Luckily, the authors of code2vec have generously given us their code and a tutorial on how to use and test their model. Sadly, however, this is geared towards their task of predicting a function’s name from its body, so we must still figure out a way to adapt it to our needs.

They also give us a preprocessed Java dataset of around 14 million examples, already split into training, testing and validation subsets, and pre-trained models both in ready for use and trainable versions. One can also choose to train a model from scratch. Either way, after training (or skipping it), what remains is to assess the accuracy of the model, which one can do with a single command:

Assessing code2vec performance.

python code2vec.py --load /path/to/model --test /path/to/data

Just this step, however, takes quite some time on a regular machine. Around 20 minutes on mine, giving a precision of 66%, recall of 53%, and so and an F score of 59%. Not bad but far from ideal, and this is consistent with the published results [1].

By the way, as this has not been said elsewhere in our machine learning for security series, let us take a break to clarify those terms above, which all refer to the accuracy of any search, and are just as applicable to vulnerability search in our context:

Precision and recall

Precision and recall via Wikipedia.

Suppose the dots are files in a repository. Some might actually contain vulnerabilities, that is the left half of the rectangle, labeled "relevant elements" above.

Ideally, a tool would detect them all, and nothing else.

Our vulnerability classifier, or maybe a human pentester, or any other tool, tags as vulnerable those contained in the circle. Some right (the true positives) and some wrong (the false positives). The files containing vulnerabilities which weren't tagged, labeled false negatives above, we like to call them escapes, those vulnerabilities which resist scrutiny. These are actually more dangerous than the false positives, since they are the ones that black hat hackers might take advantage of in the wild. But I digress.

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The precision is thus the ratio of actual vulnerabilities (true positives) found in the analysis to all those which were tagged, i.e., what percentage you can believe from what I say. The recall is how much of the reality is captured, that is, out of all the existent vulnerabilities, how many are picked up. The F1 score aims to combine both the precision and the recall and is defined as two times the ratio of their product to their sum

"F Score"

F Score.

But enough of information retrieval technicalities. Back to code2vec. We can run the model to get predictions. We say:

python code2vec.py --load /path/to/model --predict

Notice that even loading the model also takes a short while, but when ready, each prediction is really fast. Next, write a function body in the file Input.java, such as:

Input.java.

int f(Object target) {
  int i = 0;
  for (Object elem: this.elements) {
    if (elem.equals(target)) {
      return i;
    }
    i``;
  }
  return -1;
}

which clearly traverses an iterable object (v.g., a list) trying to find the given target, and returning its position within the object.

After updating the Input.java file, we get the possible function names as predicted by code2vec, each with their confidence:

Modify the file: "Input.java" and press any key when ready, or "q" / "quit" / "exit" to exit

Original name:  f
    (0.939002) predicted: ['index', 'of']
    (0.030483) predicted: ['get', 'index']
    (0.009410) predicted: ['get', 'child', 'index']
    (0.005901) predicted: ['find', 'item']
    (0.003169) predicted: ['get', 'tab', 'index']
    (0.002887) predicted: ['get', 'row', 'index']
    (0.002884) predicted: ['get', 'instruction', 'index']
    (0.002445) predicted: ['find', 'index', 'of']
    (0.002067) predicted: ['get', 'component', 'index']
    (0.001753) predicted: ['get', 'element', 'position']

All of those names make sense to me! indexOf and variations, or even getElementPosition would be good names for this particular function. Remember that this reflects on the training data, so if indexOf has the highest probability in the list, it must be because the model "remembers" a method with similar tokens whose name was indexOf.

However, it is much nicer to take a look at the demo they provide online, which has some built-in examples, including the one above. For each of those, the AST and the predictions are shown:

Online code2vec demo

Online code2vec demo.

However this is not what we’re after, but rather the vector embeddings. One can export both the token embeddings, i.e., the features, and the method name vectors, i.e., the labels, like this:

python3 code2vec.py --load /path/to/model --save_w2v tokens.txt

These are saved in word2vec format, which is quite simple:

$ head -2 token_vecs.txt
1294891 128
performrename 0.13724399 0.11635801 0.6363327 0.17189577 -0.4018513 ...

Here we see that the vocabulary size, i.e. the number of different tokens found was in the order of 1.3 million, and the tokens were embedded into vectors of length 128, which of course we snipped above. There we show only the first 5 entries in the vector for the token performrename. The size of these exported files is quite large, around 2 GiB each.

As it is at the moment, code2vec officially supports only Java and C#. There are third-party extractors for Python, C, C`` and TypeScript, too. That would easily comprise 90% of all of our clients' repositories.

In upcoming articles we will review code2seq, a new generation of code2vec which uses long short term memory (LSTM). Essentially from the user’s perspective, code2seq should be able to predict a target sequence instead of a single label at a time, which might be useful for our purposes.

References

  1. U. Alon, M. Zilberstein, O. Levy, and E. Yahav. code2vec: Learning Distributed Representations of Code Proc. ACM Program. Lang., Vol. 3, No. POPL, Article 40. January 2019.

  2. L. Thurstone (1934). The vectors of mind. Psychological Review 41 (1):1-32.

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