# Fuzzing of concurrent programs

Posted on October 6, 2017

This is a summary of the main theoretical result about Cuzz1, a random testing tool to find concurrency bugs. It was mentioned in Mayur Naik’s seminar on software analysis and testing, here at Penn.

## Debugging concurrent programs

One reason for the difficulty of concurrent programming is because there are so many possible ways for threads to interact by interleaving their executions. For two threads with 20 instructions each, there are already $$\binom{40}{20} = 137,846,528,820$$ possible interleavings to consider, and that number grows exponentially in the size of the program, so trying all interleavings isn’t a scalable solution to test concurrent programs.

The next best thing is to try only some interleavings. As always in testing, the problem is choosing good test cases.

Of course, if a bug requires our program to execute in a very specific order to manifest itself, then we most likely won’t catch it. But it will also occur very rarely in production, which diminishes its impact.

On the contrary, actual bugs are quite simple in practice, both in concurrent and sequential programming: the conditions to trigger them can be described succintly, and they involve only a small fragment of the program, e.g., “I forgot a lock at line X”. We can take advantage of that fact, called the “small bug hypothesis”.

Concurrency bugs usually happen because a few instructions were executed in an order unforeseen by the programmer. So a natural way to describe a bug is in terms of ordering constraints between these instructions.

In this post, we shall consider a simplified situation, where programs have no loops and no synchronization (threads don’t block), to focus more on the combinatorial idea at the core of the result. The formalization in the paper generalizes it to take these complications into account while maintaining the same probabilistic guarantee. I will say a bit more about it in the conclusion.

Formally, an ordering constraint is an ordered pair of instructions $$(X, Y)$$, expressing the requirement that the instruction $$X$$ must execute before instruction $$Y$$. A bug’s depth is the size of the smallest set of ordering constraints sufficient to trigger it. In this setting, the small bug hypothesis claims that most bugs that occur in practice have a small depth.

### An example

Consider the following totally artificial two-threaded program, Thread 1 does some initial computation during the first $$(r-2)$$ instructions and opens a file with handle x. Thread 2 may read it to then do some other work in the last $$(s-2)$$ instructions. However, Thread 2 is also allowed to run before Thread 1, in which case it should not attempt to read from the uninitialized file handle x. Thus we added a flag that Thread 1 switches on when it opens the file, so Thread 2 may know whether it is safe to read.

Initial:
flag = 0
x = null
y = null

|  1 to r-2:               |  r+1: if (flag == 1)    |
|     ...                  |  r+2:   y = read(x)     |
|     (r-2 instructions)   |  r+3 to r+s:            |
|     ...                  |    ...                  |
|  r-1: flag = 1           |    (s-2 instructions)   |
|  r  : x = open("file")   |    ...                  |

Since this is a post about testing, there is a bug in that program. Thread 1 sets the flag before setting x; if it gets interrupted between instructions r-1 and r, Thread 2 may still read x. This bug has depth 2: instruction r-1 must execute before r+1; instruction r+2 before r.

Naively, there are $$\binom{r+s}{r}$$ possible interleavings of these two threads. How many are buggy? There are $$(s-1)$$, depending on when r is executed. If we set $$r=s=\frac{k}{2}$$, the proportion of buggy programs is asymptotically equivalent to $$\frac{k^2}{2^k}$$, which decreases exponentially in the size of the program. In other words, if you add just one instruction in your program, expect to wait twice as long for the bug to appear!

## Cuzz: testing with guarantees

The high-level idea behind Cuzz is that there is a subset of possible interleavings which reveals shallow bugs with a much higher probability than with the whole set.

Given a program with $$n$$ threads and $$k$$ instructions, we show that we can find a bug of depth $$d$$ with probability at least $$\frac{1}{nk^{d-1}}$$. In particular, for a constant $$d$$, the denominator is a polynomial in the size of the program.

Note that the number $$nk^{d-1}$$ has a straightforward combinatorial interpretation. It’s the number of combinations2 made of:

• one thread in a set of $$n$$;
• a sequence of $$(d-1)$$ instructions3 in a set of $$k$$.

We will explain how to map those combinations to interleavings such that any bug of depth $$d$$ is triggered by at least one of them. The probabilistic guarantee which we announced earlier follows by uniform sampling of those combinations.

### Program scheduling

A combination as we described it above can be summarized in the following diagram, where * marks the chosen thread and numbers #i index the chosen instructions. Here, $$n=3$$, $$d=3$$, $$k=12$$.

|  Thread 1:      |  * Thread 2:    |  Thread 3:      |
|  ...            |  ...            |  ...            |
|  ... #1         |  ...            |  ...            |
|  ...            |  ... #2         |  ...            |
|  ...            |  ...            |  ...            |

We assign a priority number to each instruction, equal to:

• i, if #i is the nearest index above it;
• or 0, if the instruction belongs to the marked thread;
• or -t, if it belongs to an unmarked Thread t.
|  Thread 1:      |  * Thread 2:    |  Thread 3:      |
|  ...    %-1     |  ...    % 0     |  ...    %-3     |
|  ... #1 % 1     |  ...    % 0     |  ...    %-3     |
|  ...    % 1     |  ... #2 % 2     |  ...    %-3     |
|  ...    % 1     |  ...    % 2     |  ...    %-3     |

Schedule instructions with the lowest priority numbers first to produce an interleaving.4

|  Thread 1:      |  * Thread 2:    |  Thread 3:      |
|                 |                 |  ...    %-3     |
|                 |                 |  ...    %-3     |
|                 |                 |  ...    %-3     |
|                 |                 |  ...    %-3     |
|  ...    %-1     |                 |                 |
|                 |  ...    % 0     |                 |
|                 |  ...    % 0     |                 |
|  ... #1 % 1     |                 |                 |
|  ...    % 1     |                 |                 |
|  ...    % 1     |                 |                 |
|                 |  ... #2 % 2     |                 |
|                 |  ...    % 2     |                 |

Since there are more interleavings than combinations, not all interleavings can be obtained in that way. In particular, these interleavings can only context-switch $$n+d-2$$ times; in comparison, it’s theoretically possible to context-switch once for (almost) every one of the $$k$$ instructions. In the example above, $$n+d-2 = 3 + 3 - 2 = 4$$.

### Finding a bug

We now show that any bug of depth $$d$$ can be found by some interleaving of the form above.

A bug of depth $$d$$ is triggered by an interleaving $$I$$ satisfying a certain set of $$d$$ ordering constraints $$(X_1,Y_1),\dots,(X_d,Y_d)$$. We can renumber the constraints so that the $$Y_i$$ appear in the order of execution for that interleaving.

|  Thread 1:     |  Thread 2:     |  Thread 3:     |
|                |  ...           |                |
|                |  ...           |                |
|  ...           |                |                |
|  X1            |                |                |
|                |                |  X2            |
|                |                |  Y1            |
|                |  ...           |                |
|                |  X3            |                |
|  Y2            |                |                |
|                |                |  ...           |
|  ...           |                |                |
|                |                |  Y3            |

A combination resulting in an interleaving which satisfies the same constraints is given by:

• the thread containing $$Y_1$$;
• the instructions $$Y_2,\dots,Y_d$$, in that order.
|  Thread 1:     |  Thread 2:     |  * Thread 3:   |
|  ...    %-1    |                |                |
|  X1     %-1    |                |                |
|                |  ...    %-2    |                |
|                |  ...    %-2    |                |
|                |  ...    %-2    |                |
|                |  X3     %-2    |                |
|                |                |  X2     % 0    |
|                |                |  Y1     % 0    |
|                |                |  ...    % 0    |
|  Y2  #1 % 1    |                |                |
|  ...    % 1    |                |                |
|                |                |  Y3  #2 % 2    |

To prove that the constraints are indeed satisfied, we may show that for every ordering constraint $$(X_i,Y_i)$$, the instruction $$X_i$$ is above $$Y_i$$ in the same thread, or is in another thread with a smaller priority number than $$Y_i$$.

Indeed, by construction, $$Y_i$$ is assigned the priority number $$i-1$$. The only way for $$X_i$$ to be assigned a greater priority number is if there is some $$Y_j$$ above it such that $$j > i$$. This is not possible, because the original interleaving $$I$$ satisfies the ordering constraints, so $$X_i$$ must have been executed before $$Y_i$$, and the $$Y$$ were numbered in execution order, so $$Y_i$$ must have been executed before $$Y_j$$. Therefore $$X_i$$ was executed before $$Y_j$$, so the latter may not appear before the former in program order.

## Conclusion

To finish, a few words about the more general result presented by the paper. Programs with potentially blocking operations and arbitrary control flow are abstracted as trees of execution traces (i.e., interleavings) of depth $$k$$, rather than sequences of $$k$$ instructions. Then, ordering constraints relate dynamic events produced by each thread as they progress, instead of static instructions. The role of combinations is played more algorithmically by a scheduler to pick out a trace which satisfies the ordering constraints and provoke a corresponding bug.

The bound of $$\frac{1}{nk^{d-1}}$$ for that algorithm is tight. As one particular case, for the example at the beginning with a bug of depth 2, Cuzz must select the combination of Thread 2 and instruction r, which happens with probability exactly $$\frac{1}{2k}$$. That is also much greater than the $$1/\binom{k}{k/2}$$ ratio we would get by sampling all possible interleavings uniformly5. Similar extreme examples can be constructed for any triple $$(n,k,d)$$. Yet there are many other bugs with looser and more varied ordering constraints which can thus be found with probability much higher than that bound.

1. *A randomized scheduler with probabilistic guarantees of finding bugs*, Sebastian Burckhardt, Pravesh Kothari, Madanlal Musuvathi, and Santosh Nagarakatte, ASPLOS’10.

2. The word “combinations” already has a standard definition in combinatorics, but in this blog post I’m giving it a different meaning.

3. We can actually select only sequences of distinct instructions, so the denominator would actually be $$n\cdot k(k-1)\dots(k-d+2)$$, but $$k$$ is big and $$d$$ is small, so $$nk^{d-1}$$ is close enough.

4. Synchronization and control flow would cause issues here, as they prevent the instructions from being executed in an arbitrary order of priority. That is why we made a simplifying assumption at the beginning.

5. Never mind the fact that common schedulers are biased in ways that make various bugs like the one above even less likely to be found than using uniform distribution.