CBTF: Caught by the Fuzz! A minimalistic fuzz-test runner for R

The CBTF package implements a very simple mechanism for fuzz-testing functions in the public interface of an R package.

Fuzz testing helps identify functions lacking sufficient argument validation, and uncovers sets of inputs that, while valid by function signature, may cause issues within the function body.

The core functionality of the package is in the fuzz() function, which calls each provided function with a certain input and records the output produced. If an error or a warning is generated, this is captured and reported to the user, unless it matches a pattern of whitelisted messages, as specified in the ignore_patterns argument. The objects returned by fuzz() can be inspected with summary() and print().

Whitelisting can also be done after a fuzz run has been completed via the whitelist() function, so that only messages that need to be acted upon are actually shown. Using whitelist() has the advantage of not requiring the completion of a fuzz run of all functions over all inputs again.

The helper function get_exported_functions() identifies the functions in the public interface of a given package, facilitating the generation of the list of functions to be fuzzed.

The helper function test_inputs() is invoked by fuzz() if the user doesn’t specify the set of inputs to be tested. By default it generates a large set of potentially problematic inputs, but these can be limited just to the desired classes of inputs.

The helper function namify() can be used to generate automatically pretty names in the list of input object, which can improve the output, especially when structures such as data frames, matrices, and more complex objects are involved. These names are based on the deparsed representation of the unevaluated inputs.

At the moment the functionality of the package is extremely limited: it operates only on the first argument and it doesn’t introduce any randomness. However, it’s convenient when there are a large number of functions to test.

Usage

This is a simple example that displays how to use CBTF to fuzz an R package. We consider mime because it is small enough to run quickly and is likely installed on most systems.

library(CBTF)
funs <- get_exported_functions("mime")
(res <- fuzz(funs, what = list(TRUE)))

## ℹ Fuzzing 2 functions on 1 input
## ✖  🚨   CAUGHT BY THE FUZZ!   🚨
## 
## ── Test input: TRUE
##       guess_type  FAIL  a character vector argument expected
##  parse_multipart  FAIL  $ operator is invalid for atomic vectors
## 
##  [ FAIL 2 | WARN 0 | SKIP 0 | OK 0 ]

The first occurrence is a false positive, as the message returned indicates that the input was checked and the function returned cleanly. The second case instead reveals that the function didn’t validate its input: indeed, it expected an environment, and used the $ operation on it without checking.

The false positive result can be easily removed by whitelisting an appropriate pattern:

whitelist(res, "a character vector argument expected")

## ✖  🚨   CAUGHT BY THE FUZZ!   🚨
## 
## ── Test input: TRUE
##  parse_multipart  FAIL  $ operator is invalid for atomic vectors
## 
##  [ FAIL 1 | WARN 0 | SKIP 0 | OK 1 ]

Advanced uses

Better-looking output

When the inputs contains complex structures, it is better to provide a named list to the what argument of fuzz(): these names will be used instead of relying on deparsing of the input, which may be poor. A convenient way of generating names is by using the namify() helper function.

For example, compare this:

fuzz(funs, what = list(letters, data.frame(a = 1, b = "a")))

## ℹ Fuzzing 2 functions on 2 inputs
## ✖  🚨   CAUGHT BY THE FUZZ!   🚨
## 
## ── Test input: c("a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l",
##  parse_multipart  FAIL  $ operator is invalid for atomic vectors
## 
## ── Test input: structure(list(a = 1, b = "a"), class = "data.frame", row.names = c(NA,
##       guess_type  FAIL  a character vector argument expected
## 
##  [ FAIL 2 | WARN 0 | SKIP 0 | OK 2 ]

to this:

fuzz(funs, what = namify(letters, data.frame(a = 1, b = "a")))

## ℹ Fuzzing 2 functions on 2 inputs
## ✖  🚨   CAUGHT BY THE FUZZ!   🚨
## 
## ── Test input: letters
##  parse_multipart  FAIL  $ operator is invalid for atomic vectors
## 
## ── Test input: data.frame(a = 1, b = "a")
##       guess_type  FAIL  a character vector argument expected
## 
##  [ FAIL 2 | WARN 0 | SKIP 0 | OK 2 ]

Controlling the inputs tested

By default, fuzz() tests all the inputs produced by test_inputs(). However, this can be controlled by specifying the classes that should be tested:

test_inputs(use = c("scalar", "numeric", "integer", "matrix"))

Alternatively, one can specify the classes to be excluded:

test_inputs(skip = c("date", "raw"))

A vector of valid classes can be retrieved programmatically by setting this argument to “help”:

test_inputs("help")

##  [1] "all"        "scalar"     "numeric"    "integer"    "logical"   
##  [6] "character"  "factor"     "data.frame" "matrix"     "array"     
## [11] "date"       "raw"        "list"

Fuzzing list inputs automatically

It is trivial to augment a given set of inputs with list versions of the same. This effectively doubles the number of tests run with no additional coding effort.

fuzz(funs, what = namify(letters), listify_what = TRUE)

## ℹ Fuzzing 2 functions on 2 inputs
## ✖  🚨   CAUGHT BY THE FUZZ!   🚨
## 
## ── Test input: letters
##  parse_multipart  FAIL  $ operator is invalid for atomic vectors
## 
## ── Test input: list(letters)
##       guess_type  FAIL  a character vector argument expected
## 
##  [ FAIL 2 | WARN 0 | SKIP 0 | OK 2 ]

Fuzzing for arguments other than the first

At the moment, the only way to fuzz an argument other than the first is by currying the function, ensuring that the preceding arguments before are filled in.

For example, to fuzz the nrow argument of matrix(), we could do the following:

curried.matrix <- function(nrow) matrix(1:10, nrow = nrow)
fuzz("curried.matrix", what = list(NA, NULL))

## ℹ Fuzzing 1 function on 2 inputs
## ℹ Functions will be searched in the global namespace as 'package' was not specified
## ✖  🚨   CAUGHT BY THE FUZZ!   🚨
## 
## ── Test input: NA
##  curried.matrix  FAIL  invalid 'nrow' value (too large or NA)
## 
## ── Test input: NULL
##  curried.matrix  FAIL  non-numeric matrix extent
## 
##  [ FAIL 2 | WARN 0 | SKIP 0 | OK 0 ]

Funding

Development of CBTF is partially supported through the DFG programme “REPLAY: REProducible Luminescence Data AnalYses” No 528704761 led by Dr Sebastian Kreutzer (PI at Heidelberg University, DE) and Dr Thomas Kolb (PI at Justus-Liebig-University Giessen, DE). Updates on the REPLAY project at large are available at the REPLAY website.