drchaos

Fuzzing is an automated bug finding technique, where randomized inputs are fed to a target program in order to get it to crash. With fuzzing, you can increase your test coverage to find edge cases and trigger bugs more effectively.

drchaos extends the Nim interface to LLVM/Clang libFuzzer, an in-process, coverage-guided, evolutionary fuzzing engine. And adds support for structured fuzzing. The user should define the input type, as a parameter to the target function and the fuzzer is responsible for providing valid inputs. Behind the scenes it uses value profiling to guide the fuzzer past these comparisons much more efficiently than simply hoping to stumble on the exact sequence of bytes by chance.

Usage

For most cases, it is fairly trivial to define a data type and a target function that performs some operations and checks if the invariants expressed as assert conditions still hold. See What makes a good fuzz target for more information. Then call defaultMutator with that function as parameter. That fuzz target can be as basic as defining a fixed-size type and ensuring the software under test doesn't crash like:

import drchaos

proc fuzzMe(s: string, a, b, c: int32) =
  # function under test
  if a == 0xdeadc0de'i32 and b == 0x11111111'i32 and c == 0x22222222'i32:
    if s.len == 100: doAssert false

func fuzzTarget(data: (string, int32, int32, int32)) =
  let (s, a, b, c) = data
  fuzzMe(s, a, b, c)

defaultMutator(fuzzTarget)

WARNING: Fuzz targets must not modify the input variable. This can be ensured by using .noSideEffect and {.experimental: "strictFuncs".}

Or complex as shown bellow:

import drchaos

type
  ContentNodeKind = enum
    P, Br, Text
  ContentNode = object
    case kind: ContentNodeKind
    of P: pChildren: seq[ContentNode]
    of Br: discard
    of Text: textStr: string

func `==`(a, b: ContentNode): bool =
  if a.kind != b.kind: return false
  case a.kind
  of P: return a.pChildren == b.pChildren
  of Br: return true
  of Text: return a.textStr == b.textStr

func fuzzTarget(x: ContentNode) =
  # Convert or translate `x` to any format (JSON, HMTL, binary, etc...)
  # and feed it to the API you are testing.

defaultMutator(fuzzTarget)

drchaos will generate millions of inputs and run fuzzTarget under a few seconds. More articulate examples, such as fuzzing a graph library are in the examples/ directory.

Defining a == proc for the input type is necessary. proc default(_: typedesc[T]): T can also be overloaded. Which is especially useful when nil for ref is not an acceptable value.

Needed config

Compile with at least nim c --cc:clang -d:useMalloc -t:"-fsanitize=fuzzer,address,undefined" -l:"-fsanitize=fuzzer,address,undefined" -d:nosignalhandler --nomain:on -g, --mm:arc|orc is recommended.

Sample nim.cfg and .nimble files

Alternatively, drchaos provides structured input for fuzzing with nim-testutils Which includes a convenient testrunner

Post-processors

Sometimes it is necessary to adjust the random input in order to add magic values or dependencies between some fields. This is supported with a post-processing step, which for performance and clarity reasons only runs on compound types such as object/tuple/ref/seq/string/array/set and by exception distinct types.

proc postProcess(x: var ContentNode; r: var Rand) =
  if x.kind == Text:
    x.textStr = "The man the professor the student has studies Rome."

Custom mutator

Besides defaultMutator there is also customMutator which allows more fine-grained control of the mutation procedure, like uncompressing a seq[byte] then calling runMutator on the raw data and compressing the output again.

func myTarget(x: seq[byte]) =
  var data = uncompress(x)
  ...

proc myMutator(x: var seq[byte]; sizeIncreaseHint: Natural; r: var Rand) =
  var data = uncompress(x)
  runMutator(data, sizeIncreaseHint, r)
  x = compress(data)

customMutator(myTarget, myMutator)

User-defined mutate procs

It's possible to use distinct types to provide a mutate overload for fields that have interesting values, like file signatures or to limit the search space.

# Fuzzed library
when defined(runFuzzTests):
  type
    ClientId = distinct int

  proc `==`(a, b: ClientId): bool {.borrow.}
else:
  type
    ClientId = int

# In a test file
import drchaos/mutator

const
  idA = 0.ClientId
  idB = 2.ClientId
  idC = 4.ClientId

proc mutate(value: var ClientId; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
  # use `rand()` to return a new value.
  repeatMutate(r.sample([idA, idB, idC]))

For aiding the creation of mutate functions, mutators for every supported type are exported by drchaos/mutator.

User-defined serializers

User overloads must use the following proc signatures:

proc fromData(data: openArray[byte]; pos: var int; output: var T)
proc toData(data: var openArray[byte]; pos: var int; input: T)
proc byteSize(x: T): int {.inline.} ## The size that will be consumed by the serialized type in bytes.

This is only necessary for objects that contain raw pointers. mutate, default and == must also be defined. drchaos/common exports read/write procs that assist with this task.

Dos and don'ts

• Don't echo in a fuzz target as it slows down execution speed.
• Prefer -d:danger for maximum performance.
• Once you have a crash you can recompile with -d:debug and pass the crashing test case as parameter.
• Use debugEcho(x) in a target to print the crashing input.
• You could compile without sanitizers, AddressSanitizer slows down programs by ~2x, but it's not recommended.

What's not supported

• Polymorphic types, missing serialization support.
• References with cycles. A .noFuzz custom pragma will be added soon for cursors.
• Object variants work only with the lastest memory management model --mm:arc/orc.

Why choose drchaos

drchaos has several advantages over frameworks derived from FuzzDataProvider which struggle with dynamic types that in particular are nested. For a better explanation read an article written by the author of Fuzzcheck.

Bugs found with the help of drchaos

Nim reference implementation

• use-after-free bugs in object variants
• openArray on empty seq triggers UB

License

Licensed and distributed under either of

• MIT license: LICENSE-MIT or http://opensource.org/licenses/MIT

or

• Apache License, Version 2.0, (LICENSE-APACHEv2 or http://www.apache.org/licenses/LICENSE-2.0)

at your option. These files may not be copied, modified, or distributed except according to those terms.