OCaml Fuzz Testing with Crowbar

Core Philosophy

•One fuzz file per module: fuzz_foo.ml tests lib/foo.ml. Keeps tests organized and discoverable.
•Roundtrip everything: If you have encode and decode, test decode(encode(x)) = x.
•Crash-safety first: Parsers must never crash on arbitrary input, even malformed data.
•Boundary conditions matter: Test edge cases (0, max values, empty input, overflow).
•State machines need transition coverage: Test all valid and invalid state transitions.

Build Configuration

Dune setup for fuzz tests

lisp

(executable
 (name fuzz)
 (libraries crowbar borealis)
 (modules
  fuzz
  fuzz_common
  fuzz_foo
  fuzz_bar))

Main entry point (`fuzz/fuzz.ml`)

ocaml

(* Force linking of modules that register tests via side effects *)
let () =
  Fuzz_common.run ();
  Fuzz_foo.run ();
  Fuzz_bar.run ()

Each fuzz module ends with:

ocaml

let run () = ()

This ensures the module is linked and its add_test calls execute.

Test Patterns

1. Crash-safety test (parsers must not crash)

ocaml

open Crowbar
open Fuzz_common

let () =
  add_test ~name:"foo: decode" [ bytes ] @@ fun buf ->
  (match Foo.decode (to_bytes buf) with
   | Ok _ -> ()
   | Error _ -> ());
  check true

Key points:

•Use bytes generator for arbitrary binary input
•Match both Ok and Error branches (don't crash on either)
•check true signals test passed

2. Roundtrip test (encode/decode pairs)

ocaml

let () =
  add_test ~name:"foo: roundtrip" [ bytes ] @@ fun buf ->
  match Foo.decode (to_bytes buf) with
  | Error _ -> check true  (* Invalid input is fine *)
  | Ok original ->
      let encoded = Foo.encode original in
      match Foo.decode encoded with
      | Error _ -> fail "re-decode failed"
      | Ok decoded ->
          if original <> decoded then fail "roundtrip mismatch"
          else check true

Key points:

•If initial decode fails, that's OK (input was invalid)
•If re-decode fails after encode, that's a bug
•Compare original and decoded values

3. Constrained type roundtrip (smart constructors)

ocaml

(* For types with range constraints like 11-bit APID (0-2047) *)
let () =
  add_test ~name:"apid: roundtrip" [ range 2048 ] @@ fun n ->
  match Apid.of_int n with
  | None -> check (n < 0 || n > 2047)  (* Correctly rejected *)
  | Some apid ->
      let n' = Apid.to_int apid in
      if n <> n' then fail "roundtrip mismatch"
      else check true

4. Boundary tests

ocaml

(* Test boundary values explicitly *)
let () =
  add_test ~name:"apid: max_valid" [ const () ] @@ fun () ->
  match Apid.of_int 2047 with
  | None -> fail "2047 should be valid"
  | Some apid ->
      if Apid.to_int apid <> 2047 then fail "value mismatch"
      else check true

let () =
  add_test ~name:"apid: min_valid" [ const () ] @@ fun () ->
  match Apid.of_int 0 with
  | None -> fail "0 should be valid"
  | Some apid ->
      if Apid.to_int apid <> 0 then fail "value mismatch"
      else check true

Key points:

•Use [ const () ] for tests with no random input
•Never use [] as generator list (causes type error)

5. Invalid input rejection

ocaml

(* Values above max must be rejected *)
let () =
  add_test ~name:"apid: invalid_above" [ range 1000 ] @@ fun n ->
  let invalid = 2048 + n in
  match Apid.of_int invalid with
  | None -> check true
  | Some _ -> fail "should reject values > 2047"

(* Negative values must be rejected *)
let () =
  add_test ~name:"apid: invalid_negative" [ range 1000 ] @@ fun n ->
  let invalid = -(n + 1) in
  match Apid.of_int invalid with
  | None -> check true
  | Some _ -> fail "should reject negative values"

6. Pretty-printer safety

ocaml

(* pp functions must never crash *)
let () =
  add_test ~name:"foo: pp" [ bytes ] @@ fun buf ->
  match Foo.decode (to_bytes buf) with
  | Error _ -> check true
  | Ok v ->
      let _ = Format.asprintf "%a" Foo.pp v in
      check true

7. State machine transitions

ocaml

(* Test valid state transitions *)
let () =
  add_test ~name:"key: activate Pending" [ uint8; uint8; bytes ]
  @@ fun kid algo material_buf ->
  let material = to_bytes material_buf in
  if Bytes.length material > 0 then begin
    let key = Key.v ~kid ~algorithm:algo ~material in
    match Key.activate key with
    | Error _ -> check true  (* May fail if material invalid *)
    | Ok active_key ->
        if Key.state active_key <> Key.Active then fail "wrong state"
        else check true
  end
  else check true

(* Test invalid state transitions return errors *)
let () =
  add_test ~name:"key: activate Empty fails" [ uint8; uint8 ]
  @@ fun kid algo ->
  let key = Key.empty ~kid ~algorithm:algo in
  match Key.activate key with
  | Ok _ -> fail "should fail on Empty key"
  | Error (Key.Invalid_state_transition _) -> check true
  | Error _ -> fail "wrong error type"

8. Unit conversion roundtrips

ocaml

(* Test all unit conversions *)
let () =
  add_test ~name:"duration: ns_roundtrip" [ int64 ] @@ fun n ->
  let d = Duration.of_ns n in
  let n' = Duration.to_ns d in
  if n <> n' then fail "ns roundtrip mismatch"
  else check true

(* Test cross-unit conversions *)
let () =
  add_test ~name:"duration: us_to_ms" [ range 1000000 ] @@ fun n ->
  let us = Int64.of_int n in
  let d = Duration.of_us us in
  let ms = Duration.to_ms d in
  let expected = Int64.div us 1000L in
  if ms <> expected then fail "us to ms conversion failed"
  else check true

9. Filestore/resource operations

ocaml

(* Test create/exists invariant *)
let () =
  add_test ~name:"filestore: create_exists" [ bytes ] @@ fun name_buf ->
  let name = Bytes.to_string (to_bytes name_buf) in
  if String.length name = 0 then check true
  else
    let fs = Filestore.in_memory () in
    match Filestore.create fs name with
    | Error _ -> check true
    | Ok () ->
        if not (Filestore.exists fs name) then
          fail "created file should exist"
        else check true

Common Module: fuzz_common.ml

ocaml

(** Common utilities for fuzz tests. *)

open Crowbar

let to_bytes buf =
  let len = String.length buf in
  let b = Bytes.create len in
  Bytes.blit_string buf 0 b 0 len;
  b

let catch_invalid_arg f =
  try f () with Invalid_argument _ -> check true

let run () = ()

Generators Reference

Generator	Type	Use for
`bytes`	`string`	Arbitrary binary data
`uint8`	`int`	0-255
`int8`	`int`	-128 to 127
`int32`	`int32`	Full int32 range
`int64`	`int64`	Full int64 range
`range n`	`int`	0 to n-1
`bool`	`bool`	true/false
`const v`	`'a`	Fixed value (for no-input tests)
`list gen`	`'a list`	Lists of generated values
`option gen`	`'a option`	Some/None

File Organization

code

fuzz/
├── fuzz.ml              # Main entry, links all modules
├── fuzz_common.ml       # Shared utilities
├── fuzz_tc_frame.ml     # Tests for lib/frames/tc_frame.ml
├── fuzz_tm_frame.ml     # Tests for lib/frames/tm_frame.ml
├── fuzz_apid.ml         # Tests for lib/frames/apid.ml
├── fuzz_keyid.ml        # Tests for lib/sdls/keyid.ml
└── ...

Naming convention: fuzz_<module>.ml tests lib/**/<module>.ml

Running Fuzz Tests

Without AFL (quick check)

bash

dune exec fuzz/fuzz.exe

With AFL (thorough fuzzing)

bash

dune build fuzz/fuzz.exe
mkdir -p fuzz/input
echo -n "" > fuzz/input/empty
afl-fuzz -m none -i fuzz/input -o _fuzz -- \
  _build/default/fuzz/fuzz.exe @@

Check for duplicate test names

bash

grep -h 'add_test ~name:"' fuzz/fuzz_*.ml | \
  sed 's/.*~name:"\([^"]*\)".*/\1/' | sort | uniq -d

Coverage Checklist

For each module with a public API (.mli file):

• Crash safety: All decode_*, parse_*, read_*, of_* functions
• Roundtrip: All encode/decode, to_*/of_* pairs
• Boundaries: Min/max valid values, edge cases
• Invalid input: Values outside valid range rejected
• State machines: All transitions (valid and invalid)
• Pretty-printers: All pp_* functions don't crash
• Comparison: equal and compare are consistent

Priority Order

When adding fuzz tests to a codebase:

•Security-critical: Crypto primitives, authentication, key management
•Protocol parsers: Wire format decoders, frame parsers
•State machines: Lifecycle transitions, session state
•Constrained types: Smart constructors, ID validators
•Utility functions: Encoding helpers, time conversions

Common Mistakes

Wrong: Empty generator list with function

ocaml

(* ERROR: This expression should not be a function *)
add_test ~name:"test" [] @@ fun () -> ...

Right: Use `const ()` for no-input tests

ocaml

add_test ~name:"test" [ const () ] @@ fun () -> ...

Wrong: Ignoring error cases

ocaml

(* BAD: Only tests happy path *)
add_test ~name:"foo: decode" [ bytes ] @@ fun buf ->
  let Ok v = Foo.decode (to_bytes buf) in
  check true

Right: Handle both Ok and Error

ocaml

add_test ~name:"foo: decode" [ bytes ] @@ fun buf ->
  (match Foo.decode (to_bytes buf) with
   | Ok _ -> ()
   | Error _ -> ());
  check true

Wrong: Asserting on invalid input

ocaml

(* BAD: Fails on invalid input *)
add_test ~name:"foo: roundtrip" [ bytes ] @@ fun buf ->
  match Foo.decode (to_bytes buf) with
  | Error _ -> fail "decode failed"  (* Wrong! Invalid input is expected *)
  | Ok v -> ...

Right: Accept invalid input gracefully

ocaml

add_test ~name:"foo: roundtrip" [ bytes ] @@ fun buf ->
  match Foo.decode (to_bytes buf) with
  | Error _ -> check true  (* Invalid input is fine *)
  | Ok v -> ...

Expected Outputs

When adding fuzz tests, produce:

•New fuzz file: fuzz/fuzz_<module>.ml with comprehensive tests
•Update fuzz.ml: Add Fuzz_<module>.run () call
•Verify build: dune build succeeds
•No duplicates: Test names are unique across all fuzz files
•Coverage summary: List of tests added and what they cover

OCaml Fuzz Testing with Crowbar

Core Philosophy

Build Configuration

Dune setup for fuzz tests

Main entry point (fuzz/fuzz.ml)

Test Patterns

1. Crash-safety test (parsers must not crash)

2. Roundtrip test (encode/decode pairs)

3. Constrained type roundtrip (smart constructors)

4. Boundary tests

5. Invalid input rejection

6. Pretty-printer safety

7. State machine transitions

8. Unit conversion roundtrips

9. Filestore/resource operations

Common Module: fuzz_common.ml

Generators Reference

File Organization

Running Fuzz Tests

Without AFL (quick check)

With AFL (thorough fuzzing)

Check for duplicate test names

Coverage Checklist

Priority Order

Common Mistakes

Wrong: Empty generator list with function

Right: Use const () for no-input tests

Wrong: Ignoring error cases

Right: Handle both Ok and Error

Wrong: Asserting on invalid input

Right: Accept invalid input gracefully

Expected Outputs

Main entry point (`fuzz/fuzz.ml`)

Right: Use `const ()` for no-input tests