Getting started with MojoLPM

Note: Using MojoLPM to fuzz your Mojo interfaces is intended to be simple, but there are edge-cases that may require a very detailed understanding of the Mojo implementation to fix. If you run into problems that you can‘t understand readily, send an email to and cc and we’ll try and help.

Prerequisites: Knowledge of libfuzzer and basic understanding of Protocol Buffers and libprotobuf-mutator. Basic understanding of testing in Chromium.

This document will walk you through:

  • An overview of MojoLPM and what it's used for.
  • Adding a fuzzer to an existing Mojo interface using MojoLPM.

Overview of MojoLPM

MojoLPM is a toolchain for automatically generating structure-aware fuzzers for Mojo interfaces using libprotobuf-mutator as the fuzzing engine.

This tool works by using the existing “grammar” for the interface provided by the .mojom files, and translating that into a Protocol Buffer format that can be fuzzed by libprotobuf-mutator. These protocol buffers are then interpreted by a generated runtime as a sequence of mojo method calls on the targeted interface.

The intention is that using these should be as simple as plugging the generated code in to the existing unittests for those interfaces - so if you've already implemented the necessary mocks to unittest your code, the majority of the work needed to get quite effective fuzzing of your interfaces is already complete!

Choose the Mojo interface(s) to fuzz

If you‘re a developer looking to add fuzzing support for an interface that you’re developing, then this should be very easy for you!

If not, then a good starting point is to search for interfaces in codesearch. The most interesting interfaces from a security perspective are those which are implemented in the browser process and exposed to the renderer process, but there isn't a very simple way to enumerate these, so you may need to look through some of the source code to find an interesting one.

For the rest of this guide, we'll write a new fuzzer for blink.mojom.CodeCacheHost, which is defined in third_party/blink/public/mojom/loader/code_cache.mojom.

We then need to find the relevant GN build target for this mojo interface so that we know how to refer to it later - in this case that is //third_party/blink/public/mojom:mojom_platform.

Find the implementations of the interfaces

If you are developing these interfaces, then you already know where to find the implementations.

Otherwise a good starting point is to search for references to “public blink::mojom::CodeCacheHost”. Usually there is only a single implementation of a given Mojo interface (there are a few exceptions where the interface abstracts platform specific details, but this is less common). This leads us to content/browser/renderer_host/code_cache_host_impl.h and CodeCacheHostImpl.

Find the unittest for the implementation

Unfortunately, it doesn‘t look like CodeCacheHostImpl has a unittest, so we’ll have to go through the process of understanding how to create a valid instance ourselves in order to fuzz this interface.

Since this interface runs in the Browser process, and is part of /content, we're going to create our new fuzzer in /content/test/fuzzer.

Add our testcase proto

First we'll add a proto source file, code_cache_host_mojolpm_fuzzer.proto, which is going to define the structure of our testcases. This is basically boilerplate, but it allows creating fuzzers which interact with multiple Mojo interfaces to uncover more complex issues. For our case, this will be a simple file:

syntax = "proto2";

package content.fuzzing.code_cache_host.proto;

import "third_party/blink/public/mojom/loader/code_cache.mojom.mojolpm.proto";

message NewCodeCacheHost {
  required uint32 id = 1;

message RunUntilIdle {
  enum ThreadId {
    IO = 0;
    UI = 1;

  required ThreadId id = 1;

message Action {
  oneof action {
    NewCodeCacheHost new_code_cache_host = 1;
    RunUntilIdle run_until_idle = 2;
    mojolpm.blink.mojom.CodeCacheHost.RemoteMethodCall code_cache_host_call = 3;

message Sequence {
  repeated uint32 action_indexes = 1 [packed=true];

message Testcase {
  repeated Action actions = 1;
  repeated Sequence sequences = 2;
  repeated uint32 sequence_indexes = 3 [packed=true];

This specifies all of the actions that the fuzzer will be able to take - it will be able to create a new CodeCacheHost instance, perform sequences of interface calls on those instances, and wait for various threads to be idle.

In order to build this proto file, we'll need to copy it into the out/ directory so that it can reference the proto files generated by MojoLPM - this will be handled for us by the mojolpm_fuzzer_test build rule.

Add our fuzzer source

Now we're ready to create the fuzzer c++ source file, and the fuzzer build target. This target is going to depend on both our proto file, and on the c++ source file. Most of the necessary dependencies will be handled for us, but we do still need to add some directly.

Note especially the dependency on mojom_platform_mojolpm in blink, this is an autogenerated target where the target containing the generated fuzzer protocol buffer descriptions will be the name of the mojom target with _mojolpm appended.

mojolpm_fuzzer_test("code_cache_host_mojolpm_fuzzer") {
  sources = [

  proto_source = "code_cache_host_mojolpm_fuzzer.proto"

   deps = [

  proto_deps = [

Now, the minimal source code to do load our testcases:

// Copyright 2020 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include <stdint.h>
#include <utility>

#include "code_cache_host_mojolpm_fuzzer.pb.h"
#include "mojo/core/embedder/embedder.h"
#include "third_party/blink/public/mojom/loader/code_cache.mojom-mojolpm.h"
#include "third_party/libprotobuf-mutator/src/src/libfuzzer/libfuzzer_macro.h"

    const content::fuzzing::code_cache_host::proto::Testcase& testcase) {

You should now be able to build and run this fuzzer (it, of course, won't do very much) to check that everything is lined up right so far.

Handle global process setup

Now we need to add some basic setup code so that our process has something that mostly resembles a normal Browser process; if you look in the file this is CodeCacheHostFuzzerEnvironment, which adds a global environment instance that will handle setting up this basic environment, which will be reused for all of our testcases, since starting threads is expensive and slow.

Handle per-testcase setup

We next need to handle the necessary setup to instantiate CodeCacheHostImpl, so that we can actually run the testcases. At this point, we realise that it's likely that we want to be able to have multiple CodeCacheHostImpl's with different render_process_ids and different backing origins, so we need to modify our proto file to reflect this:

message NewCodeCacheHost {
  enum OriginId {
    ORIGIN_A = 0;
    ORIGIN_B = 1;

  required uint32 id = 1;
  required uint32 render_process_id = 2;
  required OriginId origin_id = 3;

Note that we‘re using an enum to represent the origin, rather than a string; it’s unlikely that the true value of the origin is going to be important, so we've instead chosen a few select values based on the cases mentioned in the source.

The first thing that we need to do is set-up the basic Browser process environment; this is what ContentFuzzerEnvironment is doing - this has a basic setup suitable for fuzzing interfaces in /content. A few things to be careful of are that we need to make sure that mojo::core::Init() is called (only once) and we probably want as much freedom as possible in terms of scheduling, so we want to use slightly different threading options than the average unittest. This is a singleton type that will live for the entire duration of the fuzzer process so we don't want to be holding any testcase-specific data here.

The next thing that we need to do is to figure out the basic setup needed to instantiate the interface we‘re interested in. Looking at the constructor for CodeCacheHostImpl we need three things; a valid render_process_id, an instance of CacheStorageContextImpl and an instance of GeneratedCodeCacheContext. CodeCacheHostFuzzerContext is our container for these per-testcase instances; and will handle creating and binding the instances of the Mojo interfaces that we’re going to fuzz. The most important thing to be careful of here is that everything happens on the correct thread/sequence. Many Browser-process objects have specific expectations, and will end up with very different behaviour if they are created or used from the wrong context.

Integrate with the generated MojoLPM fuzzer code

Finally, we need to do a little bit more plumbing, to rig up this infrastructure that we've built together with the autogenerated code that MojoLPM gives us to interpret and run our testcases. This is the CodeCacheHostTestcase, and the part where the magic happens is here:

void CodeCacheHostTestcase::NextAction() {
  if (next_idx_ < testcase_.sequence_indexes_size()) {
    auto sequence_idx = testcase_.sequence_indexes(next_idx_++);
    const auto& sequence =
      testcase_.sequences(sequence_idx % testcase_.sequences_size());
    for (auto action_idx : sequence.action_indexes()) {
      if (!testcase_.actions_size() || ++action_count_ > MAX_ACTION_COUNT) {
      const auto& action =
        testcase_.actions(action_idx % testcase_.actions_size());
      switch (action.action_case()) {
        case content::fuzzing::code_cache_host::proto::Action::kNewCodeCacheHost: {
        } break;

        case content::fuzzing::code_cache_host::proto::Action::kRunUntilIdle: {
          if (action.run_until_idle().id()) {
          } else {
        } break;

        case content::fuzzing::code_cache_host::proto::Action::kCodeCacheHostCall: {
        } break;

        case content::fuzzing::code_cache_host::proto::Action::ACTION_NOT_SET:

The key line here in integration with MojoLPM is the last case, kCodeCacheHostCall, where we're asking MojoLPM to treat this incoming proto entry as a call to a method on the CodeCacheHost interface.

There's just a little bit more boilerplate in the bottom of the file to tidy up concurrency loose ends, making sure that the fuzzer components are all running on the correct threads; those are more-or-less common to any fuzzer using MojoLPM.

Test it!

Make a corpus directory and fire up your shiny new fuzzer!

 ~/chromium/src% out/Default/code_cache_host_mojolpm_fuzzer /dev/shm/corpus
INFO: Seed: 3273881842
INFO: Loaded 1 modules   (1121912 inline 8-bit counters): 1121912 [0x559151a1aea8, 0x559151b2cd20),
INFO: Loaded 1 PC tables (1121912 PCs): 1121912 [0x559151b2cd20,0x559152c4b4a0),
INFO:      146 files found in /dev/shm/corpus
INFO: -max_len is not provided; libFuzzer will not generate inputs larger than 4096 bytes
INFO: seed corpus: files: 146 min: 2b max: 268b total: 8548b rss: 88Mb
#147  INITED cov: 4633 ft: 10500 corp: 138/8041b exec/s: 0 rss: 91Mb
#152  NEW    cov: 4633 ft: 10501 corp: 139/8139b lim: 4096 exec/s: 0 rss: 91Mb L: 98/268 MS: 8 Custom-ChangeByte-Custom-EraseBytes-Custom-ShuffleBytes-Custom-Custom-
#154  NEW    cov: 4634 ft: 10510 corp: 140/8262b lim: 4096 exec/s: 0 rss: 91Mb L: 123/268 MS: 3 CustomCrossOver-ChangeBit-Custom-
#157  NEW    cov: 4634 ft: 10512 corp: 141/8384b lim: 4096 exec/s: 0 rss: 91Mb L: 122/268 MS: 3 CustomCrossOver-Custom-CustomCrossOver-
#158  NEW    cov: 4634 ft: 10514 corp: 142/8498b lim: 4096 exec/s: 0 rss: 91Mb L: 114/268 MS: 1 CustomCrossOver-
#159  NEW    cov: 4634 ft: 10517 corp: 143/8601b lim: 4096 exec/s: 0 rss: 91Mb L: 103/268 MS: 1 Custom-
#160  NEW    cov: 4634 ft: 10526 corp: 144/8633b lim: 4096 exec/s: 0 rss: 91Mb L: 32/268 MS: 1 Custom-
#164  NEW    cov: 4634 ft: 10528 corp: 145/8851b lim: 4096 exec/s: 0 rss: 91Mb L: 218/268 MS: 4 CustomCrossOver-Custom-CustomCrossOver-Custom-

Wait for it...

Let the fuzzer run for a while, and keep periodically checking in in case it‘s fallen over. It’s likely you‘ll have made a few mistakes somewhere along the way but hopefully soon you’ll have the fuzzer running ‘clean’ for a few hours.

If your coverage isn‘t going up at all, then you’ve probably made a mistake and it likely isn‘t managing to actually interact with the interface you’re trying to fuzz - try using the code coverage output from the next step to debug what's going wrong.

(Optional) Run coverage

In many cases it's useful to check the code coverage to see if we can benefit from adding some manual testcases to get deeper coverage. For this example I used the following command:

python tools/code_coverage/ code_cache_host_mojolpm_fuzzer -b out/Coverage -o ManualReport -c "out/Coverage/code_cache_host_mojolpm_fuzzer -ignore_timeouts=1 -timeout=4 -runs=0 /dev/shm/corpus" -f content

With the CodeCacheHost, looking at the coverage after a few hours we could see that there's definitely some room for improvement:

/* 55       */ base::Optional<GURL> GetSecondaryKeyForCodeCache(const GURL& resource_url,
/* 56 53.6k */ int render_process_id) {
/* 57 53.6k */    if (!resource_url.is_valid() || !resource_url.SchemeIsHTTPOrHTTPS())
/* 58 53.6k */      return base::nullopt;
/* 59 0     */
/* 60 0     */    GURL origin_lock =
/* 61 0     */        ChildProcessSecurityPolicyImpl::GetInstance()->GetOriginLock(
/* 62 0     */            render_process_id);

(Optional) Improve corpus manually

It's fairly easy to improve the corpus manually, since our corpus files are just protobuf files that describe the sequence of interface calls to make.

There are a couple of approaches that we can take here - we‘ll try building a small manual seed corpus that we’ll use to kick-start our fuzzer. Since it's easier to edit text protos, MojoLPM can automatically convert our seed corpus from text protos to binary protos during the build, making this slightly less painful for us, and letting us store our corpus in-tree in a readable format.

So, we'll create a new folder to hold this seed corpus, and craft our first file:

actions {
  new_code_cache_host {
    id: 1
    render_process_id: 0
    origin_id: ORIGIN_A
actions {
  code_cache_host_call {
    remote {
      id: 1
    m_did_generate_cacheable_metadata {
      m_cache_type: CodeCacheType_kJavascript
      m_url {
        new {
          id: 1
          m_url: ""
      m_data {
        new {
          id: 1
          m_bytes {
      m_expected_response_time {
sequences {
  action_indexes: 0
  action_indexes: 1
sequence_indexes: 0

We can then add some new entries to our build target to have the corpus converted to binary proto directly during build.

  testcase_proto_kind = "content.fuzzing.code_cache_host.proto.Testcase"

  seed_corpus_sources = [

If we now run a new coverage report using this single file seed corpus: (note that the binary corpus files will be output in your output directory, in this case

autoninja -C out/Coverage chrome
rm -rf /tmp/corpus; mkdir /tmp/corpus; unzip out/Coverage/ -d /tmp/corpus
python tools/code_coverage/ code_cache_host_mojolpm_fuzzer -b out/Coverage -o ManualReport -c "out/Coverage/code_cache_host_mojolpm_fuzzer -ignore_timeouts=1 -timeout=4 -runs=0 /tmp/corpus" -f content

We can see that we're now getting some more coverage:

/* 118   */ void CodeCacheHostImpl::DidGenerateCacheableMetadata(
/* 119   */     blink::mojom::CodeCacheType cache_type,
/* 120   */     const GURL& url,
/* 121   */     base::Time expected_response_time,
/* 122 2 */       mojo_base::BigBuffer data) {
/* 123 2 */     if (!url.SchemeIsHTTPOrHTTPS()) {
/* 124 0 */       mojo::ReportBadMessage("Invalid URL scheme for code cache.");
/* 125 0 */       return;
/* 126 0 */     }
/* 127 2 */
/* 128 2 */     DCHECK_CURRENTLY_ON(BrowserThread::UI);
/* 129 2 */
/* 130 2 */     GeneratedCodeCache* code_cache = GetCodeCache(cache_type);
/* 131 2 */     if (!code_cache)
/* 132 0 */       return;
/* 133 2 */
/* 134 2 */     base::Optional<GURL> origin_lock =
/* 135 2 */         GetSecondaryKeyForCodeCache(url, render_process_id_);
/* 136 2 */     if (!origin_lock)
/* 137 0 */       return;
/* 138 2 */
/* 139 2 */     code_cache->WriteEntry(url, *origin_lock, expected_response_time,
/* 140 2 */                            std::move(data));
/* 141 2 */ }

Much better!