pkg/bisect: add initial testing support for cause bisection

(note: incomplete change)

Refactor existing code as follows:
* Move reusable test utility functions from git_repo_test.go to
pkg/vcs/test_util.go and make them exported.
* Split Run() into Run()+runImpl().
* Change type of bisect.go:env.inst to `instance.BuilderTester`.
Change usage inside syz-testbuild/testbuild.go accordingly.
* Move most of linux.PreviousReleaseTags() into vcs/git.go as
git.previousReleaseTags().
* Allow build.CompilerIdentity to be mocked.

Introduce the following changes:
* instance.BuilderTester is an interface with methods
	BuildSyzkaller()
	BuildKernel()
	Test()
NewEnv() now returns this interface.
* type testEnv implements instance.BuilderTester.
* type testBuilder implements builder interface. Add a entry into table
inside pkg/build/build.go:getBuilder() to return testBuilder object.

11 files changed, 369 insertions, 140 deletions

diff --git a/pkg/bisect/bisect.go b/pkg/bisect/bisect.go
index 78cb58bd4..904be4b53 100644
--- a/pkg/bisect/bisect.go
+++ b/pkg/bisect/bisect.go
@@ -55,7 +55,7 @@ type env struct {
 	repo      vcs.Repo
 	bisecter  vcs.Bisecter
 	head      *vcs.Commit
-	inst      *instance.Env
+	inst      instance.BuilderTester
 	numTests  int
 	buildTime time.Duration
 	testTime  time.Duration
@@ -85,11 +85,30 @@ func Run(cfg *Config) ([]*vcs.Commit, *report.Report, error) {
 	if !ok {
 		return nil, nil, fmt.Errorf("bisection is not implemented for %v", cfg.Manager.TargetOS)
 	}
+	inst, err := instance.NewEnv(&cfg.Manager)
+	if err != nil {
+		return nil, nil, err
+	}
+	if _, err = repo.CheckoutBranch(cfg.Kernel.Repo, cfg.Kernel.Branch); err != nil {
+		return nil, nil, err
+	}
+
+	return runImpl(cfg, repo, bisecter, inst)
+}
+
+func runImpl(cfg *Config, repo vcs.Repo, bisecter vcs.Bisecter, inst instance.BuilderTester) (
+	[]*vcs.Commit, *report.Report, error) {
 	env := &env{
 		cfg:      cfg,
 		repo:     repo,
 		bisecter: bisecter,
+		inst:     inst,
 	}
+	head, err := repo.HeadCommit()
+	if err != nil {
+		return nil, nil, err
+	}
+	env.head = head
 	if cfg.Fix {
 		env.log("bisecting fixing commit since %v", cfg.Kernel.Commit)
 	} else {
@@ -135,12 +154,6 @@ func Run(cfg *Config) ([]*vcs.Commit, *report.Report, error) {
 func (env *env) bisect() ([]*vcs.Commit, *report.Report, error) {
 	cfg := env.cfg
 	var err error
-	if env.inst, err = instance.NewEnv(&cfg.Manager); err != nil {
-		return nil, nil, err
-	}
-	if env.head, err = env.repo.CheckoutBranch(cfg.Kernel.Repo, cfg.Kernel.Branch); err != nil {
-		return nil, nil, err
-	}
 	if err := build.Clean(cfg.Manager.TargetOS, cfg.Manager.TargetVMArch,
 		cfg.Manager.Type, cfg.Manager.KernelSrc); err != nil {
 		return nil, nil, fmt.Errorf("kernel clean failed: %v", err)
diff --git a/pkg/bisect/bisect_test.go b/pkg/bisect/bisect_test.go
new file mode 100644
index 000000000..472fde289
--- /dev/null
+++ b/pkg/bisect/bisect_test.go
@@ -0,0 +1,133 @@
+// Copyright 2019 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+package bisect
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"strconv"
+	"testing"
+
+	"github.com/google/syzkaller/pkg/build"
+	"github.com/google/syzkaller/pkg/instance"
+	"github.com/google/syzkaller/pkg/mgrconfig"
+	"github.com/google/syzkaller/pkg/report"
+	"github.com/google/syzkaller/pkg/vcs"
+)
+
+// testEnv will implement instance.BuilderTester. This allows us to
+// set bisect.env.inst to a testEnv object.
+type testEnv struct {
+	repo *vcs.TestRepo
+	r    vcs.Repo
+	t    *testing.T
+	// TODO: add a "fix bool" here so that Test() can return results according to
+	// whether fix/cause bisection is happening.
+}
+
+func (env *testEnv) BuildSyzkaller(repo, commit string) error {
+	return nil
+}
+
+func (env *testEnv) BuildKernel(compilerBin, userspaceDir, cmdlineFile, sysctlFile string,
+	kernelConfig []byte) (string, error) {
+	return "", nil
+}
+
+func (env *testEnv) Test(numVMs int, reproSyz, reproOpts, reproC []byte) ([]error, error) {
+	hc, err := env.r.HeadCommit()
+	if err != nil {
+		env.t.Fatal(err)
+	}
+	// For cause bisection, if newer than or equal to 602, it crashes.
+	// -- 602 is the cause commit.
+	// TODO: for fix bisection(check env.fix), if older than 602, it crashes.
+	// -- 602 is the fix commit.
+	val, err := strconv.Atoi(hc.Title)
+	if err != nil {
+		env.t.Fatalf("invalid commit title: %v", val)
+	}
+	if val >= 602 {
+		var errors []error
+		for i := 0; i < numVMs; i++ {
+			errors = append(errors, &instance.CrashError{
+				Report: &report.Report{
+					Title: fmt.Sprintf("crashes at %v", hc.Title),
+				},
+			})
+		}
+		return errors, nil
+	}
+	var errors []error
+	for i := 0; i < numVMs; i++ {
+		errors = append(errors, nil)
+	}
+	return errors, nil
+}
+
+// TestBisectCause tests that bisection returns the correct cause
+// commit.
+func TestBisectCause(t *testing.T) {
+	t.Parallel()
+	build.CompilerIdentity = func(_ string) (string, error) {
+		return "unused-compiler-identity", nil
+	}
+	baseDir, err := ioutil.TempDir("", "syz-git-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(baseDir)
+	originRepo := vcs.CreateTestRepo(t, baseDir, "originRepo")
+	for rv := 4; rv < 10; rv++ {
+		for i := 0; i < 6; i++ {
+			originRepo.CommitChange(fmt.Sprintf("%v", rv*100+i))
+			if i == 0 {
+				originRepo.SetTag(fmt.Sprintf("v%v.0", rv))
+			}
+		}
+	}
+	repo := vcs.CloneTestRepo(t, baseDir, "repo", originRepo)
+	r, err := vcs.NewRepo("test", "64", repo.Dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	head, err := r.HeadCommit()
+	if err != nil {
+		t.Fatal(err)
+	}
+	cfg := &Config{
+		Fix:   false,
+		Trace: new(bytes.Buffer),
+		Manager: mgrconfig.Config{
+			TargetOS:     "test",
+			TargetVMArch: "64",
+			Type:         "qemu",
+			KernelSrc:    repo.Dir,
+		},
+		Kernel: KernelConfig{
+			Commit: head.Hash,
+			Repo:   originRepo.Dir,
+		},
+	}
+	inst := &testEnv{
+		repo: repo,
+		r:    r,
+		t:    t,
+	}
+	commits, rep, err := runImpl(cfg, r, r.(vcs.Bisecter), inst)
+	if err != nil {
+		t.Fatalf("returned error: '%v'", err)
+	}
+	if len(commits) != 1 {
+		t.Fatalf("Got %d commits: %v", len(commits), commits)
+	}
+	if commits[0].Title != "602" {
+		t.Fatalf("Expected commit '602' got '%v'", commits[0].Title)
+	}
+	if rep == nil {
+		t.Fatal("returned rep==nil, report should not be empty")
+	}
+}
diff --git a/pkg/build/build.go b/pkg/build/build.go
index 7e2a71ae4..7fce681df 100644
--- a/pkg/build/build.go
+++ b/pkg/build/build.go
@@ -79,6 +79,7 @@ func getBuilder(targetOS, targetArch, vmType string) (builder, error) {
 		{"openbsd", "amd64", []string{"gce", "vmm"}, openbsd{}},
 		{"netbsd", "amd64", []string{"gce", "qemu"}, netbsd{}},
 		{"freebsd", "amd64", []string{"gce", "qemu"}, freebsd{}},
+		{"test", "64", []string{"qemu"}, testBuilder{}},
 	}
 	for _, s := range supported {
 		if targetOS == s.OS && targetArch == s.arch {
@@ -92,7 +93,7 @@ func getBuilder(targetOS, targetArch, vmType string) (builder, error) {
 	return nil, fmt.Errorf("unsupported image type %v/%v/%v", targetOS, targetArch, vmType)
 }
 
-func CompilerIdentity(compiler string) (string, error) {
+var CompilerIdentity = func(compiler string) (string, error) {
 	if compiler == "" {
 		return "", nil
 	}
diff --git a/pkg/build/testlinux.go b/pkg/build/testlinux.go
new file mode 100644
index 000000000..482c162c5
--- /dev/null
+++ b/pkg/build/testlinux.go
@@ -0,0 +1,16 @@
+// Copyright 2019 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+package build
+
+// TypeBuilder implements the builder interface.
+type testBuilder struct{}
+
+func (tb testBuilder) build(targetArch, vmType, kernelDir, outputDir, compiler, userspaceDir,
+	cmdlineFile, sysctlFile string, config []byte) error {
+	return nil
+}
+
+func (tb testBuilder) clean(string, string) error {
+	return nil
+}
diff --git a/pkg/instance/instance.go b/pkg/instance/instance.go
index cc1b147f1..b3e24aace 100644
--- a/pkg/instance/instance.go
+++ b/pkg/instance/instance.go
@@ -27,11 +27,17 @@ import (
 	"github.com/google/syzkaller/vm"
 )
 
-type Env struct {
+type BuilderTester interface {
+	BuildSyzkaller(string, string) error
+	BuildKernel(string, string, string, string, []byte) (string, error)
+	Test(numVMs int, reproSyz, reproOpts, reproC []byte) ([]error, error)
+}
+
+type env struct {
 	cfg *mgrconfig.Config
 }
 
-func NewEnv(cfg *mgrconfig.Config) (*Env, error) {
+func NewEnv(cfg *mgrconfig.Config) (BuilderTester, error) {
 	if !vm.AllowsOvercommit(cfg.Type) {
 		return nil, fmt.Errorf("test instances are not supported for %v VMs", cfg.Type)
 	}
@@ -47,13 +53,13 @@ func NewEnv(cfg *mgrconfig.Config) (*Env, error) {
 	if err := osutil.MkdirAll(cfg.Workdir); err != nil {
 		return nil, fmt.Errorf("failed to create tmp dir: %v", err)
 	}
-	env := &Env{
+	env := &env{
 		cfg: cfg,
 	}
 	return env, nil
 }
 
-func (env *Env) BuildSyzkaller(repo, commit string) error {
+func (env *env) BuildSyzkaller(repo, commit string) error {
 	cfg := env.cfg
 	srcIndex := strings.LastIndex(cfg.Syzkaller, "/src/")
 	if srcIndex == -1 {
@@ -82,7 +88,7 @@ func (env *Env) BuildSyzkaller(repo, commit string) error {
 	return nil
 }
 
-func (env *Env) BuildKernel(compilerBin, userspaceDir, cmdlineFile, sysctlFile string,
+func (env *env) BuildKernel(compilerBin, userspaceDir, cmdlineFile, sysctlFile string,
 	kernelConfig []byte) (string, error) {
 	cfg := env.cfg
 	imageDir := filepath.Join(cfg.Workdir, "image")
@@ -170,7 +176,7 @@ func (err *CrashError) Error() string {
 // Test boots numVMs VMs, tests basic kernel operation, and optionally tests the provided reproducer.
 // TestError is returned if there is a problem with kernel/image (crash, reboot loop, etc).
 // CrashError is returned if the reproducer crashes kernel.
-func (env *Env) Test(numVMs int, reproSyz, reproOpts, reproC []byte) ([]error, error) {
+func (env *env) Test(numVMs int, reproSyz, reproOpts, reproC []byte) ([]error, error) {
 	if err := mgrconfig.Complete(env.cfg); err != nil {
 		return nil, err
 	}
diff --git a/pkg/vcs/git.go b/pkg/vcs/git.go
index 7b0a621b6..359c3d68e 100644
--- a/pkg/vcs/git.go
+++ b/pkg/vcs/git.go
@@ -450,3 +450,27 @@ func (git *git) bisectInconclusive(output []byte) ([]*Commit, error) {
 	}
 	return commits, nil
 }
+
+func (git *git) previousReleaseTags(commit string, self bool) ([]string, error) {
+	var tags []string
+	if self {
+		output, err := git.git("tag", "--list", "--points-at", commit, "--merged", commit, "v*.*")
+		if err != nil {
+			return nil, err
+		}
+		tags, err = gitParseReleaseTags(output)
+		if err != nil {
+			return nil, err
+		}
+	}
+	output, err := git.git("tag", "--no-contains", commit, "--merged", commit, "v*.*")
+	if err != nil {
+		return nil, err
+	}
+	tags1, err := gitParseReleaseTags(output)
+	if err != nil {
+		return nil, err
+	}
+	tags = append(tags, tags1...)
+	return tags, nil
+}
diff --git a/pkg/vcs/git_repo_test.go b/pkg/vcs/git_repo_test.go
index 017aaa658..227834b71 100644
--- a/pkg/vcs/git_repo_test.go
+++ b/pkg/vcs/git_repo_test.go
@@ -10,10 +10,8 @@ import (
 	"path/filepath"
 	"sort"
 	"testing"
-	"time"
 
 	"github.com/google/go-cmp/cmp"
-	"github.com/google/syzkaller/pkg/osutil"
 )
 
 func init() {
@@ -21,12 +19,6 @@ func init() {
 	os.Setenv("SYZ_DISABLE_SANDBOXING", "yes")
 }
 
-const (
-	userEmail           = `test@syzkaller.com`
-	userName            = `Test Syzkaller`
-	extractFixTagsEmail = `"syzbot" <syzbot@my.mail.com>`
-)
-
 func TestGitRepo(t *testing.T) {
 	t.Parallel()
 	baseDir, err := ioutil.TempDir("", "syz-git-test")
@@ -34,30 +26,30 @@ func TestGitRepo(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(baseDir)
-	repo1 := createTestRepo(t, baseDir, "repo1")
-	repo2 := createTestRepo(t, baseDir, "repo2")
+	repo1 := CreateTestRepo(t, baseDir, "repo1")
+	repo2 := CreateTestRepo(t, baseDir, "repo2")
 	repo := newGit(filepath.Join(baseDir, "repo"), nil)
 	{
-		com, err := repo.Poll(repo1.dir, "master")
+		com, err := repo.Poll(repo1.Dir, "master")
 		if err != nil {
 			t.Fatal(err)
 		}
-		if diff := cmp.Diff(com, repo1.commits["master"]["1"]); diff != "" {
+		if diff := cmp.Diff(com, repo1.Commits["master"]["1"]); diff != "" {
 			t.Fatal(diff)
 		}
 	}
 	{
-		com, err := repo.CheckoutBranch(repo1.dir, "branch1")
+		com, err := repo.CheckoutBranch(repo1.Dir, "branch1")
 		if err != nil {
 			t.Fatal(err)
 		}
-		if diff := cmp.Diff(com, repo1.commits["branch1"]["1"]); diff != "" {
+		if diff := cmp.Diff(com, repo1.Commits["branch1"]["1"]); diff != "" {
 			t.Fatal(diff)
 		}
 	}
 	{
-		want := repo1.commits["branch1"]["0"]
-		com, err := repo.CheckoutCommit(repo1.dir, want.Hash)
+		want := repo1.Commits["branch1"]["0"]
+		com, err := repo.CheckoutCommit(repo1.Dir, want.Hash)
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -66,7 +58,7 @@ func TestGitRepo(t *testing.T) {
 		}
 	}
 	{
-		commits, err := repo.ListRecentCommits(repo1.commits["branch1"]["1"].Hash)
+		commits, err := repo.ListRecentCommits(repo1.Commits["branch1"]["1"].Hash)
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -76,8 +68,8 @@ func TestGitRepo(t *testing.T) {
 		}
 	}
 	{
-		want := repo2.commits["branch1"]["0"]
-		com, err := repo.CheckoutCommit(repo2.dir, want.Hash)
+		want := repo2.Commits["branch1"]["0"]
+		com, err := repo.CheckoutCommit(repo2.Dir, want.Hash)
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -86,8 +78,8 @@ func TestGitRepo(t *testing.T) {
 		}
 	}
 	{
-		want := repo2.commits["branch1"]["1"]
-		com, err := repo.CheckoutCommit(repo2.dir, want.Hash)
+		want := repo2.Commits["branch1"]["1"]
+		com, err := repo.CheckoutCommit(repo2.Dir, want.Hash)
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -96,16 +88,16 @@ func TestGitRepo(t *testing.T) {
 		}
 	}
 	{
-		com, err := repo.CheckoutBranch(repo2.dir, "branch2")
+		com, err := repo.CheckoutBranch(repo2.Dir, "branch2")
 		if err != nil {
 			t.Fatal(err)
 		}
-		if diff := cmp.Diff(com, repo2.commits["branch2"]["1"]); diff != "" {
+		if diff := cmp.Diff(com, repo2.Commits["branch2"]["1"]); diff != "" {
 			t.Fatal(diff)
 		}
 	}
 	{
-		want := repo2.commits["branch2"]["0"]
+		want := repo2.Commits["branch2"]["0"]
 		com, err := repo.SwitchCommit(want.Hash)
 		if err != nil {
 			t.Fatal(err)
@@ -123,9 +115,9 @@ func TestMetadata(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(repoDir)
-	repo := makeTestRepo(t, repoDir)
+	repo := MakeTestRepo(t, repoDir)
 	for i, test := range metadataTests {
-		repo.commitChange(test.description)
+		repo.CommitChange(test.description)
 		com, err := repo.repo.HeadCommit()
 		if err != nil {
 			t.Fatal(err)
@@ -297,10 +289,10 @@ func TestBisect(t *testing.T) {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(repoDir)
-	repo := makeTestRepo(t, repoDir)
+	repo := MakeTestRepo(t, repoDir)
 	var commits []string
 	for i := 0; i < 5; i++ {
-		repo.commitChange(fmt.Sprintf("commit %v", i))
+		repo.CommitChange(fmt.Sprintf("commit %v", i))
 		com, err := repo.repo.HeadCommit()
 		if err != nil {
 			t.Fatal(err)
@@ -413,80 +405,3 @@ func TestBisect(t *testing.T) {
 		}
 	}
 }
-
-type testWriter testing.T
-
-func (t *testWriter) Write(data []byte) (int, error) {
-	(*testing.T)(t).Log(string(data))
-	return len(data), nil
-}
-
-func createTestRepo(t *testing.T, baseDir, name string) *testRepo {
-	repo := makeTestRepo(t, filepath.Join(baseDir, name))
-	repo.git("checkout", "-b", "master")
-	repo.commitFileChange("master", "0")
-	for _, branch := range []string{"branch1", "branch2"} {
-		repo.git("checkout", "-b", branch, "master")
-		repo.commitFileChange(branch, "0")
-		repo.commitFileChange(branch, "1")
-	}
-	repo.git("checkout", "master")
-	repo.commitFileChange("master", "1")
-	return repo
-}
-
-type testRepo struct {
-	t       *testing.T
-	dir     string
-	name    string
-	commits map[string]map[string]*Commit
-	repo    *git
-}
-
-func makeTestRepo(t *testing.T, dir string) *testRepo {
-	if err := osutil.MkdirAll(dir); err != nil {
-		t.Fatal(err)
-	}
-	ignoreCC := map[string]bool{
-		"stable@vger.kernel.org": true,
-	}
-	repo := &testRepo{
-		t:       t,
-		dir:     dir,
-		name:    filepath.Base(dir),
-		commits: make(map[string]map[string]*Commit),
-		repo:    newGit(dir, ignoreCC),
-	}
-	repo.git("init")
-	repo.git("config", "--add", "user.email", userEmail)
-	repo.git("config", "--add", "user.name", userName)
-	return repo
-}
-
-func (repo *testRepo) git(args ...string) {
-	if _, err := osutil.RunCmd(time.Minute, repo.dir, "git", args...); err != nil {
-		repo.t.Fatal(err)
-	}
-}
-
-func (repo *testRepo) commitFileChange(branch, change string) {
-	id := fmt.Sprintf("%v-%v-%v", repo.name, branch, change)
-	file := filepath.Join(repo.dir, "file")
-	if err := osutil.WriteFile(file, []byte(id)); err != nil {
-		repo.t.Fatal(err)
-	}
-	repo.git("add", file)
-	repo.git("commit", "-m", id)
-	if repo.commits[branch] == nil {
-		repo.commits[branch] = make(map[string]*Commit)
-	}
-	com, err := repo.repo.HeadCommit()
-	if err != nil {
-		repo.t.Fatal(err)
-	}
-	repo.commits[branch][change] = com
-}
-
-func (repo *testRepo) commitChange(description string) {
-	repo.git("commit", "--allow-empty", "-m", description)
-}
diff --git a/pkg/vcs/linux.go b/pkg/vcs/linux.go
index e12682277..aaca97fe7 100644
--- a/pkg/vcs/linux.go
+++ b/pkg/vcs/linux.go
@@ -31,30 +31,10 @@ func newLinux(dir string) *linux {
 }
 
 func (ctx *linux) PreviousReleaseTags(commit string) ([]string, error) {
-	return ctx.previousReleaseTags(commit, false)
-}
-
-func (ctx *linux) previousReleaseTags(commit string, self bool) ([]string, error) {
-	var tags []string
-	if self {
-		output, err := ctx.git.git("tag", "--list", "--points-at", commit, "--merged", commit, "v*.*")
-		if err != nil {
-			return nil, err
-		}
-		tags, err = gitParseReleaseTags(output)
-		if err != nil {
-			return nil, err
-		}
-	}
-	output, err := ctx.git.git("tag", "--no-contains", commit, "--merged", commit, "v*.*")
-	if err != nil {
-		return nil, err
-	}
-	tags1, err := gitParseReleaseTags(output)
+	tags, err := ctx.git.previousReleaseTags(commit, false)
 	if err != nil {
 		return nil, err
 	}
-	tags = append(tags, tags1...)
 	for i, tag := range tags {
 		if tag == "v4.0" {
 			// Initially we tried to stop at 3.8 because:
diff --git a/pkg/vcs/test_util.go b/pkg/vcs/test_util.go
new file mode 100644
index 000000000..55bd7bccd
--- /dev/null
+++ b/pkg/vcs/test_util.go
@@ -0,0 +1,116 @@
+package vcs
+
+import (
+	"fmt"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/google/syzkaller/pkg/osutil"
+)
+
+const (
+	userEmail           = `test@syzkaller.com`
+	userName            = `Test Syzkaller`
+	extractFixTagsEmail = `"syzbot" <syzbot@my.mail.com>`
+)
+
+type testWriter testing.T
+
+func (t *testWriter) Write(data []byte) (int, error) {
+	(*testing.T)(t).Log(string(data))
+	return len(data), nil
+}
+
+type TestRepo struct {
+	t       *testing.T
+	Dir     string
+	name    string
+	Commits map[string]map[string]*Commit
+	repo    *git
+}
+
+func (repo *TestRepo) git(args ...string) {
+	if _, err := osutil.RunCmd(time.Minute, repo.Dir, "git", args...); err != nil {
+		repo.t.Fatal(err)
+	}
+}
+
+func MakeTestRepo(t *testing.T, dir string) *TestRepo {
+	if err := osutil.MkdirAll(dir); err != nil {
+		t.Fatal(err)
+	}
+	ignoreCC := map[string]bool{
+		"stable@vger.kernel.org": true,
+	}
+	repo := &TestRepo{
+		t:       t,
+		Dir:     dir,
+		name:    filepath.Base(dir),
+		Commits: make(map[string]map[string]*Commit),
+		repo:    newGit(dir, ignoreCC),
+	}
+	repo.git("init")
+	repo.git("config", "--add", "user.email", userEmail)
+	repo.git("config", "--add", "user.name", userName)
+	return repo
+}
+
+func (repo *TestRepo) CommitFileChange(branch, change string) {
+	id := fmt.Sprintf("%v-%v-%v", repo.name, branch, change)
+	file := filepath.Join(repo.Dir, "file")
+	if err := osutil.WriteFile(file, []byte(id)); err != nil {
+		repo.t.Fatal(err)
+	}
+	repo.git("add", file)
+	repo.git("commit", "-m", id)
+	if repo.Commits[branch] == nil {
+		repo.Commits[branch] = make(map[string]*Commit)
+	}
+	com, err := repo.repo.HeadCommit()
+	if err != nil {
+		repo.t.Fatal(err)
+	}
+	repo.Commits[branch][change] = com
+}
+
+func (repo *TestRepo) CommitChange(description string) {
+	repo.git("commit", "--allow-empty", "-m", description)
+}
+
+func (repo *TestRepo) SetTag(tag string) {
+	repo.git("tag", tag)
+}
+
+func CreateTestRepo(t *testing.T, baseDir, name string) *TestRepo {
+	repo := MakeTestRepo(t, filepath.Join(baseDir, name))
+	repo.git("checkout", "-b", "master")
+	repo.CommitFileChange("master", "0")
+	for _, branch := range []string{"branch1", "branch2"} {
+		repo.git("checkout", "-b", branch, "master")
+		repo.CommitFileChange(branch, "0")
+		repo.CommitFileChange(branch, "1")
+	}
+	repo.git("checkout", "master")
+	repo.CommitFileChange("master", "1")
+	return repo
+}
+
+func CloneTestRepo(t *testing.T, baseDir string, name string, originRepo *TestRepo) *TestRepo {
+	dir := filepath.Join(baseDir, name)
+	if err := osutil.MkdirAll(dir); err != nil {
+		t.Fatal(err)
+	}
+	ignoreCC := map[string]bool{
+		"stable@vger.kernel.org": true,
+	}
+	repo := &TestRepo{
+		t:       t,
+		Dir:     dir,
+		name:    filepath.Base(dir),
+		Commits: make(map[string]map[string]*Commit),
+		repo:    newGit(dir, ignoreCC),
+	}
+	repo.git("clone", originRepo.Dir, repo.Dir)
+	return repo
+}
diff --git a/pkg/vcs/testos.go b/pkg/vcs/testos.go
new file mode 100644
index 000000000..f30dd0799
--- /dev/null
+++ b/pkg/vcs/testos.go
@@ -0,0 +1,23 @@
+// Copyright 2019 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+package vcs
+
+type testos struct {
+	*git
+}
+
+func newTestos(dir string) *testos {
+	return &testos{
+		git: newGit(dir, nil),
+	}
+}
+
+func (ctx *testos) PreviousReleaseTags(commit string) ([]string, error) {
+	return ctx.git.previousReleaseTags(commit, false)
+}
+func (ctx *testos) EnvForCommit(commit string, kernelConfig []byte) (*BisectEnv, error) {
+	return &BisectEnv{
+		Compiler: "test-compiler-dont-use",
+	}, nil
+}
diff --git a/pkg/vcs/vcs.go b/pkg/vcs/vcs.go
index 77b919d4b..28264522f 100644
--- a/pkg/vcs/vcs.go
+++ b/pkg/vcs/vcs.go
@@ -104,6 +104,8 @@ func NewRepo(os, vm, dir string) (Repo, error) {
 		return newNetBSD(vm, dir), nil
 	case "freebsd":
 		return newFreeBSD(vm, dir), nil
+	case "test":
+		return newTestos(dir), nil
 	}
 	return nil, fmt.Errorf("vcs is unsupported for %v", os)
 }