3 files changed, 86 insertions, 13 deletions
diff --git a/pkg/manager/diff_test.go b/pkg/manager/diff_test.go
new file mode 100644
index 000000000..f2408f13b
--- /dev/null
+++ b/pkg/manager/diff_test.go
@@ -0,0 +1,68 @@
+// Copyright 2025 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 manager
+
+import (
+	"testing"
+
+	"github.com/google/syzkaller/pkg/mgrconfig"
+	"github.com/google/syzkaller/pkg/osutil"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPatchFocusAreas(t *testing.T) {
+	cfg := &mgrconfig.Config{
+		KernelSrc: t.TempDir(),
+	}
+	require.NoError(t, osutil.FillDirectory(cfg.KernelSrc, map[string]string{
+		"header.h": `Test`,
+		"a.c": `#include <header.h>
+int main(void) { }`,
+		"b.c": `int main(void) { }`,
+		"c.c": `int main(void) { }`,
+	}))
+
+	PatchFocusAreas(cfg, [][]byte{
+		[]byte(`diff --git a/b.c b/b.c
+index 103167d..fbf7a68 100644
+--- a/b.c
++++ b/b.c
+@@ -1 +1 @@
+-int main(void) { }
+\ No newline at end of file
++int main(void) { return 1; }
+\ No newline at end of file`),
+		// Also, emulate an update to te header.h.
+		[]byte(`diff --git a/header.h b/header.h
+index 103167d..fbf7a68 100644
+--- a/header.h
++++ b/header.h
+@@ -1 +1 @@
+-Test
+\ No newline at end of file
++Test2
+\ No newline at end of file`),
+	})
+
+	assert.Equal(t, []mgrconfig.FocusArea{
+		{
+			Name: modifiedArea,
+			Filter: mgrconfig.CovFilterCfg{
+				Files: []string{"b.c", "header.h"},
+			},
+			Weight: 3.0,
+		},
+		{
+			Name: includesArea,
+			Filter: mgrconfig.CovFilterCfg{
+				Files: []string{"a.c"},
+			},
+			Weight: 2.0,
+		},
+		{
+			Weight: 1.0,
+		},
+	}, cfg.Experimental.FocusAreas)
+}
diff --git a/pkg/osutil/fileutil.go b/pkg/osutil/fileutil.go
index 850335cfe..cd5b602cf 100644
--- a/pkg/osutil/fileutil.go
+++ b/pkg/osutil/fileutil.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"path/filepath"
 )
 
 // CopyFile atomically copies oldFile to newFile preserving permissions and modification time.
@@ -50,6 +51,21 @@ func Rename(oldFile, newFile string) error {
 	return err
 }
 
+// FillDirectory is used to fill in directory structure for tests.
+func FillDirectory(dir string, fileContent map[string]string) error {
+	for path, content := range fileContent {
+		fullPath := filepath.Join(dir, path)
+		dirPath := filepath.Dir(fullPath)
+		if err := MkdirAll(dirPath); err != nil {
+			return fmt.Errorf("mkdir %q failed: %w", dirPath, err)
+		}
+		if err := WriteFile(fullPath, []byte(content)); err != nil {
+			return fmt.Errorf("write file failed: %w", err)
+		}
+	}
+	return nil
+}
+
 // WriteTempFile writes data to a temp file and returns its name.
 func WriteTempFile(data []byte) (string, error) {
 	// Note: pkg/report knows about "syzkaller" prefix as it appears in crashes as process name.
diff --git a/pkg/osutil/tar_test.go b/pkg/osutil/tar_test.go
index a0e5a22bd..432d7c622 100644
--- a/pkg/osutil/tar_test.go
+++ b/pkg/osutil/tar_test.go
@@ -7,31 +7,20 @@ import (
 	"archive/tar"
 	"bytes"
 	"io"
-	"path/filepath"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestTarDirectory(t *testing.T) {
 	dir := t.TempDir()
-
 	items := map[string]string{
 		"file1.txt":     "first file content",
 		"dir/file2.txt": "second file content",
 		"empty.txt":     "",
 	}
-
-	for path, content := range items {
-		fullPath := filepath.Join(dir, path)
-		dirPath := filepath.Dir(fullPath)
-		if err := MkdirAll(dirPath); err != nil {
-			t.Fatalf("mkdir %q failed: %v", dirPath, err)
-		}
-		if err := WriteFile(fullPath, []byte(content)); err != nil {
-			t.Fatalf("write file failed: %v", err)
-		}
-	}
+	require.NoError(t, FillDirectory(dir, items))
 
 	var buf bytes.Buffer
 	err := tarDirectory(dir, &buf)