diff --git a/pkg/kernel/kernel.go b/pkg/kernel/kernel.go
index 77e6f6c1..c38020b5 100644
--- a/pkg/kernel/kernel.go
+++ b/pkg/kernel/kernel.go
@@ -25,35 +25,43 @@ import (
 	"github.com/google/syzkaller/pkg/osutil"
 )
 
-func Build(dir, compiler, config string, fullConfig bool) error {
-	const timeout = 10 * time.Minute // default timeout for command invocations
-	if fullConfig {
-		if err := ioutil.WriteFile(filepath.Join(dir, ".config"), []byte(config), 0600); err != nil {
-			return fmt.Errorf("failed to write config file: %v", err)
-		}
-	} else {
-		os.Remove(filepath.Join(dir, ".config"))
-		configFile := filepath.Join(dir, "syz.config")
-		if err := ioutil.WriteFile(configFile, []byte(config), 0600); err != nil {
-			return fmt.Errorf("failed to write config file: %v", err)
-		}
-		defer os.Remove(configFile)
-		if _, err := osutil.RunCmd(timeout, dir, "make", "defconfig"); err != nil {
-			return err
-		}
-		if _, err := osutil.RunCmd(timeout, dir, "make", "kvmconfig"); err != nil {
-			return err
-		}
-		if _, err := osutil.RunCmd(timeout, dir, "scripts/kconfig/merge_config.sh", "-n", ".config", configFile); err != nil {
-			return err
-		}
+func Build(dir, compiler, config string) error {
+	if err := fileutil.CopyFile(config, filepath.Join(dir, ".config")); err != nil {
+		return fmt.Errorf("failed to write config file: %v", err)
 	}
+	return build(dir, compiler)
+}
+
+// TODO(dvyukov): this is only for syz-gce, remove when syz-gce is deleted.
+func BuildWithPartConfig(dir, compiler, config string) error {
+	const timeout = 10 * time.Minute // default timeout for command invocations
+	os.Remove(filepath.Join(dir, ".config"))
+	configFile := filepath.Join(dir, "syz.config")
+	if err := ioutil.WriteFile(configFile, []byte(config), 0600); err != nil {
+		return fmt.Errorf("failed to write config file: %v", err)
+	}
+	defer os.Remove(configFile)
+	if _, err := osutil.RunCmd(timeout, dir, "make", "defconfig"); err != nil {
+		return err
+	}
+	if _, err := osutil.RunCmd(timeout, dir, "make", "kvmconfig"); err != nil {
+		return err
+	}
+	if _, err := osutil.RunCmd(timeout, dir, "scripts/kconfig/merge_config.sh", "-n", ".config", configFile); err != nil {
+		return err
+	}
+	return build(dir, compiler)
+}
+
+func build(dir, compiler string) error {
+	const timeout = 10 * time.Minute // default timeout for command invocations
 	if _, err := osutil.RunCmd(timeout, dir, "make", "olddefconfig"); err != nil {
 		return err
 	}
 	// We build only bzImage as we currently don't use modules.
 	// Build of a large kernel can take a while on a 1 CPU VM.
-	if _, err := osutil.RunCmd(3*time.Hour, dir, "make", "bzImage", "-j", strconv.Itoa(runtime.NumCPU()), "CC="+compiler); err != nil {
+	cpu := strconv.Itoa(runtime.NumCPU())
+	if _, err := osutil.RunCmd(3*time.Hour, dir, "make", "bzImage", "-j", cpu, "CC="+compiler); err != nil {
 		return err
 	}
 	return nil
diff --git a/syz-gce/syz-gce.go b/syz-gce/syz-gce.go
index ebe91ae6..ec28c5e2 100644
--- a/syz-gce/syz-gce.go
+++ b/syz-gce/syz-gce.go
@@ -354,16 +354,14 @@ func (a *LocalBuildAction) Build() error {
 		}
 	}
 	Logf(0, "building kernel on %v...", hash)
-	config, full := syzconfig, false
 	if cfg.Linux_Config != "" {
-		data, err := ioutil.ReadFile(cfg.Linux_Config)
-		if err != nil {
-			return fmt.Errorf("failed to read config file: %v", err)
+		if err := kernel.Build(dir, a.Compiler, cfg.Linux_Config); err != nil {
+			return fmt.Errorf("build failed: %v", err)
+		}
+	} else {
+		if err := kernel.BuildWithPartConfig(dir, a.Compiler, syzconfig); err != nil {
+			return fmt.Errorf("build failed: %v", err)
 		}
-		config, full = string(data), true
-	}
-	if err := kernel.Build(dir, a.Compiler, config, full); err != nil {
-		return fmt.Errorf("build failed: %v", err)
 	}
 	Logf(0, "building image...")
 	os.MkdirAll("image/obj", 0700)