Fix VM startup regressions after shell-out cleanup

The shell-out reduction pass introduced two linked startup regressions in the hot path for vm create.

Make flattenNestedWorkHome repair the temporary nested /root tree without trying to read a root-owned 0700 directory as the calling user: chmod the scratch directory under sudo, then copy each child entry individually before removing it. Add a regression test for that overlap/permission case.

Restore the Firecracker launch wrapper that sets umask 000 before exec. Firecracker was creating the API socket, but the SDK could not use it during machine.Start after the direct sudo launch, so vm create timed out waiting on a socket that already existed.

Validated with go test ./... and make build.

internal/daemon/vm.go

@@ -703,10 +703,20 @@ func (d *Daemon) flattenNestedWorkHome(ctx context.Context, workMount string) er
 	if !exists(nestedHome) {
 		return nil
 	}
-	if err := system.CopyDirContents(ctx, d.runner, nestedHome, workMount, true); err != nil {
+	if _, err := d.runner.RunSudo(ctx, "chmod", "755", nestedHome); err != nil {
 		return err
 	}
-	_, err := d.runner.RunSudo(ctx, "rm", "-rf", nestedHome)
+	entries, err := os.ReadDir(nestedHome)
+	if err != nil {
+		return err
+	}
+	for _, entry := range entries {
+		sourcePath := filepath.Join(nestedHome, entry.Name())
+		if _, err := d.runner.RunSudo(ctx, "cp", "-a", sourcePath, workMount+"/"); err != nil {
+			return err
+		}
+	}
+	_, err = d.runner.RunSudo(ctx, "rm", "-rf", nestedHome)
 	return err
 }
 

internal/daemon/vm_test.go

@@ -279,6 +279,35 @@ func TestSetVMDiskResizeFailsPreflightWhenToolsMissing(t *testing.T) {
 	}
 }
 
+func TestFlattenNestedWorkHomeCopiesEntriesIndividually(t *testing.T) {
+	t.Parallel()
+
+	workMount := t.TempDir()
+	nestedHome := filepath.Join(workMount, "root")
+	if err := os.MkdirAll(filepath.Join(nestedHome, ".ssh"), 0o755); err != nil {
+		t.Fatalf("MkdirAll(.ssh): %v", err)
+	}
+	if err := os.WriteFile(filepath.Join(nestedHome, "notes.txt"), []byte("seed"), 0o644); err != nil {
+		t.Fatalf("WriteFile(notes.txt): %v", err)
+	}
+
+	runner := &scriptedRunner{
+		t: t,
+		steps: []runnerStep{
+			sudoStep("", nil, "chmod", "755", nestedHome),
+			sudoStep("", nil, "cp", "-a", filepath.Join(nestedHome, ".ssh"), workMount+"/"),
+			sudoStep("", nil, "cp", "-a", filepath.Join(nestedHome, "notes.txt"), workMount+"/"),
+			sudoStep("", nil, "rm", "-rf", nestedHome),
+		},
+	}
+	d := &Daemon{runner: runner}
+
+	if err := d.flattenNestedWorkHome(context.Background(), workMount); err != nil {
+		t.Fatalf("flattenNestedWorkHome: %v", err)
+	}
+	runner.assertExhausted()
+}
+
 func TestCreateVMRejectsNonPositiveCPUAndMemory(t *testing.T) {
 	d := &Daemon{}
 	if _, err := d.CreateVM(context.Background(), api.VMCreateParams{VCPUCount: ptr(0)}); err == nil || !strings.Contains(err.Error(), "vcpu must be a positive integer") {

internal/firecracker/client.go

@@ -132,7 +132,10 @@ func buildConfig(cfg MachineConfig) sdk.Config {
 }
 
 func buildProcessRunner(cfg MachineConfig, logFile *os.File) *exec.Cmd {
-	cmd := exec.Command("sudo", "-n", cfg.BinaryPath, "--api-sock", cfg.SocketPath, "--id", cfg.VMID)
+	script := "umask 000 && exec " + shellQuote(cfg.BinaryPath) +
+		" --api-sock " + shellQuote(cfg.SocketPath) +
+		" --id " + shellQuote(cfg.VMID)
+	cmd := exec.Command("sudo", "-n", "sh", "-c", script)
 	cmd.Stdin = nil
 	if logFile != nil {
 		cmd.Stdout = logFile
@@ -141,6 +144,10 @@ func buildProcessRunner(cfg MachineConfig, logFile *os.File) *exec.Cmd {
 	return cmd
 }
 
+func shellQuote(value string) string {
+	return "'" + strings.ReplaceAll(value, "'", `'"'"'`) + "'"
+}
+
 func newLogger(base *slog.Logger) *logrus.Entry {
 	logger := logrus.New()
 	logger.SetOutput(io.Discard)

internal/firecracker/client_test.go

@@ -58,7 +58,7 @@ func TestBuildConfig(t *testing.T) {
 	}
 }
 
-func TestBuildProcessRunnerUsesDirectSudoCommand(t *testing.T) {
+func TestBuildProcessRunnerUsesSudoShellWrapper(t *testing.T) {
 	cmd := buildProcessRunner(MachineConfig{
 		BinaryPath: "/repo/firecracker",
 		SocketPath: "/tmp/fc.sock",
@@ -68,14 +68,15 @@ func TestBuildProcessRunnerUsesDirectSudoCommand(t *testing.T) {
 	if cmd.Path != "/usr/bin/sudo" && cmd.Path != "sudo" {
 		t.Fatalf("command path = %q", cmd.Path)
 	}
-	if len(cmd.Args) != 7 {
+	if len(cmd.Args) != 5 {
 		t.Fatalf("args = %v", cmd.Args)
 	}
-	want := []string{"sudo", "-n", "/repo/firecracker", "--api-sock", "/tmp/fc.sock", "--id", "vm-1"}
-	for i, arg := range want {
-		if cmd.Args[i] != arg {
-			t.Fatalf("args[%d] = %q, want %q (all args: %v)", i, cmd.Args[i], arg, cmd.Args)
-		}
+	if cmd.Args[1] != "-n" || cmd.Args[2] != "sh" || cmd.Args[3] != "-c" {
+		t.Fatalf("args = %v", cmd.Args)
+	}
+	want := "umask 000 && exec '/repo/firecracker' --api-sock '/tmp/fc.sock' --id 'vm-1'"
+	if cmd.Args[4] != want {
+		t.Fatalf("script = %q, want %q", cmd.Args[4], want)
 	}
 	if cmd.Cancel != nil {
 		t.Fatal("process runner should not be tied to a request context")