Fix devpts mount to enable PTY allocation for unprivileged users

Add newinstance,ptmxmode=0666,mode=0620,gid=5 options to the devpts
mount and create a /dev/ptmx -> /dev/pts/ptmx symlink. Without these,
TUI applications (e.g. Claude Code) hang inside the VM because
unprivileged users cannot allocate PTYs.

Fixes #37

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: JAORMX

guest/mount/mount.go

@@ -28,7 +28,7 @@ func Essential(logger *slog.Logger) error {
 		{"proc", "/proc", "proc", syscall.MS_NOSUID | syscall.MS_NODEV | syscall.MS_NOEXEC, ""},
 		{"sysfs", "/sys", "sysfs", syscall.MS_NOSUID | syscall.MS_NODEV | syscall.MS_NOEXEC, ""},
 		{"devtmpfs", "/dev", "devtmpfs", syscall.MS_NOSUID | syscall.MS_NOEXEC, ""},
-		{"devpts", "/dev/pts", "devpts", syscall.MS_NOSUID | syscall.MS_NOEXEC, ""},
+		{"devpts", "/dev/pts", "devpts", syscall.MS_NOSUID | syscall.MS_NOEXEC, "newinstance,ptmxmode=0666,mode=0620,gid=5"},
 		{"tmpfs", "/tmp", "tmpfs", syscall.MS_NOSUID | syscall.MS_NODEV, "size=256m"},
 		{"tmpfs", "/run", "tmpfs", syscall.MS_NOSUID | syscall.MS_NODEV | syscall.MS_NOEXEC, "size=64m"},
 	}
@@ -47,6 +47,16 @@ func Essential(logger *slog.Logger) error {
 		}
 	}
 
+	// Replace /dev/ptmx with a symlink to /dev/pts/ptmx. With the
+	// newinstance devpts mount, the PTY multiplexer lives at
+	// /dev/pts/ptmx, but glibc's posix_openpt() opens /dev/ptmx.
+	// devtmpfs may auto-create /dev/ptmx as a device node, so remove
+	// it first to ensure the symlink succeeds.
+	_ = os.Remove("/dev/ptmx")
+	if err := os.Symlink("/dev/pts/ptmx", "/dev/ptmx"); err != nil {
+		return fmt.Errorf("creating /dev/ptmx symlink: %w", err)
+	}
+
 	// Create standard /dev symlinks.
 	symlinks := [][2]string{
 		{"/proc/self/fd", "/dev/fd"},

guest/mount/mount_test.go

@@ -40,7 +40,7 @@ func TestEssentialMountPoints(t *testing.T) {
 		{"proc", "/proc", "proc", 0, ""},
 		{"sysfs", "/sys", "sysfs", 0, ""},
 		{"devtmpfs", "/dev", "devtmpfs", 0, ""},
-		{"devpts", "/dev/pts", "devpts", 0, ""},
+		{"devpts", "/dev/pts", "devpts", 0, "newinstance,ptmxmode=0666,mode=0620,gid=5"},
 		{"tmpfs", "/tmp", "tmpfs", 0, "size=256m"},
 		{"tmpfs", "/run", "tmpfs", 0, "size=64m"},
 	}