1 files changed, 20 insertions, 0 deletions
diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index 76cba9d5..1ef9a2f6 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -220,6 +220,16 @@ pub const e_control = "invalid command line: it holds a control character other
 /// the write, with its words and an err record, and no msg, as a ctl line
 /// does (runBuiltin). The refusal, or null.
 pub fn captured(p: *Pardes, req: Req, id: usize, exec: bool, text: []const u8, in_tag: bool) ?Reply {
+    // A builtin that means nothing without its argument, written bare: the
+    // write is refused, EINVAL, as a ctl's is (checkBuiltin), not run to
+    // fail or to ask.
+    if (exec) {
+        const cmd = exec_line.commandText(text);
+        if (std.meta.stringToEnum(Builtin, cmd)) |b| if (builtins.registry.requiresArg(b)) {
+            p.fs.results_len = 0;
+            return refuse(p, req, "wrong #args in control message", cmd);
+        };
+    }
     p.fs.capturing = true;
     p.fs.failure_len = 0;
     defer p.fs.capturing = false;
@@ -1799,6 +1809,16 @@ test "Tty in a directory that is not there fails and makes no pane; a shell that
     try testing.expectEqualStrings("error shell gone\n", call(p, .{ .tag = 3, .op = .read, .node = run_node, .handle = h, .size = 64 }).bytes);
 }
 
+test "a builtin that needs its argument, written bare to an exec, is EINVAL as through ctl" {
+    const p = try withFile(testing.allocator, "x\n");
+    defer p.deinit();
+    for ([_]u64{ root_exec, Node.of(serialOf(p), .exec), @intFromEnum(tree.TopFile.tagexec) }) |node| {
+        const refused = wr(p, node, "Mount\n");
+        try testing.expectEqual(E.INVAL, refused.errno());
+        try testing.expectEqualStrings("wrong #args in control message \"Mount\"", refused.reply.ename);
+    }
+}
+
 test "size is monotonic: growing is never refused, and a size once taken is taken again" {
     const p = try Pardes.init(testing.allocator, .{ .tty_only = true, .cols = 160, .rows = 60 });
     defer p.deinit();