kunit/usercopy: Disable testing on !CONFIG_MMU

Since arch_pick_mmap_layout() is an inline for non-MMU systems, disable
this test there.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406160505.uBge6TMY-lkp@intel.com/Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

lib/kunit/user_alloc.c

@@ -30,6 +30,10 @@ static int kunit_attach_mm(void)
 	if (current->mm)
 		return 0;
 
+	/* arch_pick_mmap_layout() is only sane with MMU systems. */
+	if (!IS_ENABLED(CONFIG_MMU))
+		return -EINVAL;
+
 	mm = mm_alloc();
 	if (!mm)
 		return -ENOMEM;

lib/usercopy_kunit.c

@@ -290,6 +290,11 @@ static int usercopy_test_init(struct kunit *test)
 	struct usercopy_test_priv *priv;
 	unsigned long user_addr;
 
+	if (!IS_ENABLED(CONFIG_MMU)) {
+		kunit_skip(test, "Userspace allocation testing not available on non-MMU systems");
+		return 0;
+	}
+
 	priv = kunit_kzalloc(test, sizeof(*priv), GFP_KERNEL);
 	KUNIT_ASSERT_NOT_ERR_OR_NULL(test, priv);
 	test->priv = priv;

mm/util.c

@@ -484,7 +484,9 @@ void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack)
 	clear_bit(MMF_TOPDOWN, &mm->flags);
 }
 #endif
+#ifdef CONFIG_MMU
 EXPORT_SYMBOL_IF_KUNIT(arch_pick_mmap_layout);
+#endif
 
 /**
  * __account_locked_vm - account locked pages to an mm's locked_vm