tests: arch: riscv: make sure that `gp` reg can't be corrupted

Add a test to make sure that the `gp` global pointer register used for
relative addressing when `CONFIG_RISCV_GP` is enabled can't be
corrupted by a rogue user thread.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>

tests/arch/riscv/userspace/riscv_gp/CMakeLists.txt

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(riscv_gp)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

tests/arch/riscv/userspace/riscv_gp/prj.conf

@@ -0,0 +1,3 @@
+CONFIG_ZTEST=y
+CONFIG_RISCV_GP=y
+CONFIG_TEST_USERSPACE=y

tests/arch/riscv/userspace/riscv_gp/src/main.c

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2024 Meta Platforms
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <stdio.h>
+
+#include <zephyr/arch/riscv/reg.h>
+#include <zephyr/kernel.h>
+#include <zephyr/ztest.h>
+
+#define ROGUE_USER_STACK_SZ 2048
+
+static struct k_thread rogue_user_thread;
+static K_THREAD_STACK_DEFINE(rogue_user_stack, ROGUE_USER_STACK_SZ);
+
+static void rogue_user_fn(void *p1, void *p2, void *p3)
+{
+	zassert_true(k_is_user_context());
+
+	reg_write(gp, 0xbad);
+	zassert_equal(reg_read(gp), 0xbad);
+}
+
+ZTEST_USER(riscv_gp, test_gp_value)
+{
+	uintptr_t gp_val = reg_read(gp);
+	k_tid_t th;
+
+	zassert_not_equal(gp_val, 0);
+
+	th = k_thread_create(&rogue_user_thread, rogue_user_stack, ROGUE_USER_STACK_SZ,
+			     rogue_user_fn, NULL, NULL, NULL, -1, K_USER, K_NO_WAIT);
+	zassert_ok(k_thread_join(th, K_FOREVER));
+
+	zassert_equal(reg_read(gp), gp_val, "`gp` corrupted by user thread");
+}
+
+static void *userspace_setup(void)
+{
+	k_thread_access_grant(k_current_get(), &rogue_user_thread, &rogue_user_stack);
+
+	return NULL;
+}
+
+ZTEST_SUITE(riscv_gp, NULL, userspace_setup, NULL, NULL, NULL);

tests/arch/riscv/userspace/riscv_gp/testcase.yaml

@@ -0,0 +1,8 @@
+common:
+  ignore_faults: true
+  ignore_qemu_crash: true
+  tags: kernel riscv
+  platform_allow:
+    - qemu_riscv64
+tests:
+  arch.riscv64.riscv_gp: {}