kernel: make k_is_pre_kernel safe to call from user mode

Make k_is_pre_kernel safe to call from user mode. Since
z_sys_post_kernel memory is not accessible to user threads, calling
k_is_pre_kernel would result in a memory access fault.

This change adds k_is_user_context guard. It can be assumed the system
is post-kernel if k_is_user_context is true.

Signed-off-by: Rob Barnes <[email protected]>

Author: rbbrns

include/zephyr/kernel.h

@@ -1289,6 +1289,15 @@ static inline bool k_is_pre_kernel(void)
 {
 	extern bool z_sys_post_kernel; /* in init.c */
 
+	/*
+	 * If called from user mode, it must already be post kernel.
+	 * This guard is necessary because z_sys_post_kernel memory
+	 * is not accessible to user threads.
+	 */
+	if (k_is_user_context()) {
+		return false;
+	}
+
 	return !z_sys_post_kernel;
 }
 

tests/kernel/mem_protect/userspace/src/main.c

@@ -110,6 +110,18 @@ ZTEST_USER(userspace, test_is_usermode)
 	zassert_true(k_is_user_context(), "thread left in kernel mode");
 }
 
+/**
+ * @brief Test to check if k_is_pre_kernel works from user mode
+ *
+ * @ingroup kernel_memprotect_tests
+ */
+ZTEST_USER(userspace, test_is_post_kernel)
+{
+	clear_fault();
+
+	zassert_false(k_is_pre_kernel(), "still pre-kernel in user mode");
+}
+
 /**
  * @brief Test to write to a control register
  *