kernel: Reenable userspace stack tracing, but hidden behind a config flag

Sometimes this is needed for userspace program debugging (such as ports),
but sometimes it can crash, so we leave it off by default.

kernel/config.cmake.template

@@ -15,3 +15,7 @@
 # control characters, leading/trailing spaces, problematic characters and invalid UTF-8). Keep in mind that this restriction
 # is only enforced when creating files; existing files with such illegal filenames are parsed correctly and fully usable.
 # target_compile_definitions(moon PRIVATE MOON_DISABLE_FILENAME_RESTRICTIONS)
+
+# Uncomment the line below to make the kernel also calculate stack traces for userspace addresses on program crashes. 
+# This can aid in debugging, but makes the kernel more unstable as stack tracing will access arbitrary userspace memory.
+# target_compile_definitions(moon PRIVATE MOON_ENABLE_USERSPACE_STACK_TRACES)

kernel/src/arch/x86_64/CPU.cpp

@@ -326,7 +326,10 @@ namespace CPU
     static void backtrace_impl(u64 base_pointer, void (*callback)(u64, void*), void* arg)
     {
         StackFrame* current_frame = (StackFrame*)base_pointer;
-        while (current_frame && (u64)current_frame >= 0xFFFF'FFFF'8000'0000 &&
+        while (current_frame &&
+#ifndef MOON_ENABLE_USERSPACE_STACK_TRACES
+               (u64)current_frame >= 0xFFFF'FFFF'8000'0000 &&
+#endif
                MemoryManager::validate_access(current_frame, sizeof(*current_frame), MemoryManager::DEFAULT_ACCESS) &&
                current_frame->instruction)
         {