py/persistentcode: Maintain root ptr list of imported native .mpy code.

On ports where normal heap memory can contain executable code (eg ARM-based
ports such as stm32), native code loaded from an .mpy file may be reclaimed
by the GC because there's no reference to the very start of the native
machine code block that is reachable from root pointers (only pointers to
internal parts of the machine code block are reachable, but that doesn't
help the GC find the memory).

This commit fixes this issue by maintaining an explicit list of root
pointers pointing to native code that is loaded from an .mpy file.  This
is not needed for all ports so is selectable by the new configuration
option MICROPY_PERSISTENT_CODE_TRACK_RELOC_CODE.  It's enabled by default
if a port does not specify any special functions to allocate or commit
executable memory.

A test is included to test that native code loaded from an .mpy file does
not get reclaimed by the GC.

Fixes #6045.

Signed-off-by: Damien George <damien@micropython.org>

4 files changed, 34 insertions, 1 deletions

diff --git a/py/mpconfig.h b/py/mpconfig.h
index 287b15aaef..3d9ce8bb55 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -351,6 +351,18 @@
 // Convenience definition for whether any native or inline assembler emitter is enabled
 #define MICROPY_EMIT_MACHINE_CODE (MICROPY_EMIT_NATIVE || MICROPY_EMIT_INLINE_ASM)
 
+// Whether native relocatable code loaded from .mpy files is explicitly tracked
+// so that the GC cannot reclaim it.  Needed on architectures that allocate
+// executable memory on the MicroPython heap and don't explicitly track this
+// data some other way.
+#ifndef MICROPY_PERSISTENT_CODE_TRACK_RELOC_CODE
+#if !MICROPY_EMIT_MACHINE_CODE || defined(MP_PLAT_ALLOC_EXEC) || defined(MP_PLAT_COMMIT_EXEC)
+#define MICROPY_PERSISTENT_CODE_TRACK_RELOC_CODE (0)
+#else
+#define MICROPY_PERSISTENT_CODE_TRACK_RELOC_CODE (1)
+#endif
+#endif
+
 /*****************************************************************************/
 /* Compiler configuration                                                    */
 
diff --git a/py/mpstate.h b/py/mpstate.h
index 5f6cf55936..2519c77e2d 100644
--- a/py/mpstate.h
+++ b/py/mpstate.h
@@ -167,6 +167,11 @@ typedef struct _mp_state_vm_t {
     mp_obj_dict_t *mp_module_builtins_override_dict;
     #endif
 
+    #if MICROPY_PERSISTENT_CODE_TRACK_RELOC_CODE
+    // An mp_obj_list_t that tracks relocated native code to prevent the GC from reclaiming them.
+    mp_obj_t track_reloc_code_list;
+    #endif
+
     // include any root pointers defined by a port
     MICROPY_PORT_ROOT_POINTERS
 
diff --git a/py/persistentcode.c b/py/persistentcode.c
index 386ea49477..da3234a5fe 100644
--- a/py/persistentcode.c
+++ b/py/persistentcode.c
@@ -3,7 +3,7 @@
  *
  * The MIT License (MIT)
  *
- * Copyright (c) 2013-2016 Damien P. George
+ * Copyright (c) 2013-2020 Damien P. George
  *
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
@@ -516,6 +516,18 @@ STATIC mp_raw_code_t *load_raw_code(mp_reader_t *reader, qstr_window_t *qw) {
         fun_data = MP_PLAT_COMMIT_EXEC(fun_data, fun_data_len, opt_ri);
         #else
         if (prelude.scope_flags & MP_SCOPE_FLAG_VIPERRELOC) {
+            #if MICROPY_PERSISTENT_CODE_TRACK_RELOC_CODE
+            // If native code needs relocations then it's not guaranteed that a pointer to
+            // the head of `buf` (containing the machine code) will be retained for the GC
+            // to trace.  This is because native functions can start inside `buf` and so
+            // it's possible that the only GC-reachable pointers are pointers inside `buf`.
+            // So put this `buf` on a list of reachable root pointers.
+            if (MP_STATE_PORT(track_reloc_code_list) == MP_OBJ_NULL) {
+                MP_STATE_PORT(track_reloc_code_list) = mp_obj_new_list(0, NULL);
+            }
+            mp_obj_list_append(MP_STATE_PORT(track_reloc_code_list), MP_OBJ_FROM_PTR(fun_data));
+            #endif
+            // Do the relocations.
             mp_native_relocate(&ri, fun_data, (uintptr_t)fun_data);
         }
         #endif
diff --git a/py/runtime.c b/py/runtime.c
index 78da386919..38da2f4538 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -106,6 +106,10 @@ void mp_init(void) {
     MP_STATE_VM(mp_module_builtins_override_dict) = NULL;
     #endif
 
+    #if MICROPY_PERSISTENT_CODE_TRACK_RELOC_CODE
+    MP_STATE_VM(track_reloc_code_list) = MP_OBJ_NULL;
+    #endif
+
     #if MICROPY_PY_OS_DUPTERM
     for (size_t i = 0; i < MICROPY_PY_OS_DUPTERM; ++i) {
         MP_STATE_VM(dupterm_objs[i]) = MP_OBJ_NULL;