py: Make sure that static emg-exc-buffer is aligned to size of mp_obj_t.

dpgeorge · dpgeorge · commit ee86de1f1a22 · 2017-04-10T16:02:56.000+10:00
This buffer is used to allocate objects temporarily, and such objects
require that their underlying memory be correctly aligned for their data
type.  Aligning for mp_obj_t should be sufficient for emergency exceptions,
but in general the memory buffer should aligned to the maximum alignment of
the machine (eg on a 32-bit machine with mp_obj_t being 4 bytes, a double
may not be correctly aligned).

This patch fixes a bug for certain nan-boxing builds, where mp_obj_t is 8
bytes and must be aligned to 8 bytes (even though the machine is 32 bit).
diff --git a/py/mpstate.h b/py/mpstate.h
@@ -120,8 +120,8 @@ typedef struct _mp_state_vm_t {
     // memory for exception arguments if we can't allocate RAM
     #if MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF
     #if MICROPY_EMERGENCY_EXCEPTION_BUF_SIZE > 0
-    // statically allocated buf
-    byte mp_emergency_exception_buf[MICROPY_EMERGENCY_EXCEPTION_BUF_SIZE];
+    // statically allocated buf (needs to be aligned to mp_obj_t)
+    mp_obj_t mp_emergency_exception_buf[MICROPY_EMERGENCY_EXCEPTION_BUF_SIZE / sizeof(mp_obj_t)];
     #else
     // dynamically allocated buf
     byte *mp_emergency_exception_buf;
diff --git a/py/objexcept.c b/py/objexcept.c
@@ -348,7 +348,7 @@ mp_obj_t mp_obj_new_exception_msg_varg(const mp_obj_type_t *exc_type, const char
             tuple->items[0] = MP_OBJ_FROM_PTR(str);
 
             byte *str_data = (byte *)&str[1];
-            uint max_len = MP_STATE_VM(mp_emergency_exception_buf) + mp_emergency_exception_buf_size
+            size_t max_len = (byte*)MP_STATE_VM(mp_emergency_exception_buf) + mp_emergency_exception_buf_size
                          - str_data;
 
             vstr_t vstr;
@@ -366,14 +366,14 @@ mp_obj_t mp_obj_new_exception_msg_varg(const mp_obj_type_t *exc_type, const char
 
             o->args = tuple;
 
-            uint offset = &str_data[str->len] - MP_STATE_VM(mp_emergency_exception_buf);
+            size_t offset = &str_data[str->len] - (byte*)MP_STATE_VM(mp_emergency_exception_buf);
             offset += sizeof(void *) - 1;
             offset &= ~(sizeof(void *) - 1);
 
             if ((mp_emergency_exception_buf_size - offset) > (sizeof(o->traceback_data[0]) * 3)) {
                 // We have room to store some traceback.
                 o->traceback_data = (size_t*)((byte *)MP_STATE_VM(mp_emergency_exception_buf) + offset);
-                o->traceback_alloc = (MP_STATE_VM(mp_emergency_exception_buf) + mp_emergency_exception_buf_size - (byte *)o->traceback_data) / sizeof(o->traceback_data[0]);
+                o->traceback_alloc = ((byte*)MP_STATE_VM(mp_emergency_exception_buf) + mp_emergency_exception_buf_size - (byte *)o->traceback_data) / sizeof(o->traceback_data[0]);
                 o->traceback_len = 0;
             }
         }
