py/misc: Change sizeof to offsetof for variable-length alloc.

This fixes the case where e.g.

    struct foo_t {
      mp_obj_t x;
      uint16_t y;
      char buf[];
    };

will have `sizeof(struct foo_t)==8`, but `offsetof(struct foo_t, buf)==6`.

When computing the size to allocate for `m_new_obj_var` we need to use
offsetof to avoid over-allocating. This is important especially when it
might cause it to spill over into another GC block.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

1 files changed, 7 insertions, 7 deletions

diff --git a/py/misc.h b/py/misc.h
index 32061a009..352a9f34c 100644
--- a/py/misc.h
+++ b/py/misc.h
@@ -71,26 +71,26 @@ typedef unsigned int uint;
 #define m_new0(type, num) ((type *)(m_malloc0(sizeof(type) * (num))))
 #define m_new_obj(type) (m_new(type, 1))
 #define m_new_obj_maybe(type) (m_new_maybe(type, 1))
-#define m_new_obj_var(obj_type, var_type, var_num) ((obj_type *)m_malloc(sizeof(obj_type) + sizeof(var_type) * (var_num)))
-#define m_new_obj_var0(obj_type, var_type, var_num) ((obj_type *)m_malloc0(sizeof(obj_type) + sizeof(var_type) * (var_num)))
-#define m_new_obj_var_maybe(obj_type, var_type, var_num) ((obj_type *)m_malloc_maybe(sizeof(obj_type) + sizeof(var_type) * (var_num)))
+#define m_new_obj_var(obj_type, var_field, var_type, var_num) ((obj_type *)m_malloc(offsetof(obj_type, var_field) + sizeof(var_type) * (var_num)))
+#define m_new_obj_var0(obj_type, var_field, var_type, var_num) ((obj_type *)m_malloc0(offsetof(obj_type, var_field) + sizeof(var_type) * (var_num)))
+#define m_new_obj_var_maybe(obj_type, var_field, var_type, var_num) ((obj_type *)m_malloc_maybe(offsetof(obj_type, var_field) + sizeof(var_type) * (var_num)))
 #if MICROPY_ENABLE_FINALISER
 #define m_new_obj_with_finaliser(type) ((type *)(m_malloc_with_finaliser(sizeof(type))))
-#define m_new_obj_var_with_finaliser(type, var_type, var_num) ((type *)m_malloc_with_finaliser(sizeof(type) + sizeof(var_type) * (var_num)))
+#define m_new_obj_var_with_finaliser(type, var_field, var_type, var_num) ((type *)m_malloc_with_finaliser(offsetof(type, var_field) + sizeof(var_type) * (var_num)))
 #else
 #define m_new_obj_with_finaliser(type) m_new_obj(type)
-#define m_new_obj_var_with_finaliser(type, var_type, var_num) m_new_obj_var(type, var_type, var_num)
+#define m_new_obj_var_with_finaliser(type, var_field, var_type, var_num) m_new_obj_var(type, var_field, var_type, var_num)
 #endif
 #if MICROPY_MALLOC_USES_ALLOCATED_SIZE
 #define m_renew(type, ptr, old_num, new_num) ((type *)(m_realloc((ptr), sizeof(type) * (old_num), sizeof(type) * (new_num))))
 #define m_renew_maybe(type, ptr, old_num, new_num, allow_move) ((type *)(m_realloc_maybe((ptr), sizeof(type) * (old_num), sizeof(type) * (new_num), (allow_move))))
 #define m_del(type, ptr, num) m_free(ptr, sizeof(type) * (num))
-#define m_del_var(obj_type, var_type, var_num, ptr) (m_free(ptr, sizeof(obj_type) + sizeof(var_type) * (var_num)))
+#define m_del_var(obj_type, var_field, var_type, var_num, ptr) (m_free(ptr, offsetof(obj_type, var_field) + sizeof(var_type) * (var_num)))
 #else
 #define m_renew(type, ptr, old_num, new_num) ((type *)(m_realloc((ptr), sizeof(type) * (new_num))))
 #define m_renew_maybe(type, ptr, old_num, new_num, allow_move) ((type *)(m_realloc_maybe((ptr), sizeof(type) * (new_num), (allow_move))))
 #define m_del(type, ptr, num) ((void)(num), m_free(ptr))
-#define m_del_var(obj_type, var_type, var_num, ptr) ((void)(var_num), m_free(ptr))
+#define m_del_var(obj_type, var_field, var_type, var_num, ptr) ((void)(var_num), m_free(ptr))
 #endif
 #define m_del_obj(type, ptr) (m_del(type, ptr, 1))