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, 1 insertions, 1 deletions

diff --git a/ports/unix/coverage.c b/ports/unix/coverage.c
index f3bb3d450..88dd48bfa 100644
--- a/ports/unix/coverage.c
+++ b/ports/unix/coverage.c
@@ -545,7 +545,7 @@ STATIC mp_obj_t extra_coverage(void) {
         fun_bc.context = &context;
         fun_bc.child_table = NULL;
         fun_bc.bytecode = (const byte *)"\x01"; // just needed for n_state
-        mp_code_state_t *code_state = m_new_obj_var(mp_code_state_t, mp_obj_t, 1);
+        mp_code_state_t *code_state = m_new_obj_var(mp_code_state_t, state, mp_obj_t, 1);
         code_state->fun_bc = &fun_bc;
         code_state->ip = (const byte *)"\x00"; // just needed for an invalid opcode
         code_state->sp = &code_state->state[0];