[gobject-introspection/ebassi/rebased-girffi-exec-fix: 2/8] girffi.c: fix return value for g_callable_info_prepare_closure()

[Emmanuele Bassi <ebassi src gnome org>]

commit 62c3c955547599a58786f20497748569c148379e
Author: Sergei Trofimovich <slyfox gentoo org>
Date:   Sun Jun 27 20:57:13 2021 +0100

    girffi.c: fix return value for g_callable_info_prepare_closure()
    
    The initial failure was observed on `meld` against recently released
    `libffi-3.4-rc1`. There `meld` crashes as:
    
    ```
    $ meld
    Segmentation fault (core dumped)
    
    $ gdb --args /usr/bin/python3.9 /usr/bin/meld
    (gdb) run
    ...
    Thread 1 "python3.9" received signal SIGSEGV, Segmentation fault.
    0x00007fffe9ac1ae8 in g_callable_info_free_closure (
      callable_info=0x555555d45990, closure=0x7fffe9e70c20)
        at ../gobject-introspection-1.68.0/girepository/girffi.c:428
    428       g_free (wrapper->ffi_closure.cif->arg_types);
    (gdb) bt
      callable_info=0x555555d45990, closure=0x7fffe9e70c20)
        at ../gobject-introspection-1.68.0/girepository/girffi.c:428
      data=0x555555d252d0)
        at ../pygobject-3.40.1/gi/pygi-closure.c:635
    ...
    ```
    
    The bug here is in type mismatch between expected return value of
    `g_callable_info_prepare_closure()` and actual value (executable
    code pointer):
    
    ```c
    ffi_closure * g_callable_info_prepare_closure(...) {
        gpointer exec_ptr;
        ...
        status = ffi_prep_closure_loc (&closure->ffi_closure, cif, callback, user_data, exec_ptr);
    
        return exec_ptr;
    }
    ```
    
    Note: `exec_ptr` is a code pointer that could be directly executed by
    caller, like `((rt (*)(a1,a2))exec_ptr)(1,2);` It should never be wrapped
    into an `ffi_closure*`, which is normally called via `ffi_call(closure, ...)`.
    
    We see the problem when we try to free direct code pointer instead of
    `ffi_closure()` as starting from libffi-3.4 executable trampoline and
    `ffi_closure()` don't necessarily live in the same block:
    
        https://github.com/libffi/libffi/commit/9ba559217bea0803263a9a9a0bafcf9203606f5b
    
    Signed-off-by: Sergei Trofimovich <slyfox gentoo org>

 girepository/girffi.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)
---
diff --git a/girepository/girffi.c b/girepository/girffi.c
index 86a13052..b3b92195 100644
--- a/girepository/girffi.c
+++ b/girepository/girffi.c
@@ -406,10 +406,7 @@ g_callable_info_prepare_closure (GICallableInfo       *callable_info,
       return NULL;
     }
 
-  /* Return exec_ptr, which points to the same underlying memory as
-   * closure, but via an executable-non-writable mapping.
-   */
-  return exec_ptr;
+  return &closure->ffi_closure;
 }
 
 /**