[gobject-introspection] Correctly use ffi_closure_alloc(), fixing mmap permissions error

[Owen Taylor <otaylor src gnome org>]

commit 024d7d9c94fd533472a6abdcced94ee638c0d7fc
Author: Owen W. Taylor <otaylor fishsoup net>
Date:   Wed Apr 7 17:48:52 2010 -0400

    Correctly use ffi_closure_alloc(), fixing mmap permissions error
    
    This restores the reverted-commit ed8634d:
    
        Author: Dan Winship <danw gnome org>
        Date:   Mon Apr 5 14:05:52 2010 -0400
    
        g_callable_info_prepare_closure: handle mmap permissions error
    
        When SELinux or similar is active, a process may not be able to mmap()
        a segment that is both writable and executable, which would causing
        g_callable_info_prepare_closure() to fail. Libffi has a workaround for
        this problem though (it maps the same non-anonymous region twice, once
        writable and once executable, and returns two separate pointers to
        it), so use that.
    
    But with an added fix, when using ffi_closure_alloc(), we need to use
    ffi_prep_closure_loc() so we can pass in the executable address of the
    trampoline separately from the writable address, rather than the
    deprecated ffi_prep_closure().
    
    https://bugzilla.gnome.org/show_bug.cgi?id=615105

 girepository/girffi.c |   44 ++++++++++++++++++++++----------------------
 1 files changed, 22 insertions(+), 22 deletions(-)
---
diff --git a/girepository/girffi.c b/girepository/girffi.c
index 88cafec..3f0b670 100644
--- a/girepository/girffi.c
+++ b/girepository/girffi.c
@@ -297,6 +297,11 @@ g_function_invoker_destroy (GIFunctionInvoker    *invoker)
   g_free (invoker->cif.arg_types);
 }
 
+typedef struct {
+  ffi_closure ffi_closure;
+  gpointer writable_self;
+} GIClosureWrapper;
+
 /**
  * g_callable_info_prepare_closure:
  * @callable_info: a callable info from a typelib
@@ -306,11 +311,8 @@ g_function_invoker_destroy (GIFunctionInvoker    *invoker)
  *
  * Prepares a callback for ffi invocation.
  *
- * Note: this function requires the heap to be executable, which
- *       might not function properly on systems with SELinux enabled.
- *
  * Return value: the ffi_closure or NULL on error.
- * The return value should be freed by calling g_callable_info_prepare_closure().
+ * The return value should be freed by calling g_callable_info_free_closure().
  */
 ffi_closure *
 g_callable_info_prepare_closure (GICallableInfo       *callable_info,
@@ -318,21 +320,21 @@ g_callable_info_prepare_closure (GICallableInfo       *callable_info,
                                  GIFFIClosureCallback  callback,
                                  gpointer              user_data)
 {
-  ffi_closure *closure;
+  gpointer exec_ptr;
+  GIClosureWrapper *closure;
   ffi_status status;
 
   g_return_val_if_fail (callable_info != NULL, FALSE);
   g_return_val_if_fail (cif != NULL, FALSE);
   g_return_val_if_fail (callback != NULL, FALSE);
 
-  closure = mmap (NULL, sizeof (ffi_closure),
-                  PROT_EXEC | PROT_READ | PROT_WRITE,
-                  MAP_ANON | MAP_PRIVATE, -1, sysconf (_SC_PAGE_SIZE));
+  closure = ffi_closure_alloc (sizeof (GIClosureWrapper), &exec_ptr);
   if (!closure)
     {
-      g_warning("mmap failed: %s\n", strerror(errno));
+      g_warning ("could not allocate closure\n");
       return NULL;
     }
+  closure->writable_self = closure;
 
   status = ffi_prep_cif (cif, FFI_DEFAULT_ABI,
                          g_callable_info_get_n_args (callable_info),
@@ -340,27 +342,23 @@ g_callable_info_prepare_closure (GICallableInfo       *callable_info,
                          g_callable_info_get_ffi_arg_types (callable_info));
   if (status != FFI_OK)
     {
-      g_warning("ffi_prep_cif failed: %d\n", status);
-      munmap(closure, sizeof (closure));
+      g_warning ("ffi_prep_cif failed: %d\n", status);
+      ffi_closure_free (closure);
       return NULL;
     }
 
-  status = ffi_prep_closure (closure, cif, callback, user_data);
+  status = ffi_prep_closure_loc (&closure->ffi_closure, cif, callback, user_data, exec_ptr);
   if (status != FFI_OK)
     {
       g_warning ("ffi_prep_closure failed: %d\n", status);
-      munmap(closure, sizeof (closure));
+      ffi_closure_free (closure);
       return NULL;
     }
 
-  if (mprotect(closure, sizeof (closure), PROT_READ | PROT_EXEC) == -1)
-    {
-      g_warning ("ffi_prep_closure failed: %s\n", strerror(errno));
-      munmap(closure, sizeof (closure));
-      return NULL;
-    }
-
-  return closure;
+  /* Return exec_ptr, which points to the same underlying memory as
+   * closure, but via an executable-non-writable mapping.
+   */
+  return exec_ptr;
 }
 
 /**
@@ -374,5 +372,7 @@ void
 g_callable_info_free_closure (GICallableInfo *callable_info,
                               ffi_closure    *closure)
 {
-  munmap(closure, sizeof (closure));
+  GIClosureWrapper *wrapper = (GIClosureWrapper *)closure;
+
+  ffi_closure_free (wrapper->writable_self);
 }