libcrun: document vfork shared error

src/libcrun/linux.c

@@ -6198,24 +6198,31 @@ run_in_container_namespace (libcrun_container_status_t *status, int (*callback)
       ret = setns (pidfd, CLONE_NEWNS);
       if (UNLIKELY (ret < 0))
         {
+          /* Create an error for the parent proc.  */
           crun_make_error (err, 0, "setns to target pid");
           _safe_exit (ret);
         }
       ret = chdir ("/");
       if (UNLIKELY (ret < 0))
         {
+          /* Create an error for the parent proc.  */
           crun_make_error (err, errno, "chdir to `/`");
           _safe_exit (ret);
         }
 
       ret = callback (arg, err);
+      /* On failure (ret < 0) the created error will be
+         used by the parent proc */
       _safe_exit (ret);
     }
 
   ret = waitpid_ignore_stopped (pid, &wait_status, 0);
   if (UNLIKELY (ret < 0))
     return crun_make_error (err, errno, "waitpid for exec child pid");
 
+  /* The vfork() child shares the parent's memory space, so the error
+     object populated by the child is available to the parent.
+     Do not call crun_make_error() here. */
   return get_process_exit_status (wait_status);
 }
 

src/libcrun/net_device.c

@@ -474,7 +474,10 @@ move_network_device (const char *ifname, const char *newifname, int netns_fd, li
     {
       ret = setup_network_device_in_ns_helper (buffer, buffer_size, netns_fd, newifname, ips, err);
       if (UNLIKELY (ret < 0))
-        _safe_exit (-ret);
+        {
+          /* Do not release the error as it will be used by the parent proc.  */
+          _safe_exit (-ret);
+        }
 
       _safe_exit (0);
     }
@@ -484,7 +487,11 @@ move_network_device (const char *ifname, const char *newifname, int netns_fd, li
     return crun_make_error (err, errno, "waitpid for exec child pid");
 
   if (wait_status != 0)
-    return -get_process_exit_status (wait_status);
+    {
+      /* Reuse the error from the child proc.
+         In other words, do not call crun_make_error() here. */
+      return -get_process_exit_status (wait_status);
+    }
 
   return 0;
 }