[SSSD] [sssd PR#740][opened] sbus: avoid using invalid stack point in SBUS_INTERFACE

[pbrezina]

   URL: https://github.com/SSSD/sssd/pull/740
Author: pbrezina
 Title: #740: sbus: avoid using invalid stack point in SBUS_INTERFACE
Action: opened

PR body:
"""
SBUS_INTERFACE macros expanded as:
struct sbus_interface bus =
    ({ sbus_interface(
        "org.freedesktop.DBus",
        ((void *)0),
        (((const struct sbus_method[])
        {
            ({
                /* ... compile time check of function signature omitted */ ;
                sbus_method_sync(/* ... full list of params omitted */);
            }),
  ...

This however includes an issue that methods/properties/signals are returned
by value, however stored in sbus_interface as pointers. Once we return out
of the top-level block and assign resulting sbus_interface into 'bus' variable
those objects allocated on stack becomes invalid and can be overwritten by other
allocations on stack.

This patch overcomes this issue by changing declaration of SBUS_INTERFACE and
avoiding using this top-level block. This still keeps the declarative structure
and simplifies the code as it does not require any memory handling and
tests for successful allocations.

    const struct sbus_method __ ## varname ## _m[] = methods;                 \
    const struct sbus_signal __ ## varname ## _s[] = signals;                 \
    const struct sbus_property __ ## varname ## _p[] = properties;            \
    struct sbus_interface varname = SBUS_IFACE_ ## iface(                     \
        (__ ## varname ## _m),                                                \
        (__ ## varname ## _s),                                                \
        (__ ## varname ## _p)                                                 \
    )

Resolves:
https://pagure.io/SSSD/sssd/issue/3924
"""

To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/740/head:pr740
git checkout pr740