Verify out-of-bound accesses with region.c buffers

region_watermark.c

static const uint64_t watermark_before = 0x74f31d37285c4c37;
static const uint64_t watermark_after = 0xb10269a05bf10c29;

static void *
region_alloc_with_watermark(struct region *region, size_t size)
{
        size_t real_size = size + 2 * sizeof(uint64_t);
        void *real_buf = region_alloc(region, real_size);
        if (real_buf == NULL)
                return real_buf;
        *(uint64_t *)real_buf = watermark_before;
        *((uint64_t *)((char *)real_buf + real_size) - 1) = watermark_after;
        return (char *)real_buf + sizeof(uint64_t);
}

static void
region_check_watermark(void *buf, size_t size)
{
        void *real_buf = (char *)buf - sizeof(uint64_t);
        size_t real_size = size + 2 * sizeof(uint64_t);
        assert(*(uint64_t *)real_buf == watermark_before);
        assert(*((uint64_t *)((char *)real_buf + real_size) - 1) ==
               watermark_after);
}