bus-message: add macro for calculation of offset from the page

src/basic/memory-util.h

@@ -13,6 +13,7 @@
 size_t page_size(void) _pure_;
 #define PAGE_ALIGN(l) ALIGN_TO((l), page_size())
 #define PAGE_ALIGN_DOWN(l) ((l) & ~(page_size() - 1))
+#define PAGE_OFFSET(l) ((l) & (page_size() - 1))
 
 /* Normal memcpy requires src to be nonnull. We do nothing if n is 0. */
 static inline void memcpy_safe(void *dst, const void *src, size_t n) {

src/libsystemd/sd-bus/bus-message.c

@@ -3022,7 +3022,7 @@ int bus_body_part_map(struct bus_body_part *part) {
                 return 0;
         }
 
-        shift = part->memfd_offset - ((part->memfd_offset / page_size()) * page_size());
+        shift = PAGE_OFFSET(part->memfd_offset);
         psz = PAGE_ALIGN(part->size + shift);
 
         if (part->memfd >= 0)