mm: Assert that we preserve header vs region alignment

grub_mm_region_init() does:

  h = (grub_mm_header_t) (r + 1);

where h is a grub_mm_header_t and r is a grub_mm_region_t.

Cells are supposed to be GRUB_MM_ALIGN aligned, but while grub_mm_dump
ensures this vs the region header, grub_mm_region_init() does not.

It's better to be explicit than implicit here: rather than changing
grub_mm_region_init() to ALIGN_UP(), require that the struct is
explicitly a multiple of the header size.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Tested-by: Patrick Steinhardt <ps@pks.im>

include/grub/mm_private.h

@@ -20,6 +20,7 @@
 #define GRUB_MM_PRIVATE_H	1
 
 #include <grub/mm.h>
+#include <grub/misc.h>
 
 /* For context, see kern/mm.c */
 
@@ -89,4 +90,17 @@ typedef struct grub_mm_region
 extern grub_mm_region_t EXPORT_VAR (grub_mm_base);
 #endif
 
+static inline void
+grub_mm_size_sanity_check (void) {
+  /* Ensure we preserve alignment when doing h = (grub_mm_header_t) (r + 1). */
+  COMPILE_TIME_ASSERT ((sizeof (struct grub_mm_region) %
+		        sizeof (struct grub_mm_header)) == 0);
+
+  /*
+   * GRUB_MM_ALIGN is supposed to represent cell size, and a mm_header is
+   * supposed to be 1 cell.
+   */
+  COMPILE_TIME_ASSERT (sizeof (struct grub_mm_header) == GRUB_MM_ALIGN);
+}
+
 #endif