mbr: Warn if MBR gap is small and user uses advanced modules

We don't want to support small MBR gap in pair with anything but the
simplest config of biosdisk + part_msdos + simple filesystem. In this
path "simple filesystems" are all current filesystems except ZFS and
Btrfs.

Signed-off-by: Vladimir Serbinenko <phcoder@google.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/partmap/gpt.c

@@ -25,6 +25,9 @@
 #include <grub/msdos_partition.h>
 #include <grub/gpt_partition.h>
 #include <grub/i18n.h>
+#ifdef GRUB_UTIL
+#include <grub/emu/misc.h>
+#endif
 
 GRUB_MOD_LICENSE ("GPLv3+");
 
@@ -169,7 +172,8 @@ static grub_err_t
 gpt_partition_map_embed (struct grub_disk *disk, unsigned int *nsectors,
 			 unsigned int max_nsectors,
 			 grub_embed_type_t embed_type,
-			 grub_disk_addr_t **sectors)
+			 grub_disk_addr_t **sectors,
+			 int warn_short)
 {
   struct gpt_partition_map_embed_ctx ctx = {
     .start = 0,
@@ -191,6 +195,9 @@ gpt_partition_map_embed (struct grub_disk *disk, unsigned int *nsectors,
 		       N_("this GPT partition label contains no BIOS Boot Partition;"
 			  " embedding won't be possible"));
 
+  if (ctx.len < GRUB_MIN_RECOMMENDED_MBR_GAP)
+    grub_util_warn ("Your BIOS Boot Partition is under 1 MiB, please increase its size.");
+
   if (ctx.len < *nsectors)
     return grub_error (GRUB_ERR_OUT_OF_RANGE,
 		       N_("your BIOS Boot Partition is too small;"