memdisk: don't key FAT vs MBR on the type of disk

We can't rely on the disk type to say if this is FAT vs MBR, since we
haven't determined that yet.  Thus, move the FAT test up before the
MBR test; both are heuristic, but the FAT test has much less
probability of false positives, and can be further tightened if
needed.

diff --git a/memdisk/setup.c b/memdisk/setup.c
index c30a11f..e380435 100644 (file)
--- a/memdisk/setup.c
+++ b/memdisk/setup.c
@@ -533,32 +533,10 @@ const struct geometry *get_disk_image_geometry(uint32_t where, uint32_t size)
     
     max_h = max_s = 0;
 
-    if ( hd_geometry.driveno & 0x80 ) {
-      /* Hard disk image, need to examine the partition table for geometry */
-      const struct ptab_entry *ptab = (const struct ptab_entry *)
-       ((char *)where+hd_geometry.offset+(512-2-4*16));
-
-      for ( i = 0 ; i < 4 ; i++ ) {
-       if ( ptab[i].type ) {
-         s = (ptab[i].start_s & 0x3f);
-         h = ptab[i].start_h + 1;
-         
-         if ( max_h < h ) max_h = h;
-         if ( max_s < s ) max_s = s;
-         
-         s = (ptab[i].end_s & 0x3f);
-         h = ptab[i].end_h + 1;
-         
-         if ( max_h < h ) max_h = h;
-         if ( max_s < s ) max_s = s;
-       }
-      }
-    }
-
     if (!max_h && !max_s) {
-      /* Floppy image, or unpartitioned hard disk... look for a FAT
-        superblock and if we find something that looks enough like one,
-        use geometry from that. */
+      /* Look for a FAT superblock and if we find something that looks
+        enough like one, use geometry from that.  This takes care of
+        megafloppy images and unpartitioned hard disks. */
       const struct fat_extra *extra = NULL;
       const struct fat_super *fs = (const struct fat_super *)
        ((char *)where+hd_geometry.offset);
@@ -582,6 +560,32 @@ const struct geometry *get_disk_image_geometry(uint32_t where, uint32_t size)
       }
     }
 
+    if (!max_h && !max_s) {
+      /* Not FAT, assume it is a hard disk image and scan for a
+        partition table */
+      const struct ptab_entry *ptab = (const struct ptab_entry *)
+       ((char *)where+hd_geometry.offset+(512-2-4*16));
+
+      if (*(uint16_t *)((char *)where+512) == 0xaa55)
+       for ( i = 0 ; i < 4 ; i++ ) {
+         if ( ptab[i].type && !(ptab[i].active & 0x7f) ) {
+           s = (ptab[i].start_s & 0x3f);
+           h = ptab[i].start_h + 1;
+           
+           if ( max_h < h ) max_h = h;
+           if ( max_s < s ) max_s = s;
+           
+           s = (ptab[i].end_s & 0x3f);
+           h = ptab[i].end_h + 1;
+           
+           if ( max_h < h ) max_h = h;
+           if ( max_s < s ) max_s = s;
+           
+           hd_geometry.driveno = 0x80; /* Assume hard disk */
+         }
+       }
+    }
+
     if (!max_h)
       max_h = sectors > 2097152 ? 255 : 64;
     if (!max_s)