fs/fat/: add support for DOS 1.x formatted volumes

Add structure for parsed BPB information, struct fat_bios_param_block,
and move all of the deserialization and validation logic from
fat_fill_super() into fat_read_bpb().

Add a 'dos1xfloppy' mount option to infer DOS 2.x BIOS Parameter Block
defaults from block device geometry for ancient floppies and floppy
images, as a fall-back from the default BPB parsing logic.

When fat_read_bpb() finds an invalid FAT filesystem and dos1xfloppy is
set, fall back to fat_read_static_bpb().  fat_read_static_bpb()
validates that the entire BPB is zero, and that the floppy has a
DOS-style 8086 code bootstrapping header.  Then it fills in default BPB
values from media size and a table.[0]

Media size is assumed to be static for archaic FAT volumes.  See also:
[1].

Fixes kernel.org bug #42617.

[0]: https://en.wikipedia.org/wiki/File_Allocation_Table#Exceptions
[1]: http://www.win.tue.nl/~aeb/linux/fs/fat/fat-1.html

[[email protected]: fix missed error code]
Signed-off-by: Conrad Meyer <[email protected]>
Acked-by: OGAWA Hirofumi <[email protected]>
Signed-off-by: OGAWA Hirofumi <[email protected]>
Tested-by: Alan Cox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Documentation/filesystems/vfat.txt

@@ -172,6 +172,11 @@ nfs=stale_rw|nostale_ro
 		To maintain backward compatibility, '-o nfs' is also accepted,
 		defaulting to stale_rw
 
+dos1xfloppy  -- If set, use a fallback default BIOS Parameter Block
+		configuration, determined by backing device size. These static
+		parameters match defaults assumed by DOS 1.x for 160 kiB,
+		180 kiB, 320 kiB, and 360 kiB floppies and floppy images.
+
 
 <bool>: 0,1,yes,no,true,false
 

fs/fat/fat.h

@@ -52,7 +52,8 @@ struct fat_mount_options {
 		 usefree:1,	   /* Use free_clusters for FAT32 */
 		 tz_set:1,	   /* Filesystem timestamps' offset set */
 		 rodir:1,	   /* allow ATTR_RO for directory */
-		 discard:1;	   /* Issue discard requests on deletions */
+		 discard:1,	   /* Issue discard requests on deletions */
+		 dos1xfloppy:1;	   /* Assume default BPB for DOS 1.x floppies */
 };
 
 #define FAT_HASH_BITS	8