fs: fat: calculate FAT type based on cluster count

This fixes an issue where the FAT type (FAT12, FAT16) is not
correctly detected, e.g. when the BPB field BS_FilSysType contains the
valid value "FAT     ".

According to the FAT spec the field BS_FilSysType has only
informational character and does not determine the FAT type.

The logic of this code is based on the linux kernel implementation
from the file fs/fat/inode.c function fat_fill_super().

For details about FAT see http://elm-chan.org/docs/fat_e.html

Signed-off-by: Christian Taedcke <christian.taedcke@weidmueller.com>

diff --git a/fs/fat/fat.c b/fs/fat/fat.c
index a3522340efc478c21d2cbfb716a55b3f69fcd2ba..c368c3b076589376f8eb65abcdd4f4825bc33465 100644 (file)
--- a/fs/fat/fat.c
+++ b/fs/fat/fat.c
@@ -26,6 +26,9 @@
 #include <linux/compiler.h>
 #include <linux/ctype.h>
 
+/* maximum number of clusters for FAT12 */
+#define MAX_FAT12      0xFF4
+
 /*
  * Convert a string to lowercase.  Converts at most 'len' characters,
  * 'len' may be larger than the length of 'str' if 'str' is NULL
@@ -484,6 +487,27 @@ static __u8 mkcksum(struct nameext *nameext)
        return ret;
 }
 
+/*
+ * Determine if the FAT type is FAT12 or FAT16
+ *
+ * Based on fat_fill_super() from the Linux kernel's fs/fat/inode.c
+ */
+static int determine_legacy_fat_bits(const boot_sector *bs)
+{
+       u16 fat_start = bs->reserved;
+       u32 dir_start = fat_start + bs->fats * bs->fat_length;
+       u32 rootdir_sectors = get_unaligned_le16(bs->dir_entries) *
+                             sizeof(dir_entry) /
+                             get_unaligned_le16(bs->sector_size);
+       u32 data_start = dir_start + rootdir_sectors;
+       u16 sectors = get_unaligned_le16(bs->sectors);
+       u32 total_sectors = sectors ? sectors : bs->total_sect;
+       u32 total_clusters = (total_sectors - data_start) /
+                            bs->cluster_size;
+
+       return (total_clusters > MAX_FAT12) ? 16 : 12;
+}
+
 /*
  * Read boot sector and volume info from a FAT filesystem
  */
@@ -518,7 +542,7 @@ read_bootsectandvi(boot_sector *bs, volume_info *volinfo, int *fatsize)
        bs->total_sect = FAT2CPU32(bs->total_sect);
 
        /* FAT32 entries */
-       if (bs->fat_length == 0) {
+       if (!bs->fat_length && bs->fat32_length) {
                /* Assume FAT32 */
                bs->fat32_length = FAT2CPU32(bs->fat32_length);
                bs->flags = FAT2CPU16(bs->flags);
@@ -529,25 +553,10 @@ read_bootsectandvi(boot_sector *bs, volume_info *volinfo, int *fatsize)
                *fatsize = 32;
        } else {
                vistart = (volume_info *)&(bs->fat32_length);
-               *fatsize = 0;
+               *fatsize = determine_legacy_fat_bits(bs);
        }
        memcpy(volinfo, vistart, sizeof(volume_info));
-
-       if (*fatsize == 32) {
-               if (strncmp(FAT32_SIGN, vistart->fs_type, SIGNLEN) == 0)
-                       goto exit;
-       } else {
-               if (strncmp(FAT12_SIGN, vistart->fs_type, SIGNLEN) == 0) {
-                       *fatsize = 12;
-                       goto exit;
-               }
-               if (strncmp(FAT16_SIGN, vistart->fs_type, SIGNLEN) == 0) {
-                       *fatsize = 16;
-                       goto exit;
-               }
-       }
-
-       debug("Error: broken fs_type sign\n");
+       goto exit;
 fail:
        ret = -1;
 exit:
@@ -1157,9 +1166,8 @@ int file_fat_detectfs(void)
 
        memcpy(vol_label, volinfo.volume_label, 11);
        vol_label[11] = '\0';
-       volinfo.fs_type[5] = '\0';
 
-       printf("Filesystem: %s \"%s\"\n", volinfo.fs_type, vol_label);
+       printf("Filesystem: FAT%d \"%s\"\n", fatsize, vol_label);
 
        return 0;
 }

diff --git a/include/fat.h b/include/fat.h
index a9756fb4cd1b51797001af5f11d7f4541ddff59e..3dce99a23cf5d9722f260e89d34aafd654952279 100644 (file)
--- a/include/fat.h
+++ b/include/fat.h
@@ -34,12 +34,6 @@ struct disk_partition;
 /* Maximum number of entry for long file name according to spec */
 #define MAX_LFN_SLOT   20
 
-/* Filesystem identifiers */
-#define FAT12_SIGN     "FAT12   "
-#define FAT16_SIGN     "FAT16   "
-#define FAT32_SIGN     "FAT32   "
-#define SIGNLEN                8
-
 /* File attributes */
 #define ATTR_RO        1
 #define ATTR_HIDDEN    2