[PATCH v1 2/2] fs: fat: calculate FAT type based on cluster count

[christian.taedcke-oss at weidmueller.com]

From: Christian Taedcke <christian.taedcke at weidmueller.com>

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 at weidmueller.com>
---

 fs/fat/fat.c             | 58 +++++++++++++++++++++++-----------------
 include/fat.h            |  6 -----
 test/image/spl_load_fs.c |  2 +-
 3 files changed, 35 insertions(+), 31 deletions(-)

diff --git a/fs/fat/fat.c b/fs/fat/fat.c
index 8a0f4e4e6c..9179257ff2 100644
--- a/fs/fat/fat.c
+++ b/fs/fat/fat.c
@@ -27,6 +27,9 @@
 
 #define FATU8ARRAY2CPU16(x) (((x)[1] << 8) + (x)[0])
 
+/* 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
@@ -485,6 +488,32 @@ static __u8 mkcksum(struct nameext *nameext)
 	return ret;
 }
 
+/*
+ * Determine the FAT type
+ *
+ * Based on fat_fill_super() from the Linux kernel's fs/fat/inode.c
+ */
+static int
+determine_fat_bits(const boot_sector *bs)
+{
+	if (!bs->fat_length && bs->fat32_length) {
+		return 32;
+	} else {
+		u16 fat_start = bs->reserved;
+		u32 dir_start = fat_start + bs->fats * bs->fat_length;
+		u32 rootdir_sectors = FATU8ARRAY2CPU16(bs->dir_entries) *
+				      sizeof(dir_entry) /
+				      FATU8ARRAY2CPU16(bs->sector_size);
+		u32 data_start = dir_start + rootdir_sectors;
+		u16 sectors = FATU8ARRAY2CPU16(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
  */
@@ -493,7 +522,6 @@ read_bootsectandvi(boot_sector *bs, volume_info *volinfo, int *fatsize)
 {
 	__u8 *block;
 	volume_info *vistart;
-	int ret = 0;
 
 	if (cur_dev == NULL) {
 		debug("Error: no device selected\n");
@@ -508,7 +536,8 @@ read_bootsectandvi(boot_sector *bs, volume_info *volinfo, int *fatsize)
 
 	if (disk_read(0, 1, block) < 0) {
 		debug("Error: reading block\n");
-		goto fail;
+		free(block);
+		return -1;
 	}
 
 	memcpy(bs, block, sizeof(boot_sector));
@@ -527,33 +556,15 @@ read_bootsectandvi(boot_sector *bs, volume_info *volinfo, int *fatsize)
 		bs->info_sector = FAT2CPU16(bs->info_sector);
 		bs->backup_boot = FAT2CPU16(bs->backup_boot);
 		vistart = (volume_info *)(block + sizeof(boot_sector));
-		*fatsize = 32;
 	} else {
 		vistart = (volume_info *)&(bs->fat32_length);
-		*fatsize = 0;
 	}
 	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;
-		}
-	}
+	*fatsize = determine_fat_bits(bs);
 
-	debug("Error: broken fs_type sign\n");
-fail:
-	ret = -1;
-exit:
 	free(block);
-	return ret;
+	return 0;
 }
 
 static int get_fs_info(fsdata *mydata)
@@ -1158,9 +1169,8 @@ 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 a9756fb4cd..3dce99a23c 100644
--- 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
diff --git a/test/image/spl_load_fs.c b/test/image/spl_load_fs.c
index 297ab08a82..90e640b5de 100644
--- a/test/image/spl_load_fs.c
+++ b/test/image/spl_load_fs.c
@@ -220,7 +220,7 @@ static size_t create_fat(void *dst, size_t size, const char *filename,
 	bs->root_cluster = cpu_to_le32(root_sector);
 
 	vi->ext_boot_sign = 0x29;
-	memcpy(vi->fs_type, FAT32_SIGN, sizeof(vi->fs_type));
+	memcpy(vi->fs_type, "FAT32   ", sizeof(vi->fs_type));
 
 	memcpy(dst + 0x1fe, "\x55\xAA", 2);
 
-- 
2.34.1