volume_id: make FAT32 recognition more robust

FAT32 volumes should never have a cluster count, that fits
into a 16 bit value, but mkdosfs can create such volumes.
No sane formatter or Windows will ever do this, but the
Linux kernel as Windows can read/write it.

Thanks to Ted Ts'o <tytso@mit.edu> for convincing me.

Signed-off-by: Kay Sievers <kay.sievers@suse.de>

1 files changed, 22 insertions, 9 deletions

diff --git a/extras/volume_id/volume_id/fat.c b/extras/volume_id/volume_id/fat.c
index d5f84ece4c..74f2bb9194 100644
--- a/extras/volume_id/volume_id/fat.c
+++ b/extras/volume_id/volume_id/fat.c
@@ -138,7 +138,8 @@ int volume_id_probe_vfat(struct volume_id *id, uint64_t off)
 	uint32_t root_cluster;
 	uint32_t dir_size;
 	uint32_t cluster_count;
-	uint32_t fat_length;
+	uint16_t fat_length;
+	uint32_t fat32_length;
 	uint64_t root_start;
 	uint32_t start_data_sect;
 	uint16_t root_dir_entries;
@@ -222,11 +223,18 @@ valid:
 	dbg("sect_count 0x%x", sect_count);
 
 	fat_length = le16_to_cpu(vs->fat_length);
-	if (fat_length == 0)
-		fat_length = le32_to_cpu(vs->type.fat32.fat32_length);
 	dbg("fat_length 0x%x", fat_length);
+	fat32_length = le32_to_cpu(vs->type.fat32.fat32_length);
+	dbg("fat32_length 0x%x", fat32_length);
+
+	if (fat_length)
+		fat_size = fat_length * vs->fats;
+	else if (fat32_length)
+		fat_size = fat32_length * vs->fats;
+	else
+		return -1;
+	dbg("fat_size 0x%x", fat_size);
 
-	fat_size = fat_length * vs->fats;
 	dir_size = ((dir_entries * sizeof(struct vfat_dir_entry)) +
 			(sector_size-1)) / sector_size;
 	dbg("dir_size 0x%x", dir_size);
@@ -235,14 +243,17 @@ valid:
 	cluster_count /= vs->sectors_per_cluster;
 	dbg("cluster_count 0x%x", cluster_count);
 
-	if (cluster_count < FAT12_MAX) {
+	/* must be FAT32 */
+	if (!fat_length && fat32_length)
+		goto fat32;
+
+	/* cluster_count tells us the format */
+	if (cluster_count < FAT12_MAX)
 		strcpy(id->type_version, "FAT12");
-	} else if (cluster_count < FAT16_MAX) {
+	else if (cluster_count < FAT16_MAX)
 		strcpy(id->type_version, "FAT16");
-	} else {
-		strcpy(id->type_version, "FAT32");
+	else
 		goto fat32;
-	}
 
 	/* the label may be an attribute in the root directory */
 	root_start = (reserved + fat_size) * sector_size;
@@ -274,6 +285,8 @@ valid:
 	goto found;
 
 fat32:
+	strcpy(id->type_version, "FAT32");
+
 	/* FAT32 root dir is a cluster chain like any other directory */
 	buf_size = vs->sectors_per_cluster * sector_size;
 	root_cluster = le32_to_cpu(vs->type.fat32.root_cluster);