feat: check more BPB fields when mounting fs

Fail mounting if:
* root_entries is zero (only on FAT12/FAT16)
* backup_boot_sector is outside of the reserved region (only on FAT32)
* fs_info_sector is outside of the reserved region (only on FAT32)

src/boot_sector.rs

@@ -202,6 +202,13 @@ impl BiosParameterBlock {
             ));
         }
 
+        if !is_fat32 && self.root_entries == 0 {
+            return Err(Error::new(
+                ErrorKind::Other,
+                "Empty root directory region defined in FAT12/FAT16 BPB",
+            ));
+        }
+
         if (u32::from(self.root_entries) * DIR_ENTRY_SIZE as u32) % u32::from(self.bytes_per_sector) != 0 {
             warn!("Root entries should fill sectors fully");
         }
@@ -235,6 +242,14 @@ impl BiosParameterBlock {
             return Err(Error::new(ErrorKind::Other, "Invalid BPB (total_sectors field value is too small)"));
         }
 
+        if is_fat32 && self.backup_boot_sector() >= self.reserved_sectors() {
+            return Err(Error::new(ErrorKind::Other, "Invalid BPB (backup boot-sector not in a reserved region)"));
+        }
+
+        if is_fat32 && self.fs_info_sector() >= self.reserved_sectors() {
+            return Err(Error::new(ErrorKind::Other, "Invalid BPB (FSInfo sector not in a reserved region)"));
+        }
+
         let total_clusters = self.total_clusters();
         let fat_type = FatType::from_clusters(total_clusters);
         if is_fat32 != (fat_type == FatType::Fat32) {

src/fs.rs

@@ -913,7 +913,6 @@ pub fn format_volume<T: ReadWriteSeek>(mut disk: T, options: FormatVolumeOptions
         write_zeros_until_end_of_sector(&mut disk, bytes_per_sector)?;
 
         // backup boot sector
-        debug_assert!(boot.bpb.backup_boot_sector() < boot.bpb.reserved_sectors());
         disk.seek(SeekFrom::Start(boot.bpb.bytes_from_sectors(boot.bpb.backup_boot_sector())))?;
         boot.serialize(&mut disk)?;
         write_zeros_until_end_of_sector(&mut disk, bytes_per_sector)?;