#2944: FAT data corruption during unmount()
-----------------------------+----------------------
 Reporter:  Sebastian Huber  |      Owner:  chrisj@…
     Type:  defect           |     Status:  new
 Priority:  normal           |  Milestone:  4.12
Component:  filesystem       |    Version:  4.11
 Severity:  normal           |   Keywords:
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 https://lists.rtems.org/pipermail/users/2017-March/031101.html

 In msdos_shut_down ( msdos_fsunmount.c ) there is a call to
 fat_file_close( .. ) which attempts to close a file
 descriptor and write a range of metadata to that file's director entry
 located in another cluster:

 * fat_file_write_first_cluster_num
 * fat_file_write_file_size
 * fat_file_write_time_and_date

 The problem is that this is the root node, and of course doesn't have a
 corresponding parent directory entry.

 In addition, the "parent directory entry" cluster number is initialised to
 0x1 (FAT_ROOTDIR_CLUSTER_NUM)
 which is not working according to the FAT specification (cluster numbering
 starts at 2).
 This actually creates a critical bug that overwrites random data to above
 sectors, because 2 is subtracted from 1
 to calculate the sector number of the cluster -> through a series of
 function calls -> leads to a sector number at
 the end of FAT2 (just below the start of the cluster region). The driver
 believes this is a FAT region (in fat_buf_release),
 writes the sector to what it "thinks" is FAT1, proceeds to copy the
 changes to FAT2 -> adds FAT_LENGTH (8161) to sector,
 leading to a write well into the cluster region, randomly overwriting
 files.

 The three function calls above lead to fsck complaining about disk
 structure:

 #######

 fsck from util-linux 2.27.1
 fsck.fat 3.0.28 (2015-05-16)
 0x41: Dirty bit is set. Fs was not properly unmounted and some data may be
 corrupt.
 1) Remove dirty bit
 2) No action
 ? 2
 There are differences between boot sector and its backup.
 This is mostly harmless. Differences: (offset:original/backup)
   65:01/00
 1) Copy original to backup
 2) Copy backup to original
 3) No action
 ? 3
 /  and
 /APPLICAT.ION
   share clusters.
   Truncating second to 0 bytes because first is FAT32 root dir.
 /APPLICAT.ION
   File size is 4096 bytes, cluster chain length is 0 bytes.
   Truncating file to 0 bytes.
 Perform changes ? (y/n) n
 /dev/sdm1: 14 files, 1600/1044483 clusters

 ########

 In particular the "shared cluster" problem is caused by
 fat_file_write_first_cluster_num, which adds a directory
 entry to the root directory cluster pointing at itself; e.g. there is a
 directory entry in cluster 2 pointing to
 a file in cluster 2. (Note: this occurs because we have fixed the "point
 to cluster # 1 issue" by reading the relative
 location of the root cluster node from the FAT volume info strcture).

 Removing the function call in msdos_shut_down ( .. ) to close the root
 file descriptor solves the problem perfectly
 (clean fsck). However, we're a bit unsure about the intent behind closing
 the root directory.

--
Ticket URL: <http://devel.rtems.org/ticket/2944>
RTEMS Project <http://www.rtems.org/>
RTEMS Project
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