blkdev/dosfs/emmc issues.

Tasslehoff Kjappfot tasskjapp at
Fri Mar 17 13:01:08 UTC 2017

Thanks for the notice. I'll try to get around to writing a test case.

Here's a bit more detailed info from the dev who has done the deep
digging into this:


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)
1) Copy original to backup
2) Copy backup to original
3) No action
? 3
/  and
  share clusters.
  Truncating second to 0 bytes because first is FAT32 root dir.
  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. 

On 03/17/17 10:29, Sebastian Huber wrote:
> I fixed a couple of FAT file system bugs yesterday on the Git master.
> It would be great if you could provide a self-contained test case for
> your issue. See for example "testsuite/fstests/fsdosfsname02".
> On 17/03/17 10:20, Tasslehoff Kjappfot wrote:
>> We have narrowed this down a bit, and I want to run something by you. It
>> seems the unmount of a FAT filesystem can cause random overwrites.
>> The sequence msdos_shutdown -> fat_file_close -> fat_file_update causes
>> the driver to operate on cluster #1 (set in (fat_fd->dir_pos.sname.cln).
>> The rootdir cluster is not #1, and it seems to be taken from the define
>> FAT_ROOTDIR_CLUSTER_NUM that is used a couple of places in the code.
>> The rootdir cluster found in fat.c is #2. // vol->rdir_cl =
>> We seem to get no corruption if we add the following line at the top of
>> msdos_shutdown:
>> fat_fd->dir_pos.sname.cln = 2 // should get this from rdir_cl
>> I suspect that the other places FAT_ROOTDIR_CLUSTER_NUM is used can also
>> cause problems.
>> Are we on to something?
>> Tasslehoff
>> On 03/13/17 16:36, Gedare Bloom wrote:
>>> On Mon, Mar 13, 2017 at 11:05 AM, Tasslehoff Kjappfot
>>> <tasskjapp at> wrote:
>>>> On Mon, Mar 13, 2017 at 3:48 PM, Gedare Bloom <gedare at>
>>>> wrote:
>>>>> On Mon, Mar 13, 2017 at 9:42 AM, Tasslehoff Kjappfot
>>>>> <tasskjapp at> wrote:
>>>>>> A little update on this. I found out that if I do the following, the
>>>>>> md5sum
>>>>>> is wrong the second time I check it.
>>>>>> 1. Write upgrade files
>>>>>> 2. Check MD5
>>>>>> 3. Unmount
>>>>>> 4. Mount
>>>>>> 5. Check MD5
>>>>> What is the return value from unmount?
>>>> unmount  is successful every time.
>>> I did not think dosfs supports unmount() function so this is
>>> surprising to me. How do you call it?
>>>>>> If I do not unmount/mount, the MD5 is ok, even after a reboot.
>>>>>> With JTAG I discovered that after I have initiated an unmount, the
>>>>>> bdbuf_swapout_task tries to do 3 writes into blocks inside the file
>>>>>> where
>>>>>> the MD5 check fails. If I just ignore those writes, it also works.
>>>>> Now that is strange. It may be worth it to inspect the
>>>>> bdbuf_cache.modified and bdbuf_cache.sync chains. Those are what the
>>>>> swapout task processes. A guess is maybe there is a race condition
>>>>> between the two lists when the sync happens, and you are getting a
>>>>> couple of extra writes.
>>>> Sounds plausible. Is it possible to bypass/disable the bdbuf cache
>>>> altogether? I have not configured anything related to SWAPOUT in my
>>>> application, and the BDBUF setup is the following.
>>> You can't entirely avoid it without changing the filesystem you use.
>>>> #define CONFIGURE_BDBUF_MAX_WRITE_BLOCKS       (64)
>>>> #define CONFIGURE_BDBUF_BUFFER_MIN_SIZE        (512)
>>>> #define CONFIGURE_BDBUF_BUFFER_MAX_SIZE        (32 * 1024)
>>>> #define CONFIGURE_BDBUF_CACHE_MEMORY_SIZE      (4 * 1024 * 1024)
>>> You may like to define a smaller CONFIGURE_SWAPOUT_BLOCK_HOLD
>>> These two control the delay before swapout writes to disk.
>>>>> You might also like to enable RTEMS_BDBUF_TRACE at the top of bdbuf.c
>>>>> file.
>>>> Thanks for the tip.
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