Re: [Yaffs] Scanning yaffs2

On Monday 28 March 2011, Ross Younger wrote:
> * Serguei Miridonov <mirsev@cicese.mx> wrote:
> > What are the key parameters of mtd device
> > which are necessary to start the scanning? I know that page size
> > is 4096 bytes. With hex dump I can see object headers and data
> > chunks. But I could not find anything like spare areas, so I
> > don't even know where tags, like objectId, are located.
>
> Having the contents of the spare areas is (usually) essential, in
> order to get the YAFFS tags; without those you don't know the order
> in which chunks were written and it would be impossible to
> properly recreate the filesystem.

Is there anything like a manual or simplified description, how the
kernel driver scans mtd partition? What part of kernel driver should I
check to get the idea how mtd partition is scanned and where it takes
the image format?

> You need to know the exact format of the mtd dump - what tool has
> created it?

cat /dev/block/mtdblockN > /sdcard/b-mtdN.img

or

cat /dev/mtd/mtdN > /sdcard/c-mtdN.img

Both commands produce the same files.

> It's just about conceivable that the spare areas for
> all the chunks are sitting at the end of the file.

Where? Is there any magic string which can be used to find this area?

> It's also just about possible that the filesystem was created with
> the inband_tags mode, in which case you don't need the spare areas
> and you'll find the tags occupying the last 16 bytes of each
> chunk's data.

This is `cat /proc/yaffs`

YAFFS built:Nov 14 2010 17:12:19
$Id$
$Id$

Device 0 "system"
startBlock......... 0
endBlock........... 299
totalBytesPerChunk. 4096
nDataBytesPerChunk. 4096
chunkGroupBits..... 0
chunkGroupSize..... 1
nErasedBlocks...... 72
nReservedBlocks.... 5
blocksInCheckpoint. 1
nTnodesCreated..... 2600
nFreeTnodes........ 61
nObjectsCreated.... 800
nFreeObjects....... 94
nFreeChunks........ 9530
nPageWrites........ 0
nPageReads......... 0
nBlockErasures..... 0
nGCCopies.......... 0
garbageCollections. 0
passiveGCs......... 0
nRetriedWrites..... 0
nShortOpCaches..... 10
nRetireBlocks...... 0
eccFixed........... 0
eccUnfixed......... 0
tagsEccFixed....... 0
tagsEccUnfixed..... 0
cacheHits.......... 0
nDeletedFiles...... 0
nUnlinkedFiles..... 1
nBackgroudDeletions 0
useNANDECC......... 1
isYaffs2........... 1
inbandTags......... 0

Device 1 "userdata"
startBlock......... 0
endBlock........... 629
totalBytesPerChunk. 4096
nDataBytesPerChunk. 4096
chunkGroupBits..... 0
chunkGroupSize..... 1
nErasedBlocks...... 9
nReservedBlocks.... 5
blocksInCheckpoint. 1
nTnodesCreated..... 4800
nFreeTnodes........ 53
nObjectsCreated.... 2600
nFreeObjects....... 98
nFreeChunks........ 35286
nPageWrites........ 0
nPageReads......... 0
nBlockErasures..... 0
nGCCopies.......... 0
garbageCollections. 0
passiveGCs......... 0
nRetriedWrites..... 0
nShortOpCaches..... 10
nRetireBlocks...... 0
eccFixed........... 0
eccUnfixed......... 0
tagsEccFixed....... 0
tagsEccUnfixed..... 0
cacheHits.......... 0
nDeletedFiles...... 0
nUnlinkedFiles..... 367
nBackgroudDeletions 0
useNANDECC......... 1
isYaffs2........... 1
inbandTags......... 0

> unyaffs is probably the best tool for the job, but you'll need to
> tweak it when you've figured out where the tags are.

Is it at all possible to write a yaffs tool that can read any yaffs2
image? If not, why?

Serguei.