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+++ b/doc/TIFFS-old-description	Sat Jun 11 00:13:35 2016 +0000
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+The description of TIFFS that follows was originally written in the summer of
+SE52 (A.D. 2013), before the major TI source discoveries which happened later
+that year.  The text following the dividing line below has not been edited in
+content since it was written; for a newer write-up based on the current source-
+enabled understanding and reflecting the current FreeCalypso plans with respect
+to this FFS, see TIFFS-Overview.
+
+-------------------------------------------------------------------------------
+
+This is a description, based on reverse engineering, of the flash file system
+(FFS) implemented in Pirelli's original firmware for the DP-L10 GSM/WiFi dual
+mode mobile phone, and in the Closedmoko GTA0x modem firmware.  Not knowing the
+"proper" name for this FFS, and needing _some_ identifier to refer to it, I
+have named it Mokopir-FFS, from "Moko" and "Pirelli" - sometimes abbreviated
+further to MPFFS.
+
+(I have previously called the FFS in question MysteryFFS; but now that I've
+ successfully reverse-engineered it, it isn't as much of a mystery any more :-)
+
+At a high functional level, Mokopir-FFS presents the following features:
+
+* Has a directory tree structure like UNIX file systems;
+
+* The file system API that must be implemented inside the proprietary firmware
+  appears to use UNIX-style pathnames; doing strings on firmware images reveals
+  pathname strings like these:
+
+  /var/dbg/dar
+  /gsm/l3/rr_white_list
+  /gsm/l3/rr_medium_rxlev_thr
+  /gsm/l3/rr_upper_rxlev_thr
+  /gsm/l3/shield
+
+  Parsing the corresponding FFS image with tools included in the present
+  package has confirmed that the directory structure implied by these pathnames
+  does indeed exist in the FFS.
+
+* Absolutely no DOS-ish semantics seen anywhere: no 8.3 filenames and no
+  colon-separated device names (seen in the TSM30 file system source, for
+  example) are visible in the Closedmoko/Pirelli FFS.
+
+* File contents are stored uncompressed, but not necessarily contiguous: one
+  could probably store a file in FFS which is bigger than the flash sector
+  size, it which case it can never be contiguous in a writable FFS (see below),
+  and the firmware implementation seems to limit chunk sizes to a fairly small
+  number: on the Pirelli phones all largish files are divided into chunks of
+  8 KiB each, and on my GTA02 the largest observed chunk size is only 2 KiB.
+
+  The smaller files, like the IMEI and the firmware ID strings in my GTA02 FFS,
+  are contiguous.
+
+* The FFS structure is such that the length of "user" payload data stored in
+  each chunk (and consequently, in each file) can be known exactly in bytes,
+  with the files/chunks able to contain arbitrary binary data.  (This property
+  may seem obvious or trivial, as all familiar UNIX and DOS file systems have
+  it, but contrast with RT-11 for example.)
+
+* The flash file system is a writable one: the running firmware can create,
+  delete and overwrite files (and possibly directories too) in the live FFS;
+  thus the FFS design is such that allows these operations to be performed
+  within the physical constraints of NOR flash write operations.
+
+I have reverse-engineered this Mokopir-FFS on a read-only level.  What it means
+is that I, or anyone else who can read this document and the accompanying
+source for the listing/extraction utilities, can take a Mokopir-FFS image read
+out of a device and see/extract its full content: the complete directory tree
+and the exact binary byte content of all files contained therein.
+
+However, the knowledge possessed by the present hacker (and conveyed in this
+document and the accompanying source code) is NOT sufficient for constructing a
+valid Mokopir-FFS image "in vitro" given a tree of directories and files, or
+for making modifications to the file or directory content of an existing image
+and producing a content-modified image that is also valid; valid as in suitable
+for the original proprietary firmware to make its normal read and write
+operations without noticing anything amiss.
+
+Constructing "de novo" Mokopir-FFS images or modifying existing images in such
+a way that they remain 100% valid for all read and write operations of the
+original proprietary firmware would, at the very minimum, require an
+understanding of the meaning of *all* fields of the on-media FFS format.  Some
+of these fields are still left as "non-understood" for now though: a read-only
+implementation can get away with simply ignoring them, but a writer/generator
+would have to put *something* in those fields.
+
+As you read the "read-only" description of the Mokopir-FFS on-media format in
+the remainder of this document, it should become fairly obvious which pieces
+are missing before our understanding of this FFS can be elevated to a
+"writable" level.
+
+However, when it comes to writing new code to run on the two Calypso phones in
+question (Closedmoko and Pirelli), it seems, at least to the present hacker,
+that a read-only understanding of Mokopir-FFS should be sufficient:
+
+* In the case of Closedmoko GTA0x modems, the FFS is seen to contain the IMEI
+  and the RF calibration data.  The format of the former is obvious; the latter
+  not so much - but in any case, the information of interest is clearly of a
+  read-only nature.  It's difficult to tell (or rather, I haven't bothered to
+  experiment enough) whether the Closedmoko firmware does any writes to FFS or
+  if the FFS is treated as read-only outside of the production line environment,
+  but in any case, it seems to me that for any 3rd party replacement firmware,
+  the best strategy would be to treat the FFS as a read-only source of IMEI and
+  RF calibration data, and nothing more.
+
+* In the case of Pirelli phones, the FFS is used to store user data: sent and
+  received SMS (and MMS/email/whatever), call history, UI settings, pictures
+  taken with the camera, and whatever else.  It also stores a ton of files
+  which I can only presume were meant to be immutable except at the time of
+  firmware updates: graphics for the UI, ringtones, i18n UI strings, and even
+  "helper" firmware images for the WiFi and VoIP processors.  However, no IMEI
+  or RF calibration data are anywhere to be found in the FFS - instead this
+  information appears to be stored in the "factory block" at the end of the
+  flash (in its own sector) outside of the FFS.
+
+  Being able to parse FFS images extracted out of Pirelli phones "in vitro"
+  allows us to steal some of these helper files (UI artwork, ringtones,
+  WiFi/VoIP helpers), and some of these might even come useful to firmware
+  replacement projects, but it seems to me that a replacement firmware would
+  be better off using its own FFS design for storing user data, and as to
+  retrieving the original IMEI and RF calibration data, the original FFS isn't
+  of any use for that anyway.
+
+=======================
+Moko/Pirelli FFS format
+=======================
+
+OK, now that I'm done with the introduction, we can get to the actual
+Mokopir-FFS format.
+
+* On the GTA0x modem (or at least on my GTA02; my sample size is 1) the FFS
+  occupies 7 flash sectors of 64 KiB each at offsets 0x380000 through 0x3E0000,
+  inclusive.
+
+(The 4 MiB NOR flash chip used by Closedmoko has an independent R/W bank
+ division between the first 3 MiB and the last 1 MiB.  The first 3 MiB are used
+ to hold the field-flashable closed firmware images distributed as *.m0 files;
+ the independent last megabyte holds the FFS, and thus the FW could be
+ implemented to do FFS writes while running from flash in the main bank.
+ Less than half of that last megabyte appears to be used for the FFS though;
+ the rest appears to be unused - blank flash observed.)
+
+* On the Pirelli the FFS occupies 18 sectors of 256 KiB each at offsets 0
+  through 0x440000 (inclusive) of the 2nd flash chip select, the one wired to
+  nCS3 on the Calypso.
+
+Each flash sector allocated to FFS begins with the following signature:
+
+00000000:  46 66 73 23 10 02 xx yy  zz FF FF FF FF FF FF FF  Ffs#............
+
+The bytes shown as xx and yy above serve a non-understood purpose; as a guess,
+they may hold some info for the flash wear leveling algorithm: in a "virgin"
+FFS image like that found in my GTA02 (which never had a SIM card in it and
+never made or received a call) or read out of a "virgin" Pirelli phone that
+hasn't seen any active use yet, both of these bytes are FFs, but when I look at
+FFS images read out of the Pirelli which I currently use as my everyday-use
+cellphone, I see other values in sectors which must have been erased and
+rewritten.  A read-only implementation can ignore these bytes, as mine does.
+
+The byte shown as zz is more important though, even to a read-only
+implementation.  The 3 values I've encountered in this byte so far are AB, BD
+and BF.  Per my current understanding, in a "healthy" FFS exactly one sector
+will have AB in its header, exactly one will have BF, and the rest will have
+BD.  The meanings are (or appear to be):
+
+AB: the sector holds a vital data structure which I have called the active
+    index block;
+BD: the sector holds regular data;
+BF: the sector is blank except for the header, can be turned into a new AB or
+    BD.
+
+(Note that a flash program operation, which can turn 1s into 0s but not the
+ other way around, can turn BF into either AB or BD - but neither AB nor BD can
+ be turned into any other valid value.)
+
+In a "virgin" FFS image (as explained above) the first FFS sector is AB, the
+last one is BF, and the ones in between are BDs.
+
+An FFS read operation (a search for a given pathname, or a listing of all
+present directories and files) needs to start with locating the active index
+block - the FFS sector with AB in the header.  Following this header, which is
+treated as being 16 bytes long (almost everything in Mokopir-FFS is aligned on
+16-byte boundaries), the active index block contains a linear array of 16-byte
+records, each record describing an FFS object: directory, file or file
+continuation chunk.
+
+Here is my current understanding of the 16-byte index block record structure:
+
+2 bytes: Length of the described chunk in bytes
+1 byte:	 Purpose/meaning not understood, ignored by my current code
+1 byte:	 Object type
+2 bytes: Descendant pointer
+2 bytes: Sibling pointer
+4 bytes: Data pointer
+4 bytes: Purpose/meaning not understood, ignored by my current code
+
+(On the Calypso phones of interest, all multibyte fields are in the native
+ little-endian byte order of the ARM7TDMI processor.)
+
+The active index block gets filled with these records as objects are created;
+the first record goes right after the 'Ffs#'...AB header (padded to 16 bytes);
+the last record (at any given moment) is followed by blank flash for the
+remainder of the sector.  Records thus appear in the order in which they are
+created, which bears no direct relation to the directory tree structure.
+
+The objects, each described by a record in the index block, are organized into
+a tree structure by the descendant and sibling pointers, plus the object type
+indicator byte.  Let's start with the latter; the following objtype byte values
+have been observed:
+
+00: deleted object - a read-only implementation should ignore everything except
+    the descendant and sibling pointers.  (A write-capable implementation would
+    need more care - it would need a way of reclaiming dirty flash space taken
+    up by deleted/overwritten files.)
+
+E1: a special file - see the description of the /.journal file further down
+F1: a regular file (head chunk thereof)
+F2: a directory
+F4: file continuation chunk (explained below)
+
+Each record in the index block has an associated chunk in one of the data
+sectors; the index record contains fields giving the address and length of this
+chunk.  The length of a chunk is always a nonzero multiple of 16 bytes, and is
+stored (as a number in bytes) in the first 16-bit field of the 16-byte index
+entry.  The address of each chunk is given by the data pointer field of the
+index record, and it is reckoned in 16-byte units (thereby 16-byte alignment is
+required) from the beginning of the FFS sector group in the flash address space.
+
+For objects of type F1 and F2 (regular files and directories) the just-described
+chunk begins with the name of the file or subdirectory as a NUL-terminated ASCII
+string.  This name is just for the current level of the directory tree, just
+like in UNIX directories, thus one will have chunk names like gsm, l3, eplmn
+etc, rather than /gsm/l3/eplmn.  One practical effect is that one can't readily
+see pathnames or any of the directory structure by looking at an FFS image as a
+raw hex dump; the structure is only revealed when one uses a parsing program
+like those which accompany this document.
+
+In the case of directories, the "chunk" part of the object contains only the
+name of the directory itself, padded with FFs to a 16-byte boundary.  For
+example, an FFS directory named /gsm would be represented by an object
+consisting of two flash writes: a 16-byte entry in the active index block, with
+the object type byte set to F2, and a corresponding 16-byte chunk in one of the
+data sectors, with the 16 bytes containing "gsm", a terminating NUL byte, and
+12 FF bytes to pad up to 16.  In the case of files, this name may be followed
+by the first chunk of file data content, as explained further down.
+
+In order to parse the FFS directory tree (whether the objective is to dump the
+whole thing recursively or to find a specific file given a pathname), one needs
+to first (well, after finding the active AB block) find the root directory node.
+The root directory object is similar to other directory objects: it has a type
+of F2, and an associated chunk of 16 bytes in one of the data sectors.  The
+latter contains the name of the root node: on the Pirelli it is "/", whereas on
+my GTA02 it is "/ffs-root".
+
+The astute reader should notice that it really makes no sense to store a name
+for the root node, and indeed, this name plays no part in the traversal of the
+directory tree given an absolute pathname.  But instead this name, or rather
+its first character, appears to be used for the purpose of locating the root
+node itself.  At first I had assumed that the index record for the root node is
+always the first record in the active index block right after the signature
+header - that is how it is in "virgin" FFS images, and also in some quite non-
+virgin ones I have pulled from my daily-use Pirelli.  Naturally my first version
+of the Mokopir-FFS (then called MysteryFFS) extraction utility expected the root
+node to always be at index #1.  But then I got some additional Pirelli phones,
+and discovered that in certain cases, index record #1 is a deleted object (the
+original root node which has been deleted), and the new active root node is
+somewhere in the middle of the index!
+
+Thus it appears that in order to find the active root node, one needs to scan
+the active index block linearly from the beginning (disregarding the tree
+structure pointers in this initial pass), looking for a non-deleted object of
+type F2 (a directory) whose corresponding name chunk sports a name beginning
+with the '/' character.  (Anyone who's been raised in UNIX will immediately
+know that the path separator character '/' is the only character other than NUL
+that's absolutely forbidden in the individual filenames - so this special
+"root node name" is the only case of a '/' character appearing in what would
+otherwise be a regular filename.)
+
+[What causes the root node to be somewhere other than at index #1?  I assume it
+ has to do with the dirty space reclamation / data movement algorithm.  In a
+ "virgin" FFS image the very first sector is the active index block, and the
+ following sector is the first to hold chunks, beginning with the name chunk of
+ the root node.  Now what happens if all data in that sector aside from the
+ root node name and some other mostly-static directory names becomes dirty,
+ i.e., belonging to deleted or overwritten files?  How would that flash space
+ get reclaimed?  I assume that the FFS firmware algorithm moves all still-active
+ chunks to a new flash sector, invalidating the old copies - turning the latter
+ into deleted objects.  The root node will be among them.  Then at some point
+ the active index block is going to fill up too, and will need to be rewritten
+ into a new sector - at which point the previously-deleted index entries are
+ omitted and the root node becomes #1 again...]
+
+Tree structure
+
+Once the root node has been found, the descendant and sibling pointers are used
+to traverse the tree structure.  For each directory object, including the root
+node, the descendant pointer points to the first child object of this directory:
+the first file or subdirectory contained therein.  (Descendant and sibling
+pointers take the form of index numbers in the active index block.  A "nil"
+pointer is indicated by all 1s (FFFF) - the usual all-0s NULL pointer convention
+couldn't be used because it's flash, where the blank state is all 1s.)  If the
+descendant pointer of a directory object is nil, that means an empty directory.
+The sibling pointer of each file or directory points to its next sibling, i.e.,
+the next member of the same parent directory.  The sibling pointer of the root
+node is nil.
+
+Data content of files
+
+Objects of type F1 are the head chunks of files.  Each file has a head chunk,
+and may or may not have continuation chunks.  More precisely, the head chunk
+may contain only the name (or viewed alternatively, 0 bytes of data), or it may
+contain a nonzero number of payload bytes; orthogonally to this variability,
+there may or may not be continuation chunk(s) present.
+
+Continuation chunks
+
+The descendant pointer of each file head object (the object of type F1, the one
+reached by traversing the directory tree) indicates whether or not there are
+any continuation chunks present.  If this descendant pointer is nil, there are
+no continuation chunks; otherwise it points to the first continuation chunk
+object.  File continuation objects have type F4, don't have any siblings (the
+sibling pointer is nil - but see below regarding relocated chunks), and the
+descendant pointer of each continuation object points to the next continuation
+object, if there is one - nil otherwise.
+
+Payload data delineation
+
+Each chunk, whether head or continuation, always has a length that is a nonzero
+multiple of 16 bytes.  The length of the chunk here means the amount of flash
+space it occupies in its data sector - which is NOT equal to the payload data
+length.
+
+The head chunk of each file begins with the filename, terminated by a NUL byte.
+If there are any payload data bytes present in this head chunk (I'll explain
+momentarily how you would tell), the byte immediately after the NUL that
+terminates the filename is the first byte of the payload.  In the case of a
+continuation chunk, there is no filename and the first byte of the chunk is the
+first byte of that chunk's portion of the user data payload.
+
+Each data-containing chunk (head or continuation) has the following termination
+after the last byte of that chunk's payload data: one byte of 00, followed by
+however many bytes are needed ([0,15] range) of FFs to pad to a 16-byte
+boundary.  A file head chunk that has no payload data has the same format as a
+directory name chunk: filename followed by its terminating NUL followed by
+[0,15] bytes of FFs to pad to the next 16-byte boundary.
+
+When working with a head chunk, find the beginning of possible payload data (1
+byte after the filename terminating NUL) and find the end per the standard
+termination logic: scanning from the end of the chunk, skip FFs until 00 is
+found (encountering anything else is an error).  If the head chunk has no data,
+the effective data length (end_pointer - start_pointer) will be 0 or -1.  (The
+latter possibility is the most likely, as there will normally be a "shared" 00
+byte, serving as both the filename terminator and the 00 before the padding
+FF bytes.)
+
+Relocated chunks
+
+Let's go back to the scenario in which a particular data sector is full (no more
+usable free space left) and contains a mixture of active and dirty (deleted or
+invalidated) data.  How does the dirty flash space get reclaimed, so that the
+amount of available space (blank flash ready to hold new data) becomes equal to
+the total FFS size minus the total size of active files and overhead?  It can
+only be done by relocating the still-active objects from the full sector to a
+new one, invalidating the old copies, and once the old sector consists of
+nothing but invalidated data, subjecting it to flash erasure.
+
+So how do the active FFS objects get relocated from a "condemned" sector to a
+new one?  If the object is a directory, a new index entry is created, pointing
+to the newly relocated name chunk, but it is then made to fit into the old tree
+structure without disrupting the latter: the new index entry is added at the
+tail of the sibling-chain of the parent directory's descendants, the old index
+entry for the same directory is invalidated (as if the directory were rmdir'ed),
+and the descendant pointer of the newly written index entry is set to a copy of
+the descendant pointer from the old index entry for the same directory.  The
+same approach is used when the head chunk of a file needs to be relocated; in
+both cases a read-only FFS implementation doesn't need to do anything special to
+support reading file and directory objects that have been relocated in this
+manner.
+
+However, if the relocated object is a file continuation chunk, then the manner
+in which such objects get relocated does affect file reading code.  What if a
+chunk in the middle of a chain linked by "descend" pointers needs to be moved?
+What happens in this case is that the old copy of the chunk gets invalidated
+(the object type byte turned to 00) like in the other object relocating cases,
+and the sibling pointer of that old index entry (which was originally FFFF as
+continuation objects have no siblings) is set to point to the new index entry
+for the same chunk.  The "descend" pointer in the new index entry is a copy of
+that pointer from the old index entry.
+
+The manner of chunk relocation just described has been observed in the FFS
+images read out of my most recent batch of Pirelli phones - the same ones in
+which the root directory object is not at index #1.  Thinking about it as I
+write this, I've realized that the way in which continuation objects get
+relocated is exactly the same as for other object types - thus the compaction
+code in the firmware doesn't need to examine what object type it is moving.
+However, the case of continuation chunk relocation deserves special attention
+because it affects a read-only implementation like ours - the utilities whose
+source accompanies this document used to fail on these FFS images until I
+implemented the following additional handling:
+
+When following the chunk chain of a file, normally the only object type that's
+expected is F4 - any other object type is an error.  However, as a result of
+chunk relocation, one can also encounter deleted objects, i.e., type == 00.
+If such a deleted object is encountered, follow its sibling pointer, which must
+be non-nil.
+
+Journal file
+
+Every Mokopir-FFS image I've seen so far contains a special file named
+/.journal; this file is special in the following ways:
+
+* The object type byte is E1 instead of F1;
+* Unlike regular files, this special file is internally-writable.
+
+What I mean by the above is that regular files are mostly immutable: once a
+file has been created with some data content in the head chunk, it can only be
+either appended to (one or more continuation chunks added), or overwritten by
+creating a new file with the same name at the same level in the tree hierarchy
+and invalidating the old one.  But the special /.journal file is different: I
+have never observed it to consist of more than the head chunk, and this head
+chunk is pre-allocated with some largish and apparently fixed length (4 KiB on
+my GTA02, 16 KiB on the Pirelli).  This pre-allocated chunk contains what look
+like 16-byte records at the beginning (on the first 4-byte boundary after the
+NUL terminating the ".journal" name), followed by blank flash for the remainder
+of the pre-allocated chunk - so it surely looks like new flash writes happen
+within this chunk.
+
+I do not currently know the purpose of this /.journal file or the meaning of the
+records it seems to contain.  This understanding would surely be needed if one
+wanted to create FFS images from scratch or to implement FFS write operations,
+but I reason that a read-only implementation can get away with simply ignoring
+this file.  I reason that this file can't be necessary in order to parse an FFS
+image for reading because one needs to parse the tree structure first in order
+to locate this journal file itself.
+
+-------------------------------------------------------------------------------
+
+That's all I can think of right now.  If anything is unclear, see the
+accompanying source code for the listing/extraction utilities: with the general
+explanation given by this document, it should be clear what my code does and
+why.  And if a given piece of knowledge is found neither in this document nor
+in my source code, then I don't know it myself either, and my read-only
+Mokopir-FFS implementation makes do without it.
+
+All knowledge contained herein has been recovered by reverse engineering.
+Believe it or not, I have figured it out by staring at the hex dump of FFS
+sectors, reasoning about how one could possibly implement an FFS given the
+requirement of dynamic writability and the physical constraints of flash memory,
+and writing listing/extraction test code iteratively until I got something that
+appears to correctly parse all FFS images available to me - the result is the
+code in this package.
+
+I never got as far as attempting to locate the FFS implementation routines
+within the proprietary firmware binary code images, and I haven't found an
+implementation of this particular FFS in any of the leaked sources yet either.
+The TSM30 code doesn't seem to be of any use as its FFS appears to be totally
+different.  As to the more recently found LoCosto code leak, I found that one a
+few days *after* I got the Moko/Pirelli "MysteryFFS" reverse-engineered on my
+own, and when I did look at the FFS in the LoCosto code later, I saw what seems
+to be a different FFS as well.
+
+Michael Spacefalcon
+SE 52 Mes 16