## EXT2ED - The Extended-2 filesystem editor - Design and implementation

Programmed by Gadi Oxman, with the guide of Avner Lottem
v0.1, August 3 1995

## About EXT2ED documentation

The EXT2ED documentation consists of three parts:

- The ext2 filesystem overview.

- The EXT2ED user's guide.

- The EXT2ED design and implementation.

This document is not the user's guide. If you just intend to use EXT2ED, you may not want to read it.

However, if you intend to browse and modify the source code, this document is for you.

In any case, If you intend to read this article, I strongly suggest that you will be familiar with the material presented in the other two articles as well.

## Preface

In this document I will try to explain how EXT2ED is constructed. At this time of writing, the initial version is finished and ready for distribution; It is fully functional. However, this was not always the case.

At first, I didn't know much about Unix, much less about Unix filesystems, and even less about Linux and the extended-2 filesystem. While working on this project, I gradually acquired knowledge about all of the above subjects. I can think of two ways in which I could have made my project:

1. The "Engineer" way

   Learn the subject thoroughly before I get to the programming itself. Then, I could easily see the entire picture and select the best course of action, taking all the factors into account.

2. The "Explorer - Progressive" way.

   Jump immediately into the cold water - Start programming and learning the material in parallel.

I guess that the above dilemma is typical and appears all through science and technology.

However, I didn't have the luxury of choice when I started my project - Linux is a relatively new (and great!) operating system. The extended-2 filesystem is even newer - Its first release lies somewhere in 1993 - Only passed two years until I started working on my project.

The situation I found myself at the beginning was that I didn't have a fully detailed document which describes the ext2 filesystem. In fact, I didn't have any ext2 document at all. When I asked Avner about documentation, he suggested two references:

- A general Unix book - THE DESIGN OF THE UNIX OPERATING SYSTEM, by Maurice J. Bach.

- The kernel sources.

I read the relevant parts of the book before I started my project - It is a bit old now, but the principles are still the same. However, I needed more than just the principles.

The kernel sources are a rare bonus! You don't get everyday the full sources of the operating system. There is so much that can be learned from them, and it is the ultimate source - The exact answer how the kernel works is there, with all the fine details. At the first week I started to look at random at the relevant parts of the sources. However, it is difficult to understand the global picture from direct reading of over one hundred page sources. Then, I started to do some programming. I didn't know yet what I was looking for, and I started to work on the project like a kid who starts to build a large puzzle.

However, this was exactly the interesting part! It is frustrating to know it all from advance - I think that the discovery itself, bit by bit, is the key to a true learning and understanding.

Now, in this document, I am trying to present the subject. Even though I developed EXT2ED progressively, I now can see the entire subject much brighter than I did before, and though I do have the option of presenting it only in the "engineer" way. However, I will not do that.

My presentation will be mixed - Sometimes I will present a subject with an incremental perspective, and sometimes from a "top down" view. I'll leave you to decide if my presentation choice was wise :-)

In addition, you'll notice that the sections tend to get shorter as we get closer to the end. The reason is simply that I started to feel that I was repeating myself so I decided to present only the new ideas.

## Getting started ...

Getting started is almost always the most difficult task. Once you get started, things start "running" ...

### Before the actual programming

From mine talking with Avner, I understood that Linux, like any other Unix system, provides accesses to the entire disk as though it were a general file - Accessing the device. It is surely a nice idea. Avner suggested two ways of action:

- Opening the device like a regular file in the user space.

- Constructing a device driver which will run in the kernel space and provide hooks for the user space program. The advantage is that it will be a part of the kernel, and would be able to use the ext2 kernel functions to do some of the work.

I chose the first way. I think that the basic reason was simplicity - Learning the ext2 filesystem was complicated enough, and adding to it the task of learning how to program in the kernel space was too much. I still don't know how to program a device driver, and this is perhaps the bad part, but concerning the project in a back-perspective, I think that the first way is superior to the second; Ironically, because of the very reason I chose it - Simplicity. EXT2ED can now run entirely in the user space (which I think is a point in favor, because it doesn't require the user to recompile its kernel), and the entire hard work is mine, which fitted nicely into the learning experience - I didn't use other code to do the job (aside from looking at the sources, of-course).

### Jumping into the cold water

I didn't know almost anything of the structure of the ext2 filesystem. Reading the sources was not enough - I needed to experiment. However, a tool for experiments in the ext2 filesystem was exactly my project! - Kind of a paradox.

I started immediately with constructing a simple `hex editor` - It would open the device as a regular file, provide means of moving inside the filesystem with a simple `offset` method, and just show a `hex dump` of the contents at this point. Programming this was trivially simple of-course. At this point, the user-interface didn't matter to me - I wanted a fast way to interact. As a result, I chose a simple command line parser. Of course, there where no windows at this point.

A hex editor is nice, but is not enough. It indeed enabled me to see each part of the filesystem, but the format of the viewed data was difficult to analyze. I wanted to see the data in a more intuitive way.

At this point of time, the most helpful file in the sources was the ext2 main include file - `/usr/include/linux/ext2&lowbar;fs.h`. Among its contents there were various structures which I assumed they are disk images - Appear exactly like that on the disk.

I wanted a `quick` way to get going. I didn't have the patience to learn each of the structures use in the code. Rather, I wanted to see them in action, so that I could explore the connections between them - Test my assumptions, and reach other assumptions.

So after the `hex editor`, EXT2ED progressed into a tool which has some elements of a compiler. I programmed EXT2ED to `dynamically read the kernel ext2 main include file in run time`, and process the information. The goal was to `imply a structure-definition on the current offset at the filesystem`. EXT2ED would then display the structure as a list of its variables names and contents, instead of a meaningless hex dump.

The format of the include file is not very complicated - The structures are mostly `flat` - Didn't contain a lot of recursive structure; Only a global structure definition, and some variables. There were cases of structures inside structures, I treated them in a somewhat non-elegant way - I made all the structures flat, and expanded the arrays. As a result, the parser was very simple. After all, this was not an exercise in compiling, and I wanted to quickly get some results.

To handle the task, I constructed the `struct&lowbar;descriptor` structure. Each `struct&lowbar;descriptor instance` contained information which is needed in order to format a block of data according to the C structure contained in the kernel source. The information contained:

- The descriptor name, used to reference to the structure in EXT2ED.

- The name of each variable.

- The relative offset of the each variable in the data block.

- The length, in bytes, of each variable.

Since I didn't want to limit the number of structures, I chose a simple double linked list to store the information. One variable contained the `current structure type` - A pointer to the relevant `struct&lowbar;descriptor`.

Now EXT2ED contained basically three command line operations:

- setdevice

  Used to open a device for reading only. Write access was postponed to a very advanced state in the project, simply because I didn't know a thing of the filesystem structure, and I believed that making actual changes would do nothing but damage :-)

- setoffset

  Used to move in the device.

- settype

  Used to imply a structure definition on the current place.

- show

  Used to display the data. It displayed the data in a simple hex dump if there was no type set, or in a nice formatted way - As a list of the variable contents, if there was.

Command line analyzing was primitive back then - A simple switch, as far as I can remember - Nothing alike the current flow control, but it was enough at the time.

At the end, I had something to start working with. It knew to format many structures - None of which I understood - and provided me, without too much work, something to start with.

## Starting to explore

With the above tool in my pocket, I started to explore the ext2 filesystem structure. From the brief reading in Bach's book, I got familiar to some basic concepts - The `superblock`, for example. It seems that the superblock is an important part of the filesystem. I decided to start exploring with that.

I realized that the superblock should be at a fixed location in the filesystem - Probably near the beginning. There can be no other way - The kernel should start at some place to find it. A brief looking in the kernel sources revealed that the superblock is signed by a special signature - A `magic number` - EXT2&lowbar;SUPER&lowbar;MAGIC (0xEF53 - EF probably stands for Extended Filesystem). I quickly found the superblock at the fixed offset 1024 in the filesystem - The `s&lowbar;magic` variable in the superblock was set exactly to the above value.

It seems that starting with the `superblock` was a good bet - Just from the list of variables, one can learn a lot. I didn't understand all of them at the time, but it seemed that the following keywords were repeating themselves in various variables:

- block

- inode

- group

At this point, I started to explore the block groups. I will not detail here the technical design of the ext2 filesystem. I have written a special article which explains just that, in the "engineering" way. Please refer to it if you feel that you are lacking knowledge in the structure of the ext2 filesystem.

I was exploring the filesystem in this way for some time, along with reading the sources. This lead naturally to the next step.

## Object specific commands

What has become clear is that the above way of exploring is not powerful enough - I found myself doing various calculations manually in order to pass between related structures. I needed to replace some tasks with an automated procedure.

In addition, it also became clear that (of-course) each key object in the filesystem has its special place in regard to the overall ext2 filesystem design, and needs a `fine tuned handling`. It is at this point that the structure definitions `came to life` - They became `object definitions`, making EXT2ED `object oriented`.

The actual meaning of the breathtaking words above, is that each structure now had a list of `private commands`, which ended up in `calling special fine-tuned C functions`. This approach was found to be very powerful and is `the heart of EXT2ED even now`.

In order to implement the above concepts, I added the structure `struct&lowbar;commands`. The role of this structure is to group together a group of commands, which can be later assigned to a specific type. Each structure had:

- A list of command names.

- A list of pointers to functions, which binds each command to its special fine-tuned C function.

In order to relate a list of commands to a type definition, each `struct&lowbar;descriptor` structure (explained earlier) was added a private `struct&lowbar;commands` structure.

Follows the current definitions of `struct&lowbar;descriptor` and of `struct&lowbar;command`:

    struct struct_descriptor {
            unsigned long length;
            unsigned char name [60];
            unsigned short fields_num;
            unsigned char field_names [MAX_FIELDS][80];
            unsigned short field_lengths [MAX_FIELDS];
            unsigned short field_positions [MAX_FIELDS];
            struct struct_commands type_commands;
            struct struct_descriptor *prev,*next;
    };

    typedef void (*PF) (char *);

    struct struct_commands {
            int last_command;
            char *names [MAX_COMMANDS_NUM];
            char *descriptions [MAX_COMMANDS_NUM];
            PF callback [MAX_COMMANDS_NUM];
    };

## Program flow control

Obviously the above approach lead to a major redesign of EXT2ED. The main engine of the resulting design is basically the same even now.

I redesigned the program flow control. Up to now, I analyzed the user command line with the simple switch method. Now I used the far superior callback method.

I divided the available user commands into two groups:

1. General commands.

2. Type specific commands.

As a result, at each point in time, the user was able to enter a `general command`, selectable from a list of general commands which was always available, or a `type specific command`, selectable from a list of commands which `changed in time` according to the current type that the user was editing. The special `type specific command` "knew" how to handle the object in the best possible way - It was "fine tuned" for the object's place in the ext2 filesystem design.

In order to implement the above idea, I constructed a global variable of type `struct&lowbar;commands`, which contained the `general commands`. The `type specific commands` were accessible through the `struct descriptors`, as explained earlier.

The program flow was now done according to the following algorithm:

1. Ask the user for a command line.

2. Analyze the user command - Separate it into `command` and `arguments`.

3. Trace the list of known objects to match the command name to a type. If the type is found, call the callback function, with the arguments as a parameter. Then go back to step (1).

4. If the command is not type specific, try to find it in the general commands, and call it if found. Go back to step (1).

5. If the command is not found, issue a short error message, and return to step (1).

Note the `order` of the above steps. In particular, note that a command is first assumed to be a type-specific command and only if this fails, a general command is searched. The "`side-effect`" (main effect, actually) is that when we have two commands with the `same name` - One that is a type specific command, and one that is a general command, the dispatching algorithm will call the `type specific command`. This allows `overriding` of a command to provide `fine-tuned` operation. For example, the `show` command is overridden nearly everywhere, to accommodate for the different ways in which different objects are displayed, in order to provide an intuitive fine-tuned display.

The above is done in the `dispatch` function, in `main.c`. Since it is a very important function in EXT2ED, and it is relatively short, I will list it entirely here. Note that a redesign was made since then - Another level was added between the two described, but I'll elaborate more on this later. However, the basic structure follows the explanation described above.

    int dispatch (char *command_line)

    {
    	int i,found=0;
    	char command [80];

    	parse_word (command_line,command);

    	if (strcmp (command,"quit")==0) return (1);

    	/* 1. Search for type specific commands FIRST - Allows overriding of a general command */

    	if (current_type != NULL)
    		for (i=0;i<=current_type->type_commands.last_command && !found;i++) {
    			if (strcmp (command,current_type->type_commands.names [i])==0) {
    				(*current_type->type_commands.callback [i]) (command_line);
    				found=1;
    			}
    		}

    	/* 2. Now search for ext2 filesystem general commands */

    	if (!found)
    		for (i=0;i<=ext2_commands.last_command && !found;i++) {
    			if (strcmp (command,ext2_commands.names [i])==0) {
    				(*ext2_commands.callback [i]) (command_line);
    				found=1;
    			}
    		}

    	/* 3. If not found, search the general commands */

    	if (!found)
    		for (i=0;i<=general_commands.last_command && !found;i++) {
    			if (strcmp (command,general_commands.names [i])==0) {
    				(*general_commands.callback [i]) (command_line);
    				found=1;
    			}
    		}

    	if (!found) {
    		wprintw (command_win,"Error: Unknown command\n");
    		refresh_command_win ();
    	}

    	return (0);
    }

## Source files in EXT2ED

The project was getting large enough to be split into several source files. I split the source as much as I could into self-contained source files. The source files consist of the following blocks:

- `Main include file - ext2ed.h`

  This file contains the definitions of the various structures, variables and functions used in EXT2ED. It is included by all source files in EXT2ED.

- `Main block - main.c`

  `main.c` handles the upper level of the program flow control. It contains the `parser` and the `dispatcher`. Its task is to ask the user for a required action, and to pass control to other lower level functions in order to do the actual job.

- `Initialization - init.c`

  The init source is responsible for the various initialization actions which need to be done through the program. For example, auto detection of an ext2 filesystem when selecting a device and initialization of the filesystem-specific structures described earlier.

- `Disk activity - disk.c`

  `disk.c` is handles the lower level interaction with the device. All disk activity is passed through this file - The various functions through the source code request disk actions from the functions in this file. In this way, for example, we can easily block the write access to the device.

- `Display output activity - win.c`

  In a similar way to `disk.c`, the user-interface functions and most of the interaction with the `ncurses library` are done here. Nothing will be actually written to a specific window without calling a function from this file.

- `Commands available through dispatching - *&lowbar;com.c`

  The above file name is generic - Each file which ends with `&lowbar;com.c` contains a group of related commands which can be called through `the dispatching function`.

  Each object typically has its own file. A separate file is also available for the general commands.

The entire list of source files available at this time is:

- blockbitmap&lowbar;com.c

- dir&lowbar;com.c

- disk.c

- ext2&lowbar;com.c

- file&lowbar;com.c

- general&lowbar;com.c

- group&lowbar;com.c

- init.c

- inode&lowbar;com.c

- inodebitmap&lowbar;com.c

- main.c

- super&lowbar;com.c

- win.c

## User interface

The user interface is text-based only and is based on the following libraries:

- The `ncurses` library, developed by `Zeyd Ben-Halim`.

- The `GNU readline` library.

The user interaction is command line based - The user enters a command line, which consists of a `command` and of `arguments`. This fits nicely with the program flow control described earlier - The `command` is used by `dispatch` to select the right function, and the `arguments` are interpreted by the function itself.

### The ncurses library

The `ncurses` library enables me to divide the screen into "windows". The main advantage is that I treat the "window" in a virtual way, asking the ncurses library to "write to a window". However, the ncurses library internally buffers the requests, and nothing is actually passed to the terminal until an explicit refresh is requested. When the refresh request is made, ncurses compares the current terminal state (as known in the last time that a refresh was done) with the new to be shown state, and passes to the terminal the minimal information required to update the display. As a result, the display output is optimized behind the scenes by the `ncurses` library, while I can still treat it in a virtual way.

There are two basic concepts in the `ncurses` library:

- A window.

- A pad.

A window can be no bigger than the actual terminal size. A pad, however, is not limited in its size.

The user screen is divided by EXT2ED into three windows and one pad:

- Title window.

- Status window.

- Main display pad.

- Command window.

The `title window` is static - It just displays the current version of EXT2ED.

The user interaction is done in the `command window`. The user enters a `command line`, feedback is usually displayed there, and then relevant data is usually displayed in the main display and in the status window.

The `main display` is using a `pad` instead of a window because the amount of information which is written to it is not known in advance. Therefor, the user treats the main display as a "window" into a bigger display and can `scroll vertically` using the `pgdn` and `pgup` commands. Although the `pad` mechanism enables me to use horizontal scrolling, I have not utilized this.

When I need to show something to the user, I use the ncurses `wprintw` command. Then an explicit refresh command is required. As explained before, the refresh commands is piped through `win.c`. For example, to update the command window, `refresh&lowbar;command&lowbar;win ()` is used.

### The readline library

Avner suggested me to integrate the GNU `readline` library in my project. The `readline` library is designed specifically for programs which use command line interface. It provides a nice package of `command line editing tools` - Inserting, deleting words, and the whole package of editing tools which are normally available in the `bash` shell (Refer to the readline documentation for details). In addition, I utilized the `history` feature of the readline library - The entered commands are saved in a `command history`, and can be called later by whatever means that the readline package provides. Command completion is also supported - When the user enters a partial command name, EXT2ED will provide the readline library with the possible completions.

## Possible support of other filesystems

The entire ext2 layer is provided through specific objects. Given another set of objects, support of other filesystem can be provided using the same dispatching mechanism. In order to prepare the surface for this option, I added yet another layer to the two-layer structure presented earlier. EXT2ED commands now consist of three layers:

- The general commands.

- The ext2 general commands.

- The ext2 object specific commands.

The general commands are provided by the `general&lowbar;com.c` source file, and are always available. The two other levels are not present when EXT2ED loads - They are dynamically added by `init.c` when EXT2ED detects an ext2 filesystem on the device.

The abstraction levels presented above helps to extend EXT2ED to fully support a new filesystem, with its own specific type commands.

Even without any source code modification, the user is free to add structure definitions in a separate file (specified in the configuration file), which will be added to the list of available objects. The added objects will consist only of variables, of-course, and will be used through the more primitive `setoffset` and `settype` commands.

## On the implementation of the various commands

This section points out some typical programming style that I used in many places at the code.

### The explicit use of the dispatch function

The various commands are reached by the user through the `dispatch` function. This is not surprising. The fact that can be surprising, at least in a first look, is that `you'll find the dispatch call in many of my own functions!`.

I am in fact using my own implemented functions to construct higher level operations. I am heavily using the fact that the dispatching mechanism is object oriented ant that the `overriding` principle takes place and selects the proper function to call when several commands with the same name are accessible.

Sometimes, however, I call the explicit command directly, without passing through `dispatch`. This is typically done when I want to bypass the `overriding` effect.

This is used, for example, in the interaction between the global cd command and the dir object specific cd command. You will see there that in order to implement the "entire" cd command, the type specific cd command uses both a dispatching mechanism to call itself recursively if a relative path is used, or a direct call of the general cd handling function if an explicit path is used.

### Passing information between handling functions

Typically, every source code file which handles one object type has a global structure specifically designed for it which is used by most of the functions in that file. This is used to pass information between the various functions there, and to physically provide the link to other related objects, typically for initialization use.

For example, in order to edit a file, information about the inode is needed - The file command is available only when editing an inode. When the file command is issued, the handling function (found, according to the source division outlined above, in inode_com.c) will store the necessary information about the inode in a specific structure of type struct_file_info which will be available for use by the file_com.c functions. Only then it will set the type to file. This is also the reason that a direct asynchronous set of the object type to a file through a settype command will fail - The above data structure will not be initialized properly because the user never was at the inode of the file.

### A very simplified overview of a typical command handling function

This is a very simplified overview. Detailed information will follow where appropriate.

#### The prototype of a typical handling function

1. I chose a unified `naming convention` for the various object specific commands. It is perhaps best showed with an example:

   The prototype of the handling function of the command `next` of the type `file` is:

       		extern void type_file___next (char *command_line);

   For other types and commands, the words `file` and `next` should be replaced accordingly.

2. The ext2 general commands syntax is similar. For example, the ext2 general command `super` results in calling:

       		extern void type_ext2___super (char *command_line);

   Those functions are available in `ext2&lowbar;com.c`.

3. The general commands syntax is even simpler - The name of the handling function is exactly the name of the commands. Those functions are available in `general&lowbar;com.c`.

#### "Typical" algorithm

This section can't of-course provide meaningful information - Each command is handled differently, but the following frame is typical:

1. Parse command line arguments and analyze them. Return with an error message if the syntax is wrong.

2. "Act accordingly", perhaps making use of the global variable available to this type.

3. Use some `dispatch / direct` calls in order to pass control to other lower-level user commands.

4. Sometimes `dispatch` to the object's `show` command to display the resulting data to the user.

I told you it is meaningless :-)

## Initialization overview

In this section I will discuss some aspects of the various initialization routines available in the source file `init.c`.

### Upon startup

Follows the function `main`, appearing of-course in `main.c`:

    int main (void)

    {
    	if (!init ()) return (0);	/* Perform some initial initialization */
    					/* Quit if failed */

    	parser ();			/* Get and parse user commands */

    	prepare_to_close ();		/* Do some cleanup */
    	printf ("Quitting ...\n");
    	return (1);			/* And quit */
    }

The two initialization functions, which are called by `main`, are:

- init

- prepare&lowbar;to&lowbar;close

#### The init function

`init` is called from `main` upon startup. It initializes the following tasks / subsystems:

1. Processing of the `user configuration file`, by using the `process&lowbar;configuration&lowbar;file` function. Failing to complete the configuration file processing is considered a `fatal error`, and EXT2ED is aborted. I did it this way because the configuration file has some sensitive user options like write access behavior, and I wanted to be sure that the user is aware of them.

2. Registration of the `general commands` through the use of the `add&lowbar;general&lowbar;commands` function.

3. Reset of the object memory rotating lifo structure.

4. Reset of the device parameters and of the current type.

5. Initialization of the windows subsystem - The interface between the ncurses library and EXT2ED, through the use of the `init&lowbar;windows` function, available in `win.c`.

6. Initialization of the interface between the readline library and EXT2ED, through `init&lowbar;readline`.

7. Initialization of the `signals` subsystem, through `init&lowbar;signals`.

8. Disabling write access. Write access needs to be explicitly enabled using a user command, to prevent accidental user mistakes.

When `init` is finished, it dispatches the `help` command in order to show the available commands to the user. Note that the ext2 layer is still not added; It will be added if and when EXT2ED will detect an ext2 filesystem on a device.

#### The prepare&lowbar;to&lowbar;close function

The `prepare&lowbar;to&lowbar;close` function reverses some of the actions done earlier in EXT2ED and freeing the dynamically allocated memory. Specifically, it:

1. Closes the open device, if any.

2. Removes the first level - Removing the general commands, through the use of `free&lowbar;user&lowbar;commands`, with a pointer to the general&lowbar;commands structure as a parameter.

3. Removes of the second level - Removing the ext2 ext2 general commands, in much the same way.

4. Removes of the third level - Removing the objects and the object specific commands, by using `free&lowbar;struct&lowbar;descriptors`.

5. Closes the window subsystem, and deattaches EXT2ED from the ncurses library, through the use of the `close&lowbar;windows` function, available in `win.c`.

### Registration of commands

Addition of a user command is done through the `add&lowbar;user&lowbar;command` function. The prototype is:

    void add_user_command (struct struct_commands *ptr,char *name,char
    *description,PF callback);

The function receives a pointer to a structure of type `struct&lowbar;commands`, a desired name for the command which will be used by the user to identify the command, a short description which is utilized by the `help` subsystem, and a pointer to a C function which will be called if `dispatch` decides that this command was requested.

The `add&lowbar;user&lowbar;command` is a `low level function` used in the three levels to add user commands. For example, addition of the `ext2 general commands is done by:`

    void add_ext2_general_commands (void)

    {
    	add_user_command (&ext2_commands,"super","Moves to the superblock of the filesystem",type_ext2___super);
    	add_user_command (&ext2_commands,"group","Moves to the first group descriptor",type_ext2___group);
    	add_user_command (&ext2_commands,"cd","Moves to the directory specified",type_ext2___cd);
    }

### Registration of objects

Registration of objects is based, as explained earlier, on the "compilation" of an external user file, which has a syntax similar to the C language `struct` keyword. The primitive parser I have implemented detects the definition of structures, and calls some lower level functions to actually register the new detected object. The parser's prototype is:

    int set_struct_descriptors (char *file_name)

It opens the given file name, and calls, when appropriate:

- add&lowbar;new&lowbar;descriptor

- add&lowbar;new&lowbar;variable

`add&lowbar;new&lowbar;descriptor` is a low level function which adds a new descriptor to the doubly linked list of the available objects. It will then call `fill&lowbar;type&lowbar;commands`, which will add specific commands to the object, if the object is known.

`add&lowbar;new&lowbar;variable` will add a new variable of the requested length to the specified descriptor.

### Initialization upon specification of a device

When the general command `setdevice` is used to open a device, some initialization sequence takes place, which is intended to determine two factors:

- Are we dealing with an ext2 filesystem ?

- What are the basic filesystem parameters, such as its total size and its block size ?

This questions are answered by the `set&lowbar;file&lowbar;system&lowbar;info`, possibly using some `help from the user`, through the configuration file. The answers are placed in the `file&lowbar;system&lowbar;info` structure, which is of type `struct&lowbar;file&lowbar;system&lowbar;info`:

    struct struct_file_system_info {
    	unsigned long file_system_size;
    	unsigned long super_block_offset;
    	unsigned long first_group_desc_offset;
    	unsigned long groups_count;
    	unsigned long inodes_per_block;
    	unsigned long blocks_per_group;		/* The name is misleading; beware */
    	unsigned long no_blocks_in_group;
    	unsigned short block_size;
    	struct ext2_super_block super_block;
    };

Autodetection of an ext2 filesystem is usually recommended. However, on a damaged filesystem I can't assure a success. That's were the user comes in - He can `override` the auto detection procedure and force an ext2 filesystem, by selecting the proper options in the configuration file.

If auto detection succeeds, the second question above is automatically answered - I get all the information I need from the filesystem itself. In any case, default parameters can be supplied in the configuration file and the user can select the required behavior.

If we decide to treat the filesystem as an ext2 filesystem, `registration of the ext2 specific objects` is done at this point, by calling the `set&lowbar;struct&lowbar;descriptors` outlined earlier, with the name of the file which describes the ext2 objects, and is basically based on the ext2 sources main include file. At this point, EXT2ED can be fully used by the user.

If we do not register the ext2 specific objects, the user can still provide object definitions in a separate file, and will be able to use EXT2ED in a `limited form`, but more sophisticated than a simple hex editor.

## main.c

As described earlier, `main.c` is used as a front-head to the entire program. `main.c` contains the following elements:

### The main routine

The `main` routine was displayed above. Its task is to pass control to the initialization routines and to the parser.

### The parser

The parser consists of the following functions:

- The `parser` function, which reads the command line from the user and saves it in readline's history buffer and in the internal last-command buffer.

- The `parse&lowbar;word` function, which receives a string and parses the first word from it, ignoring whitespaces, and returns a pointer to the rest of the string.

- The `complete&lowbar;command` function, which is used by the readline library for command completion. It scans the available commands at this point and determines the possible completions.

### The dispatcher

The dispatcher was already explained in the flow control section - section . Its task is to pass control to the proper command handling function, based on the command line's command.

### The self-sanity control

This is not fully implemented.

The general idea was to provide a control system which will supervise the internal work of EXT2ED. Since I am pretty sure that bugs exist, I have double checked myself in a few instances, and issued an `internal error` warning if I reached the conclusion that something is not logical. The internal error is reported by the function `internal&lowbar;error`, available in `main.c`.

The self sanity check is compiled only if the compile time option `DEBUG` is selected.

## The windows interface

Screen handling and interfacing to the `ncurses` library is done in `win.c`.

### Initialization

Opening of the windows is done in `init&lowbar;windows`. In `close&lowbar;windows`, we just close our windows. The various window lengths with an exception to the `show pad` are defined in the main header file. The rest of the display will be used by the `show pad`.

### Display output

Each actual refreshing of the terminal monitor is done by using the appropriate refresh function from this file: `refresh&lowbar;title&lowbar;win`, `refresh&lowbar;show&lowbar;win`, `refresh&lowbar;show&lowbar;pad` and `refresh&lowbar;command&lowbar;win`.

With the exception of the `show pad`, each function simply calls the `ncurses refresh command`. In order to provide to `scrolling` in the `show pad`, some information about its status is constantly updated by the various functions which display output in it. `refresh&lowbar;show&lowbar;pad` passes this information to `ncurses` so that the correct part of the pad is actually copied to the display.

The above information is saved in a global variable of type `struct struct&lowbar;pad&lowbar;info`:

    struct struct_pad_info {
    	int display_lines,display_cols;
    	int line,col;
    	int max_line,max_col;
    	int disable_output;
    };

### Screen redraw

The `redraw&lowbar;all` function will just reopen the windows. This action is necessary if the display gets garbled from some reason.

## The disk interface

All the disk activity with regard to the filesystem passes through the file `disk.c`. This is done that way to provide additional levels of safety concerning the disk access. This way, global decisions considering the disk can be easily accomplished. The benefits of this isolation will become even clearer in the next sections.

### Low level functions

Read requests are ultimately handled by `low&lowbar;read` and write requests are handled by `low&lowbar;write`. They just receive the length of the data block, the offset in the filesystem and a pointer to the buffer and pass the request to the `fread` or `fwrite` standard library functions.

### Mounted filesystems

EXT2ED design assumes that the edited filesystem is not mounted. Even if a `reasonably simple` way to handle mounted filesystems exists, it is probably `too complicated` :-)

Write access to a mounted filesystem will be denied. Read access can be allowed by using a configuration file option. The mount status is determined by reading the file /etc/mtab.

### Write access

Write access is the most sensitive part in the program. This program is intended for `editing filesystems`. It is obvious that a small mistake in this regard can make the filesystem not usable anymore.

The following safety measures are added, of-course, to the general Unix permission protection - The user can always disable write access on the device file itself.

Considering the user, the following safety measures were taken:

1. The filesystem is `never` opened with write-access enables. Rather, the user must explicitly request to enable write-access.

2. The user can `disable` write access entirely by using a `configuration file option`.

3. Changes are never done automatically - Whenever the user makes changes, they are done in memory. An explicit `writedata` command should be issued to make the changes active in the disk.

Considering myself, I tried to protect against my bugs by:

- Opening the device in read-only mode until a write request is issued by the user.

- Limiting `actual` filesystem access to two functions only - `low&lowbar;read` for reading, and `low&lowbar;write` for writing. Those functions were programmed carefully, and I added the self sanity checks there. In addition, this is the only place in which I need to check the user options described above - There can be no place in which I can "forget" to check them.

  Note that The disabling of write-access through the configuration file is double checked here only as a `self-sanity` check - If `DEBUG` is selected, since write enable should have been refused and write-access is always disabled at startup, hence finding `here` that the user has write access disabled through the configuration file clearly indicates that I have a bug somewhere.

The following safety measure can provide protection against `both` user mistakes and my own bugs:

- I added a `logging option`, which logs every actual write access to the disk in the lowest level - In `low&lowbar;write` itself.

  The logging has nothing to do with the current type and the various other higher level operations of EXT2ED - It is simply a hex dump of the contents which will be overwritten; Both the original contents and the new written data.

  In that case, even if the user makes a mistake, the original data can be retrieved.

  Even If I have a bug somewhere which causes incorrect data to be written to the disk, the logging option will still log exactly the original contents at the place were data was incorrectly overwritten. (This assumes, of-course, that `low-write` and the `logging itself` work correctly. I have done my best to verify that this is indeed the case).

  The `logging` option is implemented in the `log&lowbar;changes` function.

### Reading / Writing objects

Usually `(not always)`, the current object data is available in the global variable `type&lowbar;data`, which is of the type:

    struct struct_type_data {
    	long offset_in_block;

    	union union_type_data {
    		char buffer [EXT2_MAX_BLOCK_SIZE];
    		struct ext2_acl_header t_ext2_acl_header;
    		struct ext2_acl_entry t_ext2_acl_entry;
    		struct ext2_old_group_desc t_ext2_old_group_desc;
    		struct ext2_group_desc t_ext2_group_desc;
    		struct ext2_inode t_ext2_inode;
    		struct ext2_super_block t_ext2_super_block;
    		struct ext2_dir_entry t_ext2_dir_entry;
    	} u;
    };

The above union enables me, in the program, to treat the data as raw data or as a meaningful filesystem object.

The reading and writing, if done to this global variable, are done through the functions `load&lowbar;type&lowbar;data` and `write&lowbar;type&lowbar;data`, available in `disk.c`.

## The general commands

The `general commands` are handled in the file `general&lowbar;com.c`.

### The help system

The help command is handled by the function `help`. The algorithm is as follows:

1. Check the command line arguments. If there is an argument, pass control to the `detailed&lowbar;help` function, in order to provide help on the specific command.

2. If general help was requested, display a list of the available commands at this point. The three levels are displayed in reverse order - First the commands which are specific to the current type (If a current type is defined), then the ext2 general commands (If we decided that the filesystem should be treated like an ext2 filesystem), then the general commands.

3. Display information about EXT2ED - Current version, general information about the project, etc.

### The setdevice command

The `setdevice` commands result in calling the `set&lowbar;device` function. The algorithm is:

1. Parse the command line argument. If it isn't available report the error and return.

2. Close the current open device, if there is one.

3. Open the new device in read-only mode. Update the global variables `device&lowbar;name` and `device&lowbar;handle`.

4. Disable write access.

5. Empty the object memory.

6. Unregister the ext2 general commands, using `free&lowbar;user&lowbar;commands`.

7. Unregister the current objects, using `free&lowbar;struct&lowbar;descriptors`

8. Call `set&lowbar;file&lowbar;system&lowbar;info` to auto-detect an ext2 filesystem and set the basic filesystem values.

9. Add the `alternate descriptors`, supplied by the user.

10. Set the device offset to the filesystem start by dispatching `setoffset 0`.

11. Show the new available commands by dispatching the `help` command.

### Basic maneuvering

Basic maneuvering is done using the `setoffset` and the `settype` user commands.

`set&lowbar;offset` accepts some alternative forms of specifying the new offset. They all ultimately lead to changing the `device&lowbar;offset` global variable and seeking to the new position. `set&lowbar;offset` also calls `load&lowbar;type&lowbar;data` to read a block ahead of the new position into the `type&lowbar;data` global variable.

`set&lowbar;type` will point the global variable `current&lowbar;type` to the correct entry in the double linked list of the known objects. If the requested type is `hex` or `none`, `current&lowbar;type` will be initialized to `NULL`. `set&lowbar;type` will also dispatch `show`, so that the object data will be re-formatted in the new format.

When editing an ext2 filesystem, it is not intended that those commands will be used directly, and it is usually not required. My implementation of the ext2 layer, on the other hand, uses this lower level commands on countless occasions.

### The display functions

The general command version of `show` is handled by the `show` function. This command is overridden by various objects to provide a display which is better suited to the object.

The general show command will format the data in `type&lowbar;data` according to the structure definition of the current type and show it on the `show pad`. If there is no current type, the data will be shown as a simple hex dump; Otherwise, the list of variables, along with their values will be shown.

A call to `show&lowbar;info` is also made - `show&lowbar;info` will provide `general statistics` on the `show&lowbar;window`, such as the current block, current type, current offset and current page.

The `pgup` and `pgdn` general commands just update the `show&lowbar;pad&lowbar;info` global variable - We just increment `show&lowbar;pad&lowbar;info.line` with the number of lines in the screen - `show&lowbar;pad&lowbar;info.display&lowbar;lines`, which was initialized in `init&lowbar;windows`.

### Changing data

Data change is done in memory only. An update to the disk if followed by an explicit `writedata` command to the disk. The `write&lowbar;data` function simple calls the `write&lowbar;type&lowbar;data` function, outlined earlier.

The `set` command is used for changing the data.

If there is no current type, control is passed to the `hex&lowbar;set` function, which treats the data as a block of bytes and uses the `type&lowbar;data.offset&lowbar;in&lowbar;block` variable to write the new text or hex string to the correct place in the block.

If a current type is defined, the requested variable is searched in the current object, and the desired new valued is entered.

The `enablewrite` commands just sets the global variable `write&lowbar;access` to `1` and re-opens the filesystem in read-write mode, if possible.

If the current type is NULL, a hex-mode is assumed - The `next` and `prev` commands will just update `type&lowbar;data.offset&lowbar;in&lowbar;block`.

If the current type is not NULL, the The `next` and `prev` command are usually overridden anyway. If they are not overridden, it will be assumed that the user is editing an array of such objects, and they will just pass to the next / prev element by dispatching to `setoffset` using the `setoffset type + / - X` syntax.

## The ext2 general commands

The ext2 general commands are contained in the `ext2&lowbar;general&lowbar;commands` global variable (which is of type `struct struct&lowbar;commands`).

The handling functions are implemented in the source file `ext2&lowbar;com.c`. I will include the entire source code since it is relatively short.

### The super command

The super command just "brings the user" to the main superblock and set the type to ext2&lowbar;super&lowbar;block. The implementation is trivial:

    void type_ext2___super (char *command_line)

    {
    	char buffer [80];

    	super_info.copy_num=0;
    	sprintf (buffer,"setoffset %ld",file_system_info.super_block_offset);dispatch (buffer);
    	sprintf (buffer,"settype ext2_super_block");dispatch (buffer);
    }

It involves only setting the `copy&lowbar;num` variable to indicate the main copy, dispatching a `setoffset` command to reach the superblock, and dispatching a `settype` to enable the superblock specific commands. This last command will also call the `show` command of the `ext2&lowbar;super&lowbar;block` type, through dispatching at the general command `settype`.

### The group command

The group command will bring the user to the specified group descriptor in the main copy of the group descriptors. The type will be set to `ext2&lowbar;group&lowbar;desc`:

    void type_ext2___group (char *command_line)

    {
    	long group_num=0;
    	char *ptr,buffer [80];

    	ptr=parse_word (command_line,buffer);
    	if (*ptr!=0) {
    		ptr=parse_word (ptr,buffer);
    		group_num=atol (buffer);
    	}

    	group_info.copy_num=0;group_info.group_num=0;
    	sprintf (buffer,"setoffset %ld",file_system_info.first_group_desc_offset);dispatch (buffer);
    	sprintf (buffer,"settype ext2_group_desc");dispatch (buffer);
    	sprintf (buffer,"entry %ld",group_num);dispatch (buffer);
    }

The implementation is as trivial as the `super` implementation. Note the use of the `entry` command, which is a command of the `ext2&lowbar;group&lowbar;desc` object, to pass to the correct group descriptor.

### The cd command

The `cd` command performs the usual cd function. The path to the global cd command is a path from `/`.

`This is one of the best examples of the power of the object oriented design and of the dispatching mechanism. The operation is complicated, yet the implementation is surprisingly short!`

    void type_ext2___cd (char *command_line)

    {
    	char temp [80],buffer [80],*ptr;

    	ptr=parse_word (command_line,buffer);
    	if (*ptr==0) {
    		wprintw (command_win,"Error - No argument specified\n");
    		refresh_command_win ();return;
    	}
    	ptr=parse_word (ptr,buffer);

    	if (buffer [0] != '/') {
    		wprintw (command_win,"Error - Use a full pathname (begin with '/')\n");
    		refresh_command_win ();return;
    	}

    	dispatch ("super");dispatch ("group");dispatch ("inode");
    	dispatch ("next");dispatch ("dir");
    	if (buffer [1] != 0) {
    		sprintf (temp,"cd %s",buffer+1);dispatch (temp);
    	}
    }

Note the number of the dispatch calls!

`super` is used to get to the superblock. `group` to get to the first group descriptor. `inode` brings us to the first inode - The bad blocks inode. A `next` is command to pass to the root directory inode, a `dir` command "enters" the directory, and then we let the `object specific cd command` to take us from there (The object is `dir`, so that `dispatch` will call the `cd` command of the `dir` type). Note that a symbolic link following could bring us back to the root directory, thus the innocent calls above treats nicely such a recursive case!

I feel that the above is `intuitive` - I was expressing myself "in the language" of the ext2 filesystem - (Go to the inode, etc), and the code was written exactly in this spirit!

I can write more at this point, but I guess I am already a bit carried away with the self compliments :-)

## The superblock

This section details the handling of the superblock.

### The superblock variables

The superblock object is `ext2&lowbar;super&lowbar;block`. The definition is just taken from the kernel ext2 main include file - /usr/include/linux/ext2&lowbar;fs.h. Those lines of source are copyrighted by `Remy Card` - The author of the ext2 filesystem, and by `Linus Torvalds` - The first author of the Linux operating system. Please cross reference the section Acknowledgments for the full copyright.

    struct ext2_super_block {
    	__u32	s_inodes_count;		/* Inodes count */
    	__u32	s_blocks_count;		/* Blocks count */
    	__u32	s_r_blocks_count;	/* Reserved blocks count */
    	__u32	s_free_blocks_count;	/* Free blocks count */
    	__u32	s_free_inodes_count;	/* Free inodes count */
    	__u32	s_first_data_block;	/* First Data Block */
    	__u32	s_log_block_size;	/* Block size */
    	__s32	s_log_frag_size;	/* Fragment size */
    	__u32	s_blocks_per_group;	/* # Blocks per group */
    	__u32	s_frags_per_group;	/* # Fragments per group */
    	__u32	s_inodes_per_group;	/* # Inodes per group */
    	__u32	s_mtime;		/* Mount time */
    	__u32	s_wtime;		/* Write time */
    	__u16	s_mnt_count;		/* Mount count */
    	__s16	s_max_mnt_count;	/* Maximal mount count */
    	__u16	s_magic;		/* Magic signature */
    	__u16	s_state;		/* File system state */
    	__u16	s_errors;		/* Behavior when detecting errors */
    	__u16	s_pad;
    	__u32	s_lastcheck;		/* time of last check */
    	__u32	s_checkinterval;	/* max. time between checks */
    	__u32	s_creator_os;		/* OS */
    	__u32	s_rev_level;		/* Revision level */
    	__u16	s_def_resuid;		/* Default uid for reserved blocks */
    	__u16	s_def_resgid;		/* Default gid for reserved blocks */
    	__u32	s_reserved[0];		/* Padding to the end of the block */
    	__u32	s_reserved[1];		/* Padding to the end of the block */
    	.
    	.
    	.
    	__u32	s_reserved[234];	/* Padding to the end of the block */
    };

Note that I `expanded` the array due to my primitive parser implementation. The various fields are described in the `technical document`.

### The superblock commands

This section explains the commands available in the `ext2&lowbar;super&lowbar;block` type. They all appear in `super&lowbar;com.c`

#### The show command

The `show` command is overridden here in order to provide more information than just the list of variables. A `show` command will end up in calling `type&lowbar;super&lowbar;block&lowbar;&lowbar;&lowbar;show`.

The first thing that we do is calling the `general show command` in order to display the list of variables.

We then add some interpretation to the various lines to make the data somewhat more intuitive (Expansion of the time variables and the creator operating system code, for example).

We also display the `backup copy number` of the superblock in the status window. This copy number is saved in the `super&lowbar;info` global variable - `super&lowbar;info.copy&lowbar;num`. Currently, this is the only variable there ... but this type of internal variable saving is typical through my implementation.

#### The backup copies handling commands

The `current copy number` is available in `super&lowbar;info.copy&lowbar;num`. It was initialized in the ext2 command `super`, and is used by the various superblock routines.

The `gocopy` routine will pass to another copy of the superblock. The new device offset will be computed with the aid of the variables in the `file&lowbar;system&lowbar;info` structure. Then the routine will `dispatch` to the `setoffset` and the `show` routines.

The `setactivecopy` routine will just save the current superblock data in a temporary variable of type `ext2&lowbar;super&lowbar;block`, and will dispatch `gocopy 0` to pass to the main superblock. Then it will place the saved data in place of the actual data.

The above two commands can be used if the main superblock is corrupted.

## The group descriptors

The group descriptors handling mechanism allows the user to take a tour in the group descriptors table, stopping at each point, and examining the relevant inode table, block allocation map or inode allocation map through dispatching to the relevant objects.

Some information about the group descriptors is available in the global variable `group&lowbar;info`, which is of type `struct&lowbar;group&lowbar;info`:

    struct struct_group_info {
    	unsigned long copy_num;
    	unsigned long group_num;
    };

`group&lowbar;num` is the index of the current descriptor in the table.

`copy&lowbar;num` is the number of the current backup copy.

### The group descriptor's variables

    struct ext2_group_desc
    {
    	__u32	bg_block_bitmap;		/* Blocks bitmap block */
    	__u32	bg_inode_bitmap;		/* Inodes bitmap block */
    	__u32	bg_inode_table;			/* Inodes table block */
    	__u16	bg_free_blocks_count;		/* Free blocks count */
    	__u16	bg_free_inodes_count;		/* Free inodes count */
    	__u16	bg_used_dirs_count;		/* Directories count */
    	__u16	bg_pad;
    	__u32	bg_reserved[3];
    };

The first three variables are used to provide the links to the `blockbitmap, inodebitmap and inode` objects.

### Movement in the table

Movement in the group descriptors table is done using the `next, prev and entry` commands. Note that the first two commands `override` the general commands of the same name. The `next and prev` command are just calling the `entry` function to do the job. I will show `next`, for example:

    void type_ext2_group_desc___next (char *command_line)

    {
    	long entry_offset=1;
    	char *ptr,buffer [80];

    	ptr=parse_word (command_line,buffer);
    	if (*ptr!=0) {
    		ptr=parse_word (ptr,buffer);
    		entry_offset=atol (buffer);
    	}

    	sprintf (buffer,"entry %ld",group_info.group_num+entry_offset);
    	dispatch (buffer);
    }

The `entry` function is also simple - It just calculates the offset using the information in `group&lowbar;info` and in `file&lowbar;system&lowbar;info`, and uses the usual `setoffset / show` pair.

### The show command

As usual, the `show` command is overridden. The implementation is similar to the superblock's show implementation - We just call the general show command, and add some information in the status window - The contents of the `group&lowbar;info` structure.

### Moving between backup copies

This is done exactly like the superblock case. Please refer to explanation there.

### Links to the available friends

From a group descriptor, one typically wants to reach an `inode`, or one of the `allocation bitmaps`. This is done using the `inode, blockbitmap or inodebitmap` commands. The implementation is again trivial - Get the necessary information from the group descriptor, initialize the structures of the next type, and issue the `setoffset / settype` pair.

For example, here is the implementation of the `blockbitmap` command:

    void type_ext2_group_desc___blockbitmap (char *command_line)

    {
    	long block_bitmap_offset;
    	char buffer [80];

    	block_bitmap_info.entry_num=0;
    	block_bitmap_info.group_num=group_info.group_num;

    	block_bitmap_offset=type_data.u.t_ext2_group_desc.bg_block_bitmap;
    	sprintf (buffer,"setoffset block %ld",block_bitmap_offset);dispatch (buffer);
    	sprintf (buffer,"settype block_bitmap");dispatch (buffer);
    }

## The inode table

The inode handling enables the user to move in the inode table, edit the various attributes of the inode, and follow to the next stage - A file or a directory.

### The inode variables

    struct ext2_inode {
    	__u16	i_mode;		/* File mode */
    	__u16	i_uid;		/* Owner Uid */
    	__u32	i_size;		/* Size in bytes */
    	__u32	i_atime;	/* Access time */
    	__u32	i_ctime;	/* Creation time */
    	__u32	i_mtime;	/* Modification time */
    	__u32	i_dtime;	/* Deletion Time */
    	__u16	i_gid;		/* Group Id */
    	__u16	i_links_count;	/* Links count */
    	__u32	i_blocks;	/* Blocks count */
    	__u32	i_flags;	/* File flags */
    	union {
    		struct {
    			__u32  l_i_reserved1;
    		} linux1;
    		struct {
    			__u32  h_i_translator;
    		} hurd1;
    	} osd1;				/* OS dependent 1 */
    	__u32	i_block[EXT2_N_BLOCKS];	/* Pointers to blocks */
    	__u32	i_version;		/* File version (for NFS) */
    	__u32	i_file_acl;		/* File ACL */
    	__u32	i_dir_acl;		/* Directory ACL */
    	__u32	i_faddr;		/* Fragment address */
    	union {
    		struct {
    			__u8	l_i_frag;	/* Fragment number */
    			__u8	l_i_fsize;	/* Fragment size */
    			__u16	i_pad1;
    			__u32	l_i_reserved2[2];
    		} linux2;
    		struct {
    			__u8	h_i_frag;	/* Fragment number */
    			__u8	h_i_fsize;	/* Fragment size */
    			__u16	h_i_mode_high;
    			__u16	h_i_uid_high;
    			__u16	h_i_gid_high;
    			__u32	h_i_author;
    		} hurd2;
    	} osd2;				/* OS dependent 2 */
    };

The above is the original source code definition. We can see that the inode supports `Operating systems specific structures`. In addition to the expansion of the arrays, I have `"flattened` the inode to support only the `Linux` declaration. It seemed that this one occasion of multiple variable aliases didn't justify the complication of generally supporting aliases. In any case, the above system specific variables are not used internally by EXT2ED, and the user is free to change the definition in `ext2.descriptors` to accommodate for his needs.

### The handling functions

The user interface to `movement` is the usual `next / prev / entry` interface. There is really nothing special in those functions - The size of the inode is fixed, the total number of inodes is known from the superblock information, and the current entry can be figured up from the device offset and the inode table start offset, which is known from the corresponding group descriptor. Those functions are a bit older then some other implementations of `next` and `prev`, and they do not save information in a special structure. Rather, they recompute it when necessary.

The `show` command is overridden here, and provides a lot of additional information about the inode - Its type, interpretation of the permissions, special ext2 attributes (Immutable file, for example), and a lot more. Again, the `general show` is called first, and then the additional information is written.

### Accessing files and directories

From the inode, a `file` or a `directory` can typically be reached. In order to treat a file, for example, its inode needs to be constantly accessed. To satisfy that need, when editing a file or a directory, the inode is still saved in memory - `type&lowbar;data` is not overwritten. Rather, the following takes place:

- An internal global structure which is used by the types `file` and `dir` handling functions is initialized by calling the appropriate function.

- The type is changed accordingly.

The result is that a `settype ext2&lowbar;inode` is the only action necessary to return to the inode - We actually never left it.

Follows the implementation of the inode's `file` command:

    void type_ext2_inode___file (char *command_line)

    {
    	char buffer [80];

    	if (!S_ISREG (type_data.u.t_ext2_inode.i_mode)) {
    		wprintw (command_win,"Error - Inode type is not file\n");
    		refresh_command_win ();	return;
    	}

    	if (!init_file_info ()) {
    		wprintw (command_win,"Error - Unable to show file\n");
    		refresh_command_win ();return;
    	}

    	sprintf (buffer,"settype file");dispatch (buffer);
    }

As we can see - We just call `init&lowbar;file&lowbar;info` to get the necessary information from the inode, and set the type to `file`. The next call to `show`, will dispatch to the `file's show` implementation.

## Viewing a file

There isn't an ext2 kernel structure which corresponds to a file - A file is just a series of blocks which are determined by its inode. As explained in the last section, the inode is never actually left - The type is changed to `file` - A type which contains no variables, and a special structure is initialized:

    struct struct_file_info {

    	struct ext2_inodes *inode_ptr;

    	long inode_offset;
    	long global_block_num,global_block_offset;
    	long block_num,blocks_count;
    	long file_offset,file_length;
    	long level;
    	unsigned char buffer [EXT2_MAX_BLOCK_SIZE];
    	long offset_in_block;

    	int display;
    	/* The following is used if the file is a directory */

    	long dir_entry_num,dir_entries_count;
    	long dir_entry_offset;
    };

The `inode&lowbar;ptr` will just point to the inode in `type&lowbar;data`, which is not overwritten while the user is editing the file, as the `setoffset` command is not internally used. The `buffer` will contain the current viewed block of the file. The other variables contain information about the current place in the file. For example, `global&lowbar;block&lowbar;num` just contains the current block number.

The general idea is that the above data structure will provide the file handling functions all the accurate information which is needed to accomplish their task.

The global structure of the above type, `file&lowbar;info`, is initialized by `init&lowbar;file&lowbar;info` in `file&lowbar;com.c`, which is called by the `type&lowbar;ext2&lowbar;inode&lowbar;&lowbar;&lowbar;file` function when the user requests to watch the file. `It is updated as necessary to provide accurate information as long as the file is edited.`

### Returning to the file's inode

Concerning the method I used to handle files, the above task is trivial:

    void type_file___inode (char *command_line)

    {
    	dispatch ("settype ext2_inode");
    }

### File movement

EXT2ED keeps track of the current position in the file. Movement inside the current block is done using `next, prev and offset` - They just change `file&lowbar;info.offset&lowbar;in&lowbar;block`.

Movement between blocks is done using `nextblock, prevblock and block`. To accomplish this, the direct blocks, indirect blocks, etc, need to be traced. This is done by `file&lowbar;block&lowbar;to&lowbar;global&lowbar;block`, which accepts a file's internal block number, and converts it to the actual filesystem block number.

    long file_block_to_global_block (long file_block,struct struct_file_info *file_info_ptr)

    {
    	long last_direct,last_indirect,last_dindirect;
    	long f_indirect,s_indirect;

    	last_direct=EXT2_NDIR_BLOCKS-1;
    	last_indirect=last_direct+file_system_info.block_size/4;
    	last_dindirect=last_indirect+(file_system_info.block_size/4) \
    		*(file_system_info.block_size/4);

    	if (file_block <= last_direct) {
    		file_info_ptr->level=0;
    		return (file_info_ptr->inode_ptr->i_block [file_block]);
    	}

    	if (file_block <= last_indirect) {
    		file_info_ptr->level=1;
    		file_block=file_block-last_direct-1;
    		return (return_indirect (file_info_ptr->inode_ptr-> \
    			i_block [EXT2_IND_BLOCK],file_block));
    	}

    	if (file_block <= last_dindirect) {
    		file_info_ptr->level=2;
    		file_block=file_block-last_indirect-1;
    		return (return_dindirect (file_info_ptr->inode_ptr-> \
    			i_block [EXT2_DIND_BLOCK],file_block));
    	}

    	file_info_ptr->level=3;
    	file_block=file_block-last_dindirect-1;
    	return (return_tindirect (file_info_ptr->inode_ptr-> \
    		i_block [EXT2_TIND_BLOCK],file_block));
    }

`last&lowbar;direct, last&lowbar;indirect, etc`, contain the last internal block number which is accessed by this method - If the requested block is smaller then `last&lowbar;direct`, for example, it is a direct block.

If the block is a direct block, its number is just taken from the inode. A non-direct block is handled by `return&lowbar;indirect, return&lowbar;dindirect and return&lowbar;tindirect`, which correspond to indirect, double-indirect and triple-indirect. Each of the above functions is constructed using the lower level functions. For example, `return&lowbar;dindirect` is constructed as follows:

    long return_dindirect (long table_block,long block_num)

    {
    	long f_indirect;

    	f_indirect=block_num/(file_system_info.block_size/4);
    	f_indirect=return_indirect (table_block,f_indirect);
    	return (return_indirect (f_indirect,block_num%(file_system_info.block_size/4)));
    }

### Object memory

The `remember` command is overridden here and in the `dir` type - We just remember the inode of the file. It is just simpler to implement, and doesn't seem like a big limitation.

### Changing data

The `set` command is overridden, and provides the same functionality like the usage of the `general set` command with no type declared. The `writedata` is overridden so that we'll write the edited block (file&lowbar;info.buffer) and not `type&lowbar;data` (Which contains the inode).

## Directories

A directory is just a file which is formatted according to a special format. As such, EXT2ED handles directories and files quite alike. Specifically, the same variable of type `struct&lowbar;file&lowbar;info` which is used in the `file`, is used here.

The `dir` type uses all the variables in the above structure, as opposed to the `file` type, which didn't use the last ones.

### The search&lowbar;dir&lowbar;entries function

The entire situation is similar to that which was described in the `file` type, with one main change:

The main function in `dir&lowbar;com.c` is `search&lowbar;dir&lowbar;entries`. This function will `"run"` on the entire entries in the directory, and will call a client's function each time. The client's function is supplied as an argument, and will check the current entry for a match, based on its own criterion. It will then signal `search&lowbar;dir&lowbar;entries` whether to `ABORT` the search, whether it `FOUND` the entry it was looking for, or that the entry is still not found, and we should `CONTINUE` searching. Follows the declaration:

    struct struct_file_info search_dir_entries \
    	(int (*action) (struct struct_file_info *info),int *status)

    /*
    	This routine runs on all directory entries in the current directory.
    	For each entry, action is called. The return code of action is one of
    	the following:

    		ABORT		-	Current dir entry is returned.
    		CONTINUE	-	Continue searching.
    		FOUND		-	Current dir entry is returned.

    	If the last entry is reached, it is returned, along with an ABORT status.

    	status is updated to the returned code of action.
    */

With the above tool in hand, many operations are simple to perform - Here is the way I counted the entries in the current directory:

    long count_dir_entries (void)

    {
    	int status;

    	return (search_dir_entries (&action_count,&status).dir_entry_num);
    }

    int action_count (struct struct_file_info *info)

    {
    	return (CONTINUE);
    }

It will just `CONTINUE` until the last entry. The returned structure (of type `struct&lowbar;file&lowbar;info`) will have its number in the `dir&lowbar;entry&lowbar;num` field, and this is exactly the required number!

### The cd command

The `cd` command accepts a relative path, and moves there ... The implementation is of-course a bit more complicated:

1. The path is checked that it is not an absolute path (from `/`). If it is, we let the `general cd` to do the job by calling directly `type&lowbar;ext2&lowbar;&lowbar;&lowbar;cd`.

2. The path is divided into the nearest path and the rest of the path. For example, cd 1/2/3/4 is divided into `1` and into `2/3/4`.

3. It is the first part of the path that we need to search for in the current directory. We search for it using `search&lowbar;dir&lowbar;entries`, which accepts the `action&lowbar;name` function as the user defined function.

4. `search&lowbar;dir&lowbar;entries` will scan the entire entries and will call our `action&lowbar;name` function for each entry. In `action&lowbar;name`, the required name will be checked against the name of the current entry, and `FOUND` will be returned when a match occurs.

5. If the required entry is found, we dispatch a `remember` command to insert the current `inode` into the object memory. This is required to easily support `symbolic links` - If we find later that the inode pointed by the entry is actually a symbolic link, we'll need to return to this point, and the above inode doesn't have (and can't have, because of `hard links`) the information necessary to "move back".

6. We then dispatch a `followinode` command to reach the inode pointed by the required entry. This command will automatically change the type to `ext2&lowbar;inode` - We are now at an inode, and all the inode commands are available.

7. We check the inode's type to see if it is a directory. If it is, we dispatch a `dir` command to "enter the directory", and recursively call ourself (The type is `dir` again) by dispatching a `cd` command, with the rest of the path as an argument.

8. If the inode's type is a symbolic link (only fast symbolic link were meanwhile implemented. I guess this is typically the case.), we note the path it is pointing at, the saved inode is recalled, we dispatch `dir` to get back to the original directory, and we call ourself again with the `link path/rest of the path` argument.

9. In any other case, we just stop at the resulting inode.

## The block and inode allocation bitmaps

The block allocation bitmap is reached by the corresponding group descriptor. The group descriptor handling functions will save the necessary information into a structure of the `struct&lowbar;block&lowbar;bitmap&lowbar;info` type:

    struct struct_block_bitmap_info {
    	unsigned long entry_num;
    	unsigned long group_num;
    };

The `show` command is overridden, and will show the block as a series of bits, each bit corresponding to a block. The main variable is the `entry&lowbar;num` variable, declared above, which is just the current block number in this block group. The current entry is highlighted, and the `next, prev and entry` commands just change the above variable.

The `allocate and deallocate` change the specified bits. Nothing special about them - They just contain code which converts between bit and byte locations.

The `inode allocation bitmap` is treated in much the same fashion, with the same commands available.

## Filesystem size limitation

While an ext2 filesystem has a size limit of `4 TB`, EXT2ED currently `can't` handle filesystems which are `bigger than 2 GB`.

This limitation results from my usage of `32 bit long variables` and of the `fseek` filesystem call, which can't seek up to 4 TB.

By looking in the `ext2 library` source code by `Theodore Ts'o`, I discovered the `llseek` system call which can seek to a `64 bit unsigned long long` offset. Correcting the situation is not difficult in concept - I need to change long into unsigned long long where appropriate and modify `disk.c` to use the llseek system call.

However, fixing the above limitation involves making changes in many places in the code and will obviously make the entire code less stable. For that reason, I chose to release EXT2ED as it is now and to postpone the above fix to the next release.

## Conclusion

Had I known in advance the structure of the ext2 filesystem, I feel that the resulting design would have been quite different from the presented design above.

EXT2ED has now two levels of abstraction - A `general` filesystem, and an `ext2` filesystem, and the surface is more or less prepared for additions of other filesystems. Had I approached the design in the "engineering" way, I guess that the first level above would not have existed.

## Copyright

EXT2ED is Copyright (C) 1995 Gadi Oxman.

EXT2ED is hereby placed under the GPL - Gnu Public License. You are free and welcome to copy, view and modify the sources. My only wish is that my copyright presented above will be left and that a list of the bug fixes, added features, etc, will be provided.

The entire EXT2ED project is based, of-course, on the kernel sources. The `ext2.descriptors` distributed with EXT2ED is a slightly modified version of the main ext2 include file, /usr/include/linux/ext2&lowbar;fs.h. Follows the original copyright:

    /*
     *  linux/include/linux/ext2_fs.h
     *
     * Copyright (C) 1992, 1993, 1994, 1995
     * Remy Card (card@masi.ibp.fr)
     * Laboratoire MASI - Institut Blaise Pascal
     * Universite Pierre et Marie Curie (Paris VI)
     *
     *  from
     *
     *  linux/include/linux/minix_fs.h
     *
     *  Copyright (C) 1991, 1992  Linus Torvalds
     */

## Acknowledgments

EXT2ED was constructed as a student project in the software laboratory of the faculty of electrical-engineering in the `Technion - Israel's institute of technology`.

At first, I would like to thank `Avner Lottem` and `Doctor Ilana David` for their interest and assistance in this project.

I would also like to thank the following people, who were involved in the design and implementation of the ext2 filesystem kernel code and support utilities:

- `Remy Card`

  Who designed, implemented and maintains the ext2 filesystem kernel code, and some of the ext2 utilities. `Remy Card` is also the author of several helpful slides concerning the ext2 filesystem. Specifically, he is the author of `File Management in the Linux Kernel` and of `The Second Extended File System - Current State, Future Development`.

- `Wayne Davison`

  Who designed the ext2 filesystem.

- `Stephen Tweedie`

  Who helped designing the ext2 filesystem kernel code and wrote the slides `Optimizations in File Systems`.

- `Theodore Ts'o`

  Who is the author of several ext2 utilities and of the ext2 library `libext2fs` (which I didn't use, simply because I didn't know it exists when I started to work on my project).

Lastly, I would like to thank, of-course, `Linus Torvalds` and the `Linux community` for providing all of us with such a great operating system.

Please contact me in a case of bug report, suggestions, or just about anything concerning EXT2ED.

Enjoy,

Gadi Oxman <tgud@tochnapc2.technion.ac.il>

Haifa, August 95