GOSFS - GeekOS FileSystem

The main part of this project is to develop a new filesystem for the GeekOS. This filesystem will reside on the second IDE disk drive in the Bochs emulator. This will allow you to continue to use your existing PFAT drive to load user programs while you test your filesystem. The second IDE disk's image is called diskd.img. It is 10MB by default.

GOSFS will provide a filesystem that includes multiple directories and long file name support.

VFS and file operations

Since GEEKOS will have two types of filesystems (PFAT and GOSFS), it will have a virtual filesystem layer (VFS) to handle sending requests to an appropriate filesystem (see figure below). We have provided an implementation of the VFS layer in the file vfs.c. The VFS layer will call the appropriate GOSFS routines when a file operation refers a file in the GOSFS filesystem.

The implementation of PFAT is in pfat.c. You will implement GOSFS in gosfs.c and relevant system calls in syscall.c. VFS picks the functions to call based on supplied structures containing function pointers, one for each operation. For example, see Init_PFAT in pfat.c: this initializes the PFAT filesystem with VFS by passing it a pointer to s_pfatFilesystemOps, a structure that contains function pointers to PFAT routines for mounting (and formatting) a filesystem.  Other PFAT functions are stored in different structures (e.g., look at the PFAT_Open routine, which passes the s_pfatFileOps structure to VFS).  You will analogously use s_gosfsFilesystemOps, s_gosfsMountPointOps, s_gosfsDirOps, and s_gosfsFileOps in gosfs.c. You should also add a call to Init_GOSFS provided gosfs.c in main.c to register the GOSFS filesystem.  In general, use the PFAT implementation as your guide to interfacing GOSFS with VFS.

Open files are tracked in memory with a file descriptor, defined as struct File.  Each user space process will have a file descriptor table that records which files the process can currently read and write. A user process can have up to USER_MAX_FILES files open at once. The file descriptor table is implemented asa struct File *fileList[USER_MAX_FILES] array that you must add to struct User_Context. Note that not all the entries in the fileList are necessarily open files, since usually a process has less than USER_MAX_FILES files open at once. If fileList[i] is NULL, it represents a free slot (file descriptor is not used). This descriptor will be filled out by the code in VFS; e.g., see Open in vfs.h, whose pFile argument is a pointer to a free slot in the table.

Your filesystem should support long filenames (at most 128 bytes, which includes a null at the end). A file's full path (which includes directories and subdirectories) will be no more than 1024 characters in total.

You should keep track of free disk blocks using a bit vector (as described in the textbook, Section 12.5.1, page 431). A library called bitset is provided (see bitset.h and bitset.c) that manages a set of bits and provides functions to find bits that are 0 (i.e. correspond to free disk blocks).

The block size for GOSFS is 4 KB; IDE sectors (a.k.a. disk blocks) are 512 bytes, so a block is 8 sectors.  Thus one bit in a bitset corresponds to a 4KB block. For example, a bitset that is 8192 bits (1024 bytes) will keep track of 8192 * 4KB = 32 MB of data.

Directory Structure

The filenode for a directory is represented by the GOSFS_DIRENTRY_ISDIRECTORY flag set in the GOSFS_Dir_Entry->flags field. The location of the block that holds the data for the directory will be stored in the first entry in the blocksList array of the directory's filenode (hence entries blocksList[1]..blocksList[GOSFS_NUM_BLOCK_PTRS-1] are unused).

Files

Mounting

The Mount system call allows you to associate a filesystem with a place in the file name hierarchy. The Mount call is implemented as part of the VFS code we supply (see Mount function in vfs.c); you will implement your Init_GOSFS function so that VFS's mount code will call your function GOSFS_Mount() in gosfs.c. Among other things, it must ensure that the filesystem being mounted is GOSFS. See new syscalls below for details.

New System Calls

You have to implement the sematics of the new system calls as described below. As you can see, the semantics is very similar to the UNIX file system.

Call	User Function	Return on success	Return on failure	Reasons for failure	Comment
SYS_MOUNT	Mount(char *dev, char *prefix, char *fstype)	0	-1	a filesystem already mounted under name illegal value for one of the parameters
SYS_OPEN	Open(char *path, int mode)	new file descriptor number	-1	name does not exist (if permissions don't include O_CREATE ) path to name does not exist (if permissions include O_CREATE ) attempt to open a directory, (should use OpenDirectory)	there's no create syscall, so setting O_CREATE will create the file. If the file exists, the call succeeds (return >= 0) but its data contents is not affected. Should NOT create directories; e.g. if you do Open("/d/d1/d2/d3/xFile", O_CREATE) and the leading path /d/d1/d2/d3 does not exist, the syscall fails, returning -1 You should use the Allocate_File in vfs.c to get a File when opening. The permissions values are flags and may be or'ed together in a call. For example: O_CREATE | O_READ O_READ | O_WRITE O_CREATE | O_READ | O_WRITE
SYS_OPENDIRECTORY	Open_Directory(char *path)	0	-1	path does not exist	Should NOT create directories, use CreateDirectory instead. You should use the Allocate_File in vfs.c to get a File when opening.
SYS_CLOSE	Close(int fd)	0	-1	fd not within range 0-(USER_MAX_FILES-1) fd is not an open file
SYS_DELETE	Delete(char *path)	0	-1	name does not exist name is a non-empty directory	if Delete(file) is called and file is still open in other threads or even in the thread that called Delete(), all the subsequent operations on that file (except Close()) should fail
SYS_READ	Read(int fd, char *buffer, int length)	number of bytes read	-1	fd not in range fd is not an open file fd was not open with O_READ flag fd is a directory	it's OK if return value < length, for instance reading close to end of file increase the filePos (the current position in the file) by the number of bytes read (if successful)
SYS_READENTRY	Read_Entry(int fd, VFS_Dir_Entry *entry)	0	-1	fd not in range fd is not an open file fd was not open with O_READ flag fd is not a directory	this reads a file out of a directory entry. increase the filePos, if successful VFS_Dir_Entry is defined in fileio.h
SYS_WRITE	Write(int fd, char *buffer, int length)	number of bytes written	-1	fd not in range fd is not an open file fd was not open with O_WRITE flag fd is a directory	increases filePos by the number of bytes written, if successful "Grow on write"- allocate blocks "on the fly" if past end of file "Grow minimum" - allocate only the needed blocks. A seek to a position past the end of the file should be allowed and a write will allocate only a single block rather than all blocks in between.
SYS_STAT	Stat(char *path, VFS_File_Stat *stat)	0	-1	path is not valid	VFS_File_Stat is defined in fileio.h
SYS_FSTAT	FStat(int fd, VFS_File_Stat *stat)	0	-1	fd not within 0-9 fd is not an open file	VFS_File_Stat is defined in fileio.h
SYS_SEEK	Seek(int fd, int offset)	0	-1	fd not within 0-9 fd is not an open file	offset is an absolute position; could be equal to fileSize or greater, then Write appends, see above
SYS_CREATEDIR	Create_Directory(char *path)	0	-1	name already exists, as file or directory regular file encountered on the path to name	Should NOT create directories recursively; e.g. CreateDirectory("/d/d1/d2/d3/d4"), will fail if /d/d1/d2/d3 does not exist already.
SYS_SYNC	Sync(void)	0	-1	None	Forces any outstanding operations to be written to disk.
SYS_FORMAT	Format(char *dev, char *fstype)	0	-1	illegal value for dev dev is in use, i.e. mounted	You only have to implement the format of the GOSFS filesystem.

Disk Layout

A guideline is provided above. The first block (0) is called the SUPERBLOCK, and it contains filesystem housekeeping data. Blocks >= 1 contain files and directories.