CSC 660: Advanced OS

1. CSC 660: Advanced OS Virtual Filesystem CSC 660: Advanced Operating Systems

2. Topics • Filesystems • Filenames and Pathnames • File Attributes • File Operations • Virtual Filesystem • VFS Objects • Processes and Files CSC 660: Advanced Operating Systems

3. Why filesystems? CSC 660: Advanced Operating Systems

4. Tree Structure / bin boot tmp usr var bin lib X11R6 ls grub less vmlinuz bin lib zip menu.lst xclock xterm CSC 660: Advanced Operating Systems

5. Filenames and Pathnames • Filenames • Human-readable identifier for data. • File suffixes have special meanings in Windows. • Directories • Store lists of filename/location mappings. • Pathnames • Identify file in directory hierarchy. • Ex: /usr/X11R6/bin/xclock CSC 660: Advanced Operating Systems

6. File Attributes • Filename and Data • Location • File Type • Regular files • Directories • Device (block + char) files • IPC files • Ownership • Access Control List • Locking • Size • Timestamps CSC 660: Advanced Operating Systems

7. Create Delete Open Close Read Write Append Seek Get Attributes Set Attributes Rename File Operations CSC 660: Advanced Operating Systems

8. File Descriptors User process reference to a file. A non-negative integer unique to process. Returned by open or creat system calls. Used as argument to other file system calls. View with: ls –l /proc/self/fd Common file descriptors: 0 STDIN 1 STDOUT 2 STDERR CSC 660: Advanced Operating Systems

9. Process File Context • Root directory • Usually /, but can change for security. • Current working directory • Default location for file commands. • Relative pathnames are relative to CWD. • Open file table • Maps file descriptors to kernel file objects. • files_struct member of task_struct CSC 660: Advanced Operating Systems

10. Open File Table struct files_struct { atomic_t count; spinlock_t file_lock; int max_fds; int max_fdset; int next_fd; struct file ** fd; fd_set *close_on_exec; fd_set *open_fds; fd_set close_on_exec_init; fd_set open_fds_init; struct file * fd_array[NR_OPEN_DEFAULT]; }; CSC 660: Advanced Operating Systems

11. Opening and Closing #include <fcntl.h> #include <unistd.h> int open(char *path, int oflag); /* Common flags: O_RDONLY, O_WRONLY, O_CREAT Returns FD on success, -1 on failure */ int close(int filedes); /* Returns 0 on success, -1 on failure */ CSC 660: Advanced Operating Systems

12. Reading and Writing #include <unistd.h> ssize_t read(int fd, void *buf, size_t nbytes); /* Returns # bytes read, 0 if EOF, -1 error */ ssize_t write(int fd, void *buf, size_t nbytes); /* Returns # bytes written, -1 on error */ off_t lseek(int fd, off_t offset, int whence) /* Whence: SEEK_SET (f/ 0) or SEEK_CUR (f/ current) Returns new file offset, -1 on error */ CSC 660: Advanced Operating Systems

13. Virtual Filesystem CSC 660: Advanced Operating Systems

14. Classes of Filesystems • Disk-based filesystems • Linux (ext2, ext3, reiserfs) • UNIX (ufs, minix, JFS, XFS) • MS (FAT, VFAT, NTFS) and other proprietary • ISO9660 CD-ROM, UDF DVD • Network filesystems • NFS, AFS, CIFS • Special filesystems • Procfs, sysfs CSC 660: Advanced Operating Systems

15. VFS Objects • superblock • Represents a mounted filesystem. • inode • Represents a specific file. • dentry • Represents a directory entry, a single path comp. • file • Represents an open file associated w/ a process. CSC 660: Advanced Operating Systems

16. VFS Objects • Directories are files • Directories are handled by file objects. • dentry objects are path components, not dirs. • Each object contains an operations object • Define methods kernel invokes on object. • Defined as struct of function pointers. • What if a fs doesn’t have a type of object? • Objects of that type made on the fly for VFS. CSC 660: Advanced Operating Systems

17. VFS Objects CSC 660: Advanced Operating Systems

18. struct super_block CSC 660: Advanced Operating Systems

19. struct inode CSC 660: Advanced Operating Systems

20. Inode Operations int create(struct inode *dir, struct dentry *dentry, int mode) struct dentry *lookup(struct inode *dir, struct dentry *dentry) int link(struct dentry *old, struct inode *dir, struct dentry *dentry) int unlink(struct inode *dir, struct dentry *dentry) int symlink(struct inode *dir, struct dentry *dentry, const char *symname) int mkdir(struct inode *dir, struct dentry *dentry, int mode) int rmdir(struct inode *dir, struct dentry *dentry) int rename(struct inode *old_dir, struct dentry *old, struct inode *new_dir, struct dentry *new) int readlink(struct dentry *dentry, char *buffer, int buflen) CSC 660: Advanced Operating Systems

21. Dentry Objects • Path components • Ex: /bin/vi has 3 dentries: /, etc, and vi • Not an on-disk data structure • Constructed on fly from pathname string. • Cached by the kernel to avoid re-construction. • States • Used: corresponds to valid inode in use. • Unused: valid inode not currently used (cached). • Negative: invalid path, no corresponding inode CSC 660: Advanced Operating Systems

22. struct dentry CSC 660: Advanced Operating Systems

23. File Objects In-memory representation of open files. Created in response to open() call. Destroyed in response to close() call. Multiple file objects can exist for same file. Different processes can open same file. File object records process-specific status info (seek point, access mode.) CSC 660: Advanced Operating Systems

24. struct file CSC 660: Advanced Operating Systems

25. References • Daniel P. Bovet and Marco Cesati, Understanding the Linux Kernel, 3rd edition, O’Reilly, 2005. • Robert Love, Linux Kernel Development, 2nd edition, Prentice-Hall, 2005. • Claudia Rodriguez et al, The Linux Kernel Primer, Prentice-Hall, 2005. • Peter Salzman et. al., Linux Kernel Module Programming Guide, version 2.6.1, 2005. • Avi Silberchatz et. al., Operating System Concepts, 7th edition, 2004. • Andrew S. Tanenbaum, Modern Operating Systems, 3rd edition, Prentice-Hall, 2005. CSC 660: Advanced Operating Systems