Transcript Kernel Modules - Northern Kentucky University

CSC 660: Advanced OS
Virtual Filesystem
CSC 660: Advanced Operating Systems
Slide #1
Topics
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6.
7.
Filesystems
Filenames and Pathnames
File Attributes
File Operations
Virtual Filesystem
VFS Objects
Processes and Files
CSC 660: Advanced Operating Systems
Slide #2
Why filesystems?
Access by
Physical Disk
Filesystem
Blocks
Bytes
Addressing Block #
Pathname
Security
ACLs
None
Reliability Corruption on crash Robust response to
system failures.
CSC 660: Advanced Operating Systems
Slide #3
Tree Structure
/
bin
boot
ls
tmp
usr
bin
grub
lib
var
X11R6
less
vmlinuz
menu.lst
bin
zip
xclock
CSC 660: Advanced Operating Systems
lib
xterm
Slide #4
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
Slide #5
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
Slide #6
File Operations
•
•
•
•
•
•
Create
Delete
Open
Close
Read
Write
•
•
•
•
•
CSC 660: Advanced Operating Systems
Append
Seek
Get Attributes
Set Attributes
Rename
Slide #7
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
Slide #8
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
Slide #9
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
Slide #10
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
Slide #11
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
Slide #12
Virtual Filesystem
CSC 660: Advanced Operating Systems
Slide #13
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
Slide #14
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
Slide #15
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
Slide #16
VFS Objects
CSC 660: Advanced Operating Systems
Slide #17
struct super_block
Type
struct list_head
Field
s_list
Description
Superblock linked list
dev_t
u_long
u_char
s_dev
Device identifier
s_blocksize Block size in bytes
s_dirt
Dirty (modified) flag
struct
s_op
super_operations *
struct semaphore s_lock
struct list_head
s_inodes
Superblock methods
struct list_head
struct list_head
Inodes waiting for write
List of file objects
s_io
s_files
CSC 660: Advanced Operating Systems
Superblock semaphore
List of all inodes
Slide #18
struct inode
Type
struct list_head
Field
i_list
Description
Inode linked list
struct list_head
inode_operations
u_long
i_dentry
*i_op
i_ino
List of dentries to this inode
Inode methods
Inode number
atomic_t
umode_t
u_long
uid_t,gid_t
i_count
i_mode
i_nlink
i_{uid,gid}
Reference count
ACL for file
Number of hard links
UID and GID of owner
loff_t
struct timespec
i_size
File size in bytes
i_[amc]time Last access, modify, change
CSC 660: Advanced Operating Systems
Slide #19
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
Slide #20
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
Slide #21
struct dentry
Type
atomic_t
Field
d_count
Description
Usage count
spinlock_t
struct inode
struct
dentry_operations
d_lock
*d_inode
*d_op
Lock for this dentry
Corresponding inode
Dentry methods
struct list_head
struct list_head
struct list_head
d_lru
d_child
d_subdirs
List of unused dentries.
Dir dentries of same parent.
Subdirectory dentries.
struct list_head
d_alias
Dentries w/ same inode.
CSC 660: Advanced Operating Systems
Slide #22
struct file
Type
struct list_head
Field
f_list
Description
Linked list of file objects
struct dentry
atomic_t
u_int
*f_dentry
f_count
f_flags
Associated dentry object
Usage count
Flags specified on open()
struct
file_operations *
mode_t
loff_t
f_op
File methods (open, read,
write, readdir, ioctl)
f_mode
f_pos
File access mode
File offset
u_int
int
f_{uid,gid} User’s UID and GID
f_error
Error code
CSC 660: Advanced Operating Systems
Slide #23
References
1.
2.
3.
4.
5.
6.
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
Slide #24