Files and Directories Filesystems from a user s perspective

Transcription

1 Files and Directories Filesystems from a user s perspective Unix Filesystems Seminar Alexander Holupirek Database and Information Systems Group Department of Computer & Information Science University of Konstanz 20. November 2007 Alexander Holupirek (U KN) Files & Directories 20. November / 41

2 Introduction Introduction Last session: I/O for regular files. opening, reading, writing a file. Today: Additional features of the filesystem. Properties of a file. The stat functions. Members of the stat structure. Next: The structure of a Unix filesystem. Symbolic links. Functions that operate on directories. The standard I/O library no seminar Alexander Holupirek (U KN) Files & Directories 20. November / 41

3 Synopsis of the stat functions The functions stat(2), fstat(2), and lstat(2) # include <sys / types.h> # include <sys / stat.h> int stat ( const char * path, struct stat * sb ); int fstat ( int fd, struct stat * sb ); int lstat ( const char * path, struct stat * sb ); Given a path, the stat(2) function returns a structure of information about the named file. fstat(2) works with a file descriptor, instead of a pathname. lstat(2) returns information about the symbolic link, not the referrenced file. Alexander Holupirek (U KN) Files & Directories 20. November / 41

4 The stat struct The stat family The structure struct stat { mode_t st_mode ; /* inode s mode */ uid_t st_uid ; /* user ID of owner */ gid_t st_gid ; /* group ID of owner */ off_t st_size ; /* file size, in bytes */ int64_t st_blocks ; /* blocks allocated for file */ u_int32_t st_blksize ;/* optimal file sys I/ O ops blocksize */ dev_t st_dev ; /* device inode resides on */ ino_t st_ino ; /* inode s number */ nlink_t st_nlink ; /* number of hard links to the file */ dev_t st_rdev ; /* device type, for special file inode */ struct timespec st_atimespec ; /* time of last access */ struct timespec st_mtimespec ; /* time of last data modification */ struct timespec st_ctimespec ; /* time of last file status change */ u_int32_t st_flags ; /* user defined flags for file */ u_int32_t st_gen ; /* file generation number */ }; This is where ls -l gets its information from. Mostly primitive system data types. Alexander Holupirek (U KN) Files & Directories 20. November / 41

5 File types The stat family File types - st mode (15-12) struct stat { mode_t st_mode ; /* inode s mode */ }; The type of a file is encoded in the st mode member of the stat structure. Regular file Text and binary data. Directory file Names of files and pointers to information on these files. Character special file Certain type of devices. Block special file Typically disk devices. FIFO Used for IPC (aka pipes). Socket Network communication. Symbolic link Pointer to another file. Alexander Holupirek (U KN) Files & Directories 20. November / 41

6 File type definition The stat family File types - st mode (15-12) struct stat { mode_t st_mode ; /* inode s mode */ }; sys / types. h : typedef mode_t mode_t ; sys / _types. h: typedef uint32_t mode_t ; /* sys / stat.h ( OpenBSD 4.2) */ # define S_IFMT /* type of file mask */ # define S_IFIFO /* named pipe ( fifo ) */ # define S_IFCHR /* character special */ # define S_IFDIR /* directory */ # define S_IFBLK /* block special */ # define S_IFREG /* regular */ # define S_IFLNK /* symbolic link */ # define S_IFSOCK /* socket */ Alexander Holupirek (U KN) Files & Directories 20. November / 41

7 File type determination The stat family File types - st mode (15-12) Determine file types with macros (pass st mode as argument). Macro Type of file S ISREG(m) Regular file S ISDIR(m) Directory file S ISCHR(m) Character special file S ISBLK(m) Block special file S ISFIFO(m) FIFO S ISLNK(m) Socket S ISSOCK(m) Symbolic link Tabelle: File type macros in <sys/stat.h> Alexander Holupirek (U KN) Files & Directories 20. November / 41

8 File types and macros The stat family File types - st mode (15-12) File types: /* sys / stat.h ( OpenBSD 4.2) */ # define S_IFMT /* type of file mask */ # define S_IFIFO /* named pipe ( fifo ) */ # define S_IFCHR /* character special */ # define S_IFDIR /* directory */ # define S_IFBLK /* block special */ # define S_IFREG /* regular */ # define S_IFLNK /* symbolic link */ # define S_IFSOCK /* socket */ File type macros: # define S_ISFIFO ( m) (( m & ) == ) /* fifo */ # define S_ISCHR ( m) (( m & ) == ) /* char special */ # define S_ISDIR ( m) (( m & ) == ) /* directory */ # define S_ISBLK ( m) (( m & ) == ) /* block special */ # define S_ISREG ( m) (( m & ) == ) /* regular file */ # define S_ISLNK ( m) (( m & ) == ) /* symbolic link */ # define S_ISSOCK ( m) (( m & ) == ) /* socket */ Alexander Holupirek (U KN) Files & Directories 20. November / 41

9 Used bits in st mode so far The stat family File types - st mode (15-12) What we have so far: # define S_IFIFO /* named pipe ( fifo ) */ # define S_IFCHR /* character special */ # define S_IFDIR /* directory */ # define S_IFBLK /* block special */ # define S_IFREG /* regular */ # define S_IFLNK /* symbolic link */ # define S_IFSOCK /* socket */ # define S_IFMT /* type of file mask */ --x xxx /* used bits st_mode */ m ode Alexander Holupirek (U KN) Files & Directories 20. November / 41

10 File permissions The stat family File permissions - st mode (11-0) /* sys / stat.h ( OpenBSD 4.2) */ # define S_IRWXU /* RWX mask for owner */ # define S_IRUSR /* R for owner */ # define S_IWUSR /* W for owner */ # define S_IXUSR /* X for owner */ # define S_IRWXG /* RWX mask for group */ # define S_IRGRP /* R for group */ # define S_IWGRP /* W for group */ # define S_IXGRP /* X for group */ # define S_IRWXO /* RWX mask for other */ # define S_IROTH /* R for other */ # define S_IWOTH /* W for other */ # define S_IXOTH /* X for other */ # define S_IREAD S_IRUSR # define S_IWRITE S_IWUSR # define S_IEXEC S_IXUSR Alexander Holupirek (U KN) Files & Directories 20. November / 41

11 Used bits in st mode so far The stat family File permissions - st mode (11-0) Missing bits 8-0: # define S_IRWXO /* RWX mask for other */ # define S_IRWXG /* RWX mask for group */ # define S_IRWXU /* RWX mask for owner */ # define S_IFMT /* type of file mask */ --x xxx --- xxx xxx xxx /* used bits st_mode */ m ode usr grp oth Alexander Holupirek (U KN) Files & Directories 20. November / 41

12 File ownership The stat family File ownership, st mode, st uid, st gid struct stat { mode_t st_mode ; /* inode s mode */ uid_t st_uid ; /* user ID of owner */ gid_t st_gid ; /* group ID of owner */ }; Every file has an owner and a group owner. The owner is stored in member st uid, the group in st gid. Alexander Holupirek (U KN) Files & Directories 20. November / 41

13 Process IDs The stat family Process IDs and their relation to stat Every process has some associated IDs: real user ID who we really are real group ID effective user ID effective group ID used for file access permission checks supplementary group IDs saved set-user-id saved by exec functions saved set-group-id real [user group] ID taken from password file on login. eff. [user group] ID depends on set-[user group]-id bits in st mode. supplementary group ID taken from /etc/group on login (max 16 grps). saved set-[user group]-id copies of eff. [user group] IDs. Alexander Holupirek (U KN) Files & Directories 20. November / 41

14 The set-id bits The stat family Process IDs and their relation to stat The missing bits 11 and 10: # define S_ISGID /* set group id on exec */ # define S_ISUID /* set user id on exec */ --x xxx xx - xxx xxx xxx /* used bits st_mode */ m ode ss - usr grp oth When we execute a program file the effective user ID of the process is usually the real user ID and the effective group ID is usually the real group ID. But, there are two special flags in the file s mode word (st mode). Set-user-ID (S ISUID) and set-group-id (S ISGID).... when this file is executed, set the effective user ID of the process to be the owner of the file (st uid). Alexander Holupirek (U KN) Files & Directories 20. November / 41

15 Example for set-user-id The stat family Process IDs and their relation to stat Example Consider passwd(1) to modify a user s password $ ls -l / usr / bin / passwd -r-sr -xr -x 1 root bin Aug 28 18:12 / usr / bin / passwd Owner is root and set-user-id bit is set. When the program file is running as a process, it has superuser privileges. This is independent of the real user ID of the process that executes the file. This is required to wirte to the password file/db. Alexander Holupirek (U KN) Files & Directories 20. November / 41

16 File access tests File access tests performed by the kernel Input: effuid - effective user id of the process Input: st uid - user id of the file Output: access ok, access deny begin if effuid equals 0 then return ok else if effuid equals st uid then if apt user permission then return ok else return deny; else if effuid equals st gid then if apt group permission then return ok else return deny; else if apt other permission then return ok else return deny; end end Alexander Holupirek (U KN) Files & Directories 20. November / 41

17 File access permissions quiz File access permissions Do any file types have permissions? Alexander Holupirek (U KN) Files & Directories 20. November / 41

18 File access permissions quiz File access permissions Do any file types have permissions? Yes. Any file types have permission (not only regular files). Alexander Holupirek (U KN) Files & Directories 20. November / 41

19 File access permissions quiz File access permissions Do any file types have permissions? Yes. Any file types have permission (not only regular files). What does execute permission for a directory grant? Alexander Holupirek (U KN) Files & Directories 20. November / 41

20 File access permissions quiz File access permissions Do any file types have permissions? Yes. Any file types have permission (not only regular files). What does execute permission for a directory grant? Permission to pass through the directory when it is a component of a pathname that we are trying to access (i.e., search the directory looking for a specific filename). Alexander Holupirek (U KN) Files & Directories 20. November / 41

21 File access permissions quiz File access permissions Do any file types have permissions? Yes. Any file types have permission (not only regular files). What does execute permission for a directory grant? Permission to pass through the directory when it is a component of a pathname that we are trying to access (i.e., search the directory looking for a specific filename). What is the search bit? Where does its name come from? Alexander Holupirek (U KN) Files & Directories 20. November / 41

22 File access permissions quiz File access permissions Do any file types have permissions? Yes. Any file types have permission (not only regular files). What does execute permission for a directory grant? Permission to pass through the directory when it is a component of a pathname that we are trying to access (i.e., search the directory looking for a specific filename). What is the search bit? Where does its name come from? Whenever we want to open any file by name we must have execute permission in each directory mentioned in the name (including the current directory if it is implied). This is why the execute permission bit for the directory is often called the search bit. Alexander Holupirek (U KN) Files & Directories 20. November / 41

23 File access permissions quiz File access permissions What does read permission for a directory grant? Alexander Holupirek (U KN) Files & Directories 20. November / 41

24 File access permissions quiz File access permissions What does read permission for a directory grant? Obtain a listing of all filenames in the directory. Alexander Holupirek (U KN) Files & Directories 20. November / 41

25 File access permissions quiz File access permissions What does read permission for a directory grant? Obtain a listing of all filenames in the directory. To create a new file what permissions do we need in the directory? Alexander Holupirek (U KN) Files & Directories 20. November / 41

26 File access permissions quiz File access permissions What does read permission for a directory grant? Obtain a listing of all filenames in the directory. To create a new file what permissions do we need in the directory? We can not create a new file in a directory unless we have write and execute permission in the directory. Alexander Holupirek (U KN) Files & Directories 20. November / 41

27 File access permissions quiz File access permissions What does read permission for a directory grant? Obtain a listing of all filenames in the directory. To create a new file what permissions do we need in the directory? We can not create a new file in a directory unless we have write and execute permission in the directory. To delete an existing file what permissions do we need in the directory? Alexander Holupirek (U KN) Files & Directories 20. November / 41

28 File access permissions quiz File access permissions What does read permission for a directory grant? Obtain a listing of all filenames in the directory. To create a new file what permissions do we need in the directory? We can not create a new file in a directory unless we have write and execute permission in the directory. To delete an existing file what permissions do we need in the directory? We (also) need write and execute permission in the directory containing the file. Alexander Holupirek (U KN) Files & Directories 20. November / 41

29 File access permissions quiz File access permissions What does read permission for a directory grant? Obtain a listing of all filenames in the directory. To create a new file what permissions do we need in the directory? We can not create a new file in a directory unless we have write and execute permission in the directory. To delete an existing file what permissions do we need in the directory? We (also) need write and execute permission in the directory containing the file. Do we need to have read and write permissions on the file to delete it? Alexander Holupirek (U KN) Files & Directories 20. November / 41

30 File access permissions quiz File access permissions What does read permission for a directory grant? Obtain a listing of all filenames in the directory. To create a new file what permissions do we need in the directory? We can not create a new file in a directory unless we have write and execute permission in the directory. To delete an existing file what permissions do we need in the directory? We (also) need write and execute permission in the directory containing the file. Do we need to have read and write permissions on the file to delete it? No. Alexander Holupirek (U KN) Files & Directories 20. November / 41

31 The history of the sticky bit File access permissions The missing bits 9: # define S_ISTXT /* sticky bit */ --x xxx xxx xxx xxx xxx /* used bits st_mode */ m ode sst usr grp oth The S ISTXT aka S VTX is known as sticky bit. On earlier versions of Unix it had an effect on executable programs. If set, a copy of the program s text was saved in the swap area on process termination. This caused the program to load into memory faster the next time. Swap area was handled as a contiguous file, compared to random data blocks in fs. S ISVTX as mnemonic for saved-text bit. Obsoleted by virtual memory and faster filesystems. Alexander Holupirek (U KN) Files & Directories 20. November / 41

32 The sticky bit today - S ISTXT File access permissions Today it has an effect on directories. If set for a directory, a file in the directory can be removed or renamed only if the user has write permission for the directory, and either owns the file, owns the directory, or is the superuser /tmp is a good candidate for the sticky bit. Any user can typically create files there. drwxrwxrwt 7 root root 4.0K :46 tmp But users should not be able to delete or rename files owned by others. Alexander Holupirek (U KN) Files & Directories 20. November / 41

33 File access permissions Functions related to st mode, st uid, st gid access(2) - check access permissions of a file or pathname umask(2) - set file creation mode mask chmod, fchmod(2) - change mode of file chown, fchown, lchown(2) - change owner/group of a file or link Alexander Holupirek (U KN) Files & Directories 20. November / 41

34 Lecture Material The tutorials are based on the following material W. Richard Stevens. Advanced Programming in the UNIX Environment. ISBN , 1999, 19th Printing. Addison-Wesley Professional Computing Series. The OpenBSD Developers. OpenBSD Manual Pages (Release 4.1). ISBN Marshall K. McKusick, Keith Bostic, Michael J. Karels, John S. Quarterman. The Design and Implementation of the 4.4BSD Operating System. ISBN , 1996, Addison-Wesley. Alexander Holupirek (U KN) Files & Directories 20. November / 41

35 Seminar Talks Lecture Material Database Driven Filesystems Patrice Matthias Brendamour, Markus Majer Survey, WinFS, BeOS and Spotlight FS Stackable FS Bastian Lemke: Stackable FS from the beginning Jochen Oekonomopulos: FiST on various platforms, Stackable FS stubs in *BSD Alexander Holupirek (U KN) Files & Directories 20. November / 41