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Full Discussion: Unix File Permissions
Top Forums UNIX for Beginners Questions & Answers Answers to Frequently Asked Questions Tips and Tutorials Unix File Permissions Post 73757 by Perderabo on Friday 3rd of June 2005 08:40:19 PM
Old 06-03-2005
Enforcement Mode File Locking/Manditory File Locking

We aren't finished with that Set Gid bit yet... Unix has a concept of file locking. File locking is beyond the scope of this thread. But you need to know that file locking comes in two flavors: advisory and manditory. Which flavor applies to a particular file depending on the permission settings. If the group execute bit is off but the setgid bit is on, any file locks on that file are manditory.

Useless Bit Combination?

Every reference that I have seen says that setgid on / group execute off is a otherwise useless combination. Even Richard Stevens (in Advanced Programming in the Unix Environment) says "Since the set-group-ID bit makes no sense when the group-execute bit is off, the designers of SVR3 chose this way to specify that the locking for a file is to be maditory locking and not advisory locking."

Well consider this case: Fred runs the Human Resources department. Fred and his group often need to lookup the vacation days used for employees. Fred decides to write a program so employees can lookup their own vacation days used. For security, Fred makes this program do a lot of logging. Fred decides that he doesn't want his group to use this program. They have other tools that won't clutter his log. So Fred does:
chown fred:hr vdays
chmod 2701 vdays
Now the vdays program cannot be run by members of hr (except fred). But it can be run by everyone else. And it will assume the gid of hr when it does run. I have written a test program, set it up like this, and have run it on both Solaris and HP-UX. It works.

Effect on ls output

While this bit combination may be useful is some limited cases, for better or worse, it will have two effects. The vdays program does work, but if a lock is attempted on the file, it will be manditory. As a practical matter, this would impact only an occasional program like a debugger. But ls may treat this bit combination differently. I have seen both of these...
Code:
chown fred:hr vdays
chmod 2701 vdays
-rwx--S--x   1 fred     hr          9938 Jul 16  2004 vdays
-rwx--l--x   1 fred     hr          9938 Jul 16  2004 vdays

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CHMOD(1)						      General Commands Manual							  CHMOD(1)

NAME
chmod - change access mode for files SYNOPSIS
chmod [-R] mode file ... OPTIONS
-R Change hierarchies recursively EXAMPLES
chmod 755 file # Owner: rwx Group: r-x Others: r-x chmod +x file1 file2 # Make file1 and file2 executable chmod a-w file # Make file read only chmod u+s file # Turn on SETUID for file chmod -R o+w dir # Allow writing for all files in dir DESCRIPTION
The given mode is applied to each file in the file list. If the -R flag is present, the files in a directory will be changed as well. The mode can be either absolute or symbolic. Absolute modes are given as an octal number that represents the new file mode. The mode bits are defined as follows: 4000 Set effective user id on execution to file's owner id 2000 Set effective group id on execution to file's group id 0400 file is readable by the owner of the file 0200 writeable by owner 0100 executable by owner 0070 same as above, for other users in the same group 0007 same as above, for all other users Symbolic modes modify the current file mode in a specified way. The form is: [who] op permissions { op permissions ...} {, [who] op ... } The possibilities for who are u, g, o, and a, standing for user, group, other and all, respectively. If who is omitted, a is assumed, but the current umask is used. The op can be +, -, or =; + turns on the given permissions, - turns them off; = sets the permissions exclu- sively for the given who. For example g=x sets the group permissions to --x. The possible permissions are r, w, x; which stand for read, write, and execute; s turns on the set effective user/group id bits. s only makes sense with u and g; o+s is harmless. SEE ALSO
ls(1), chmod(2). CHMOD(1)
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