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Old 11-08-2008, 05:58 PM   #1
raj_hcl1986@rediffma
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why a non root user can't change the permissions of /tmp ?


HI

I have a doubt, a non root user 'raj' has file permissions of /bin/chmod as 755, then why cant he change the permissions of /tmp (1777) with chmod ?

What he needs to change the same.
 
Old 11-08-2008, 06:06 PM   #2
druuna
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Hi,

/tmp needs to be accessable by all the users, that is why it needs to be owned by root (both user and group) and should have a 1777 mask (drwxrwxrwt).

This mode (1777/drwxrwxrwt) makes sure that any user can write to the /tmp (and /var/tmp) directories, but cannot remove another user's files from them. The latter is prohibited by the so-called “sticky bit,” the highest bit (1) in the 1777 bit mask.

Why would you want to change the permissions for /tmp anyway?

Hope this helps.
 
Old 11-08-2008, 06:15 PM   #3
raj_hcl1986@rediffma
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Hi I m trying to learn linux and on the infant stage. Actually I was applying chmod command. Got a query that if root can change the permissions why cant a non root user?
 
Old 11-08-2008, 06:15 PM   #4
jschiwal
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You can't change the permissions of a directory that you don't own. /tmp is a system directory owned by root. Only root should be changing the permissions of /tmp. The permissions you want (1777) are the permissions that /tmp should have.

It is the ownership on the target that prevent this and not the execute permissions of /bin/chmod. The denial comes from the kernel.
In other words, the chmod command tries to run the system command to change the permissions, and the kernel refuses and returns an error to the chmod command which in turn returns with an error when it exits.

It is the kernel which performs these types of operations. It checks the permissions and ownerships. It also checks extended permissions (see man getfacl) and extended attributes (see man lsattr) as well.

Also, bear in mind that the filesystem contains the ownership and permissions of files. A foreign filesystem such as fat32 doesn't use unix inodes, do you can't use chmod and chown.

Last edited by jschiwal; 11-08-2008 at 06:20 PM.
 
Old 11-09-2008, 04:58 AM   #5
raj_hcl1986@rediffma
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Got it. Thanks for the kind support.
 
  


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