deny rm permission for non-owner of files

AndrewBS42 · 09-25-2007, 09:31 AM

We're running Red Hat Enterprise Linux 3 on some systems, and 4 on
others.

My home directory permissions are set up thus:

Linux$ ls -ld abs
drwxrwxrwx 26 abs abs 4096 Sep 25 08:04 abs
Linux$

I have a file in my home directory called someFile.txt.
someFile.txt is owned by root. The file has permissions that allow read by all, but write only by root.

Linux$ ls -l someFile*
-rw-r--r-- 1 root root 30 Sep 25 07:59 someFile.txt
Linux$

For purposes of this demonstration, this is me:

Linux$ whoami
abs
Linux$

If I do not have write permissions, I can not over-write the file:

Linux$ cat /dev/null >| someFile.txt
bash: someFile.txt: Permission denied
Linux$

That's good.

However, if I try to remove the file...

Linux$ rm someFile.txt
rm: remove write-protected file `someFile.txt'? y
Linux$ ls -l someFile*
ls: someFile*: No such file or directory
Linux$

I am able to remove the file, even though I don't own the file, and I
don't have write permissions on the file, either. True, I do have write
permissions on the parent directory that holds the file. Maybe that's
why Linux lets me remove it?

What I'd like is to be able to set up permissions or other mechanisms
so that anybody can create a file in a directory, but if you don't own
the file or don't have write permisson on the file, then you can't
delete the file. Is that possible?

PTrenholme · 09-25-2007, 10:29 AM

From man chmod

Code:

RESTRICTED DELETION FLAG OR STICKY BIT
       The restricted deletion flag or sticky bit is a single bit, whose interpretation depends
       on  the  file  type.   For  directories, it prevents unprivileged users from removing or
       renaming a file in the directory unless they own the file  or  the  directory;  this  is
       called  the  restricted deletion flag for the directory, and is commonly found on world-
       writable directories like /tmp.  For regular files on some older systems, the bit  saves
       the  program’s text image on the swap device so it will load more quickly when run; this
       is called the sticky bit.

BPS · 10-03-2007, 02:45 AM

some files are owned by system of linux... its protected by system and denied from deleting in its directory coz, the system are using that file... But technically, if the file is write protected, you can't remove the file...:-D

jschiwal · 10-03-2007, 02:53 AM

Deleting a file writes to the directory and not the file. Since you have write access to the directory, you can delete a root owned file.

Those permissions are horrible for a home directory. You could use tmp instead as a globally writable directory, or create one for that purpose. If you look at the permissions of the /tmp directory, you will see that it has the sticky bit set.

A directory created for a globally writable samba share will have the same permissions, for the same reason. The "Samba 3 by Example" book (available in the samba-doc package) has a sample simple share including the steps to create the directory and subdirectories, including the permissions.