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Old 01-17-2007, 11:56 AM   #1
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Unhappy permissions security question

Does linux/unix have any way to distinct between write and alter permissions in it's filesystem?

I've read a bit about Standard Unix permissions and Unixes ACLs etc.

But no article clearly answers to my question. ( That or i can't undestand it right :s )
Can i permit a user / group / wtv to write new files in a folder, but never to alter already saved files in that folder?

If i put 700 in folder and 500 in file rm command only asks for confirmartion while deleting the file, but if i click 'y' or give -f it still deletes.

The thing is i wanna give apache (nobody/others) write access for new files in a folder (uploads),
but don't want other apache cgi processes existent in my server to be able to delete those files...

SUexec provides a workaround, but it is still a bit "fake" and has it's own problems.
Besides with cpanel i'd probably had to install fastCGI too and recompile php and it hurts :s

Any ideas or workarounds?

Last edited by TruthSeeker; 01-17-2007 at 11:59 AM.
Old 01-17-2007, 12:18 PM   #2
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If you have write permission on a directory, then you can add new files. For the existing files, just don't give out write permission. Suppose you have a directory "stuff" and you want the user "fred" to be able to put in files, but not change the existing ones.

Create a group "stuff" and make it the owner of the directory "stuff"
Add fred to the group stuff
set permissions so that group members can read and write
chmod 775 stuff (Gives read/write/execute to root and group, and read/execute to others)
In the stuff directory, set all permissions to read-only for group and others
Old 01-17-2007, 12:22 PM   #3
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You can use the sticky bit on the folder to protect other users files from being deleted. This is because the directory is, as far as the kernel is concerned, a file and write access gives a user the ability to delete any file (writing to the directory). The sticky bit on the directory will prevent this possibility. The file itself will need to be readonly for the other users to protect the file itself from overwriting.

ACLs in linux are useful in giving only certain users or groups permissions without needing to give others write permissions.

Last edited by jschiwal; 01-17-2007 at 12:23 PM.
Old 01-17-2007, 12:25 PM   #4
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oops, other people type/think faster


Can i permit a user / group / wtv to write new files in a folder, but never to alter already saved files in that folder?
I'm not sure if that's what you want, but it seems to me that
you could give write permission to the parent directory where all the files are (so that users could create new files), however
remove the file permissions for the files in the directory (so that users could not modify existing files +

1. [root] chmod a+wx name_of_the_directory
2. [root] chmod u+wx *
3. [root] chmod go-wx *

points 2 and 3 can be done with one command: e.g chmod 644 *

I'm not sure if it would work like that, you can wait for the
feedback of more experienced linux users



Last edited by sycamorex; 01-17-2007 at 12:27 PM.


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