Changing folder access

axolinx · 01-10-2023, 03:14 AM

Dear guys:

My scenario: Linux Mint 20.3
Two users (alex and matt)
A secondary hard drive mounted under /media/alex/2TB (mounted by alex)

I am trying to give access to a specific folder for matt. so this folder is
here: /media/alex/2TB/matt

I changed the folder ownership to matt:

Code:

 chown -R matt:matt /media/alex/2TB/matt

Now i can see:

Code:

alex@stationX:/media/alex/2TB$ ll
total 52
drwx------  10 alex  alex   4096 Jan  9 10:36  ./
drwxr-x---+  3 root root  4096 Dec 12 05:35  ../
drwxrwxr-x  27 alex  alex   4096 Aug  2 23:58 'alex data'/
drwx------   2 root root 16384 Aug  2 18:06  lost+found/
drwxrwxr-x   2 matt matt  4096 Jan 10 02:19  matt/

But then i login with the user matt, and i get permission denied to access that folder:

Code:

matt@stationX:~$ cd /media/alex/2TB/matt
-bash: cd: /media/alex/2TB/matt: Permission denied

I know i can try changing the folder permissions like:
sudo chmod -R ugo+rw /media/alex/2TB/matt
But i would like to understand what i am doing wrong and why changing the owner to that folder still does not allow access to it.

Thank you!

lvm_ · 01-10-2023, 03:53 AM

To get to /media/alex/2TB/matt user matt must have 'x' permission not to just this folder but to all folders in this path.

fatmac · 01-10-2023, 04:15 AM

As above, the 'x' (executable) permission allows you to change through the directory path.

https://linuxhandbook.com/linux-file-permissions/

axolinx · 01-10-2023, 04:26 AM

Quote:

Originally Posted by fatmac

As above, the 'x' (executable) permission allows you to change through the directory path.

https://linuxhandbook.com/linux-file-permissions/

The x was already set for UGW. I do not understand.
Now i actually changed the folder to 777 and still do not have permission for matt

Code:

drwxrwxrwx   2 matt matt  4096 Jan 10 02:19  matt/

I must add a bit more info, it may matter:
This is an encrypted drive (LUKS), so alex is the one mounting it and providing the passphrase for it.
matt needs access to that folder to pull a 40GB file that i can not just move to his home folder because the main disk is low on disk space, matt needs to have a way to access that folder.

MadeInGermany · 01-10-2023, 05:01 AM

Not that folder!
The access is denied by the top folder, marked in red:

Code:

alex@stationX:/media/alex/2TB$ ll
total 52
drwx------  10 alex  alex   4096 Jan  9 10:36  ./
drwxr-x---+  3 root root  4096 Dec 12 05:35  ../
drwxrwxr-x  27 alex  alex   4096 Aug  2 23:58 'alex data'/
drwx------   2 root root 16384 Aug  2 18:06  lost+found/
drwxrwxr-x   2 matt matt  4096 Jan 10 02:19  matt/

The suggestion was to add x (access) permission to it

Code:

chmod +x /media/alex/2TB

And perhaps to the next higher directories

Code:

chmod +x /media/alex
chmod +x /media

x without r means access without list/browse - you must know the directory names inside because you cannot read them.

The +x will allow ALL users access though. So do not have worldwide open directories and files in the tree below!

A more restrictive alternative is to just allow access for group alex and add user matt to group alex:

Code:

chmod 750 /media/alex/2TB
usermod -a -G alex matt

The 5 is g=rx and gives the group even list/browse access. The 0 denies access for all others.

axolinx · 01-10-2023, 05:39 AM

@MadeInGermany

Thank you.

Adding matt to the alex group alone did not help,
but then i set X for each folder in the path and now matt can reach the folder.
I think I knew folders need to have x permission in order to be browsed, need to pay more attention.

So when giving access to a local owned folder to another user, each step in the path needs to have the X for "others" right?
Is there a way to do this in one shot issuing the command to the specific folder, or you need to chmod each folder individually like i did?
Thanks again, my problem is solved.

hazel · 01-10-2023, 05:52 AM

If it's a folder tree, you can use chmod -R on the root of the tree to change all the modes recursively.

MadeInGermany · 01-10-2023, 07:39 AM

chmod -R is dangerous if you do not know about all of its consequences.
E.g. -x sets x bits on files and directories, this is hardly ever wanted; in Linux there is -X for only directories, but nothing for only files.

There is an x-bit for each of user,group,others.
If you are the owner then the user permissions rule. If you are not the owner but belong to the group then the group permissions rule. If you are neither owner nor group member then the permissions for others apply.

sundialsvcs · 01-10-2023, 11:03 AM

FYI: Group permissions are most commonly used for higher-level directories, because a user can belong to more than one group. Then, when you get into the target parent-folder, you use User permissions to control access.

Also for what it's worth, most Linux (and, other ...) filesystems also support ACLs = Access Control Lists, which are a parallel but much more versatile and fine-grained mechanism for controlling permissions and access. Unfortunately, they are inconsistent in their features and implementations.

MadeInGermany · 01-10-2023, 01:17 PM

Effective implementation is standardized in POSIX ACLs.
But support for copying/archiving/backup is pretty much individual.
Further, ACLs are complicated. An overuse of ACLs can make it difficult for humans to determine who has access.
I have decided to use ACLs as little as possible.
The most frequent case, a shared group-writable folder tree, I have documented here.