Hexbear

3 points

2 months ago

I decided to simply create directories within /mnt, chmod 000 them and use them as fixed mountpoints;
for manual temporary mounts I have /mnt/a, /mnt/b, … /mnt/f, but I never needed to use more than two of them at once.

While this setup doesn’t really respect the filesystem hierarchy, I wouldn’t have used /mnt at all if I were constrained by its standard purpose since having one available manual mountpoint seems pretty limiting to me.
Then again, I have 3 physical drives with ~ 10 partitions, plus one removable drive with its own dedicated mountpoint…

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gpstarman@lemmy.todayOP

2 points

2 months ago

chmod 000

What does this do? I’m a Meganoob.

Fixed mountpoints

having one available manual mountpoint

you mean the whole /mnt is meant to single mount point?

Sorry for all the questions.

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2 points

2 months ago

Adding to what the other comment explained:

I use chown 000 so that regular users fail to access a directory when no filesystem is mounted on it; in practice it never happens, because “regular users” = { me }, but I like being pedantic.

As for /mnt, it is supposed to be a single temp. mountpoint, but I use it as the parent directory of multiple mountpoints some of which are just for temporary use.

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gpstarman@lemmy.todayOP

2 points

2 months ago

I use chown 000 so that regular users fail to access a directory when no filesystem is mounted on it

My dummy brain can’t understand it man.

Isn’t someone can’t access a directory when no filesystem is mounted on it the default behaviour?

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2 points

2 months ago

No, directories without anything mounted on them are normal directories - which checks out, since you can mount anything anywhere; unlike Windows volume letters, which only exist when volumes are mounted or detected by the OS.

When you mount a filesystem onto a directory, the OS “replaces” its contents AND permissions with that of the filesystem’s root.

Here’s an example with my setup (hopefully you’re somewhat familiar with Bash and the output of ls -l).

Imagine some random filesystem in /dev/sda1 owned by “user” which only contains a file named “/Hello World.txt”:

$ # List permissions of files in /mnt:
$ # note that none of the directories have read, write nor execute permissions
$ ls -la /mnt
drwxr-xr-x   1 root root          168 May 31 23:13 .
drwxr-xr-x   1 root root          128 May 31 23:14 ..
d---------   1 root root            0 Aug  1  2020 a/
d---------   1 root root            0 Feb 11  2022 b/
d---------   1 root root            0 Aug 11  2021 vdisks/

$ # No read permission on a directory => directory entries cannot be listed
$ ls /mnt/a
cannot open directory '/mnt/a': Permission denied

$ sudo mount /dev/sda1 /mnt/a

$ # List again the permissions in /mnt: the root of /dev/sda1
$ # has rwxr-xr-x (or 755) permissions, which override the 000 of /mnt/a ...
$ ls -la /mnt
drwxr-xr-x   1 root root          168 May 31 23:13 .
drwxr-xr-x   1 root root          128 May 31 23:14 ..
drwxr-xr-x   1 root root            0 Aug  1  2020 a/
d---------   1 root root            0 Feb 11  2022 b/
d---------   1 root root            0 Aug 11  2021 vdisks/

$ # ... and its contents can be accessed by the mounted filesystem's owner:
$ ls -la /mnt/a
drwxr-xr-x   1 user user          168 May 31 23:13 .
drwxr-xr-x   1 root root          168 May 31 23:13 ..
-rw-r--r-- 1 user user   0 Jul  4 22:13 'Hello World.txt'

$ find /mnt
/mnt
/mnt/a
/mnt/a/Hello World.txt
find: ‘/mnt/b Permission denied
find: ‘/mnt/vdisks’: Permission denied

Please note that me setting permissions is just extreme pedantry, it’s not necessary at all and barely changes anything and if you’re still getting familiar with how the Linux VFS and its permissions work you can just ignore all of this.

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gpstarman@lemmy.todayOP

2 points

2 months ago

OS “replaces” its contents AND permissions with that of the filesystem’s root.

So, the original content is lost forever?

setting permissions is just extreme pedantry

So, what’s the actual use case of it though? Even though it’s pedantry, it still there has to be some benefits, right?

I mean, What’s the need for you to deny the access of /mnt/a untill has mounted with something? One can just leave it as it is, right?

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2 points

2 months ago

So, the original content is lost forever?

No, but it becomes invisible and inaccessible* as long as the filesystem is mounted over it - see this Stack Exchange question and accepted answer.

The benefits are marginal, for example I can see if a filesystem is mounted by simply typing ll /mnt (ll being an alias of ls -lA) - it comes handy with my system due to how I manage a bunch of virtual machines and their virtual disks, and it’s short and easy to type.
Some programs may refuse to write inside inaccessible directories, even if the root user can always modify regular files and directories as long as the filesystem supports it.

It’s not a matter of security, it’s more of a hint that if I’m trying to create something inside those directories then I’m doing something wrong (like forgetting to mount a filesystem) and “permission denied” errors let me know that I am.

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