this post was submitted on 14 Sep 2024

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chown on a sdcard directory impossible... (programming.dev)

submitted 4 days ago by [email protected] to c/[email protected]

12 comments fedilink hide all child comments

Hi,

I would like to change the owner of a directory on the sdcard /sdcard/aDirectory

I have a terminal installed on my Android 10 (LineageOS 17) com.android.terminal

sudo is not present so I use su and it works.

su
#Terminal was granted Superuser rights

cd /sdcard
chown 10:10 aDirectory
#I don't get any error message.

stat aDirectory
#Uid (0/root)

So the owner stay root no matter what I'm doing, any ideas ?

top 12 comments

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[–] [email protected] 4 points 4 days ago

Android users all kinds of overlays over the sdcard directory. This is part of how it enforces storage access for apps. There's probably a way to override these settings, but they sure as hell aren't easy.

There's also another layer of permissions somewhere seeing as I can't access certain files on /sdcard that were created by the recovery. I assume it's an selinux context issue (it always is).

Changing ownership should work on real SD cards with normal storage, but it won't for the emulated internal /sdcard.

[–] [email protected] 12 points 4 days ago* (last edited 4 days ago)

Nine times out of ten, running chown on Android is an astronomically bad idea. 10 times of 10, what you're trying to do right now, is an astronomically bad idea.

What is it you are trying to do? Or rather, why?

[–] [email protected] 10 points 4 days ago

If you want to change the owner of fat partition, you can do it while mounting, e.g. mount -t fat -o uid=10,gid=10 ......

[–] [email protected] 16 points 4 days ago* (last edited 4 days ago) (1 children)

Unless the SD card is using a Linux filesystem, you won't be able to use Linux access permissions.

Most SD cards are preformatted to FAT or exFAT in order to maximise compatibility. If you don't need to use the card on any non-linux devices you should be able to reformat it and gain the ability to chown/chmod (should go without saying, but back up any data on the card you want to keep before this)

[–] [email protected] 3 points 4 days ago

There are other comments here which might have some relevance but imo this one is the most likely the most applicable. I've had this experience with other filesystems over and over and it seems like it's almost always a problem. Apparently NTFS supports permissions in a similar way to Linux if you set it up exactly right, but I've never taken the time to try that.

[–] [email protected] 17 points 4 days ago (1 children)

What filesystem do you have on that SD-card? Likely FAT32, which does not support ownership.

[–] [email protected] 0 points 4 days ago (1 children)

But is it an SD card.

I mean, the directory name says so, but...

~ $ realpath /sdcard
/storage/emulated/0
~ $

...it may also not be.

[–] [email protected] 4 points 4 days ago (1 children)

It doesn't matter. FAT filesystems - which are usually the default on SD cards, simply do not support ownership or file permissions. Linux emulates these attributes at mount time, but they apply to the entire SD card. You can mount an SD card and tell Linux to act as if root owns everything on the card; you that you own everything on the card; and it will be so until you unmount it and remount it with a different ownership.

These are filesystem level attributes, not device attributes. If you have a modern internal nvme drive and you format it with vfat, you will not be able to set permissions or ownership at the file level, but only at mount time, for the entire drive.

[–] [email protected] 3 points 4 days ago (2 children)

I am talking about some devices using /sdcard to mount internal storage.

[–] [email protected] 2 points 4 days ago (1 children)

You mean, they're mounting something that isn't an SD card to the /sdcard directory? Like something truly evil, such as mount -t btrfs -o subvol=@home / /sdcard? Or do you think there's not anything mounted there; it's just a directory in the root partition? None of that would make any sense.

If they're letting whatever automount tool (eg udevil) do its thing, this is practically impossible. And if they know enough to do it by hand, I think they'd have answered the direct question of "which filesystem" with a filesystem rather than a mount point. Don't you think? We still don't know what filesystem they're working with, since they haven't answered the question.

[–] [email protected] 2 points 4 days ago

something that isn’t an SD card to the /sdcard directory?

Could be.

On my phone (Poco X3 Pro - stock Android 11, MIUI 12) the /sdcard is a symlink pointing to /storage/self/primary which itself is a symlink pointing to /storage/emulated/0, which is /data/media, the user-accessible portion of internal storage.
Though from what I can find it anyway is just emulated FAT filesystem which is actually ext4 under that.

Something about backwards compatibility as the directory used to actually be used for SD cards in the past.

[–] [email protected] 2 points 4 days ago* (last edited 4 days ago)

True, but presumably op probably has an sd card if they say they do.

Edit: I see both ways of reading their statements on re-reading them.