How to read ext4 filesystem without mounting on a non-rooted device? I perfectly know it is not possible to mount an ext4 partition from a USB key without root.

But in my case, I just need to read a password inside a .txt file (I don't have a PC right now and in the following weeks). This doesn t requires mounting it.

I know there's a Paragon plugin which allows acessing NTFS partitions without root inside Total Commander but it only works on exFAT and NTFS filesystems. But it proves it should be possible to do the same thing on ext4 without root.

So how to access my password right now without being able to root my device?
ext4 can be explored without mounting, using debugfs tool. But natively there is no way to access raw filesystem without root access on Android devices. Partitions are enumerated as block devices by Linux kernel, and default permission set by Android's init on block devices is 0600 (can be overridden in uevent.rc) or 0660 in case of vold, owned by 0:0. Also stock SELinux policy won't allow apps access files in /dev. So the block devices can't be read without root access.

Other way is to read a USB storage device in host mode through APIs. But Android's USB Host APIs provide only raw USB access, they don't have UMS filesystem code. However there are third party solutions like the one provided by Paragon, which includes raw-USB operations to achieve block-level device access, and the appropriate filesystem logic.

NTFS is a proprietary filesystem so it cannot have an open-source in-kernel Linux driver. Paragon develops drivers for filesystems including NTFS, exFAT (now open source) and HFS for Linux. They also develop exFAT/NTFS USB OTG host implementation for USB Mass Storage devices.

So what you are looking for is an app with third party implementation of USB OTG host APIs for ext4 filesystem. Paragon provides ext2/3/4 for Windows but no USB OTG solution is available for Android yet (probably because it's not of much use as ext filesystems are native to Linux/Android). However ExtFS support is part of Paragon's SDK for Android to be used by vendors and app developers. Also there's open-source Drive Mount (seems abandoned), which intends “to support HFS+ and Linux filesystems, sometime”.

As a side note, modded vold (such as vold-posix) is used on mostly custom ROMs which can mount ext4 partition from secondary external storage (SD cards, USB sticks) on non-rooted devices as well.