I use ES File Explorer to modify /system/priv-app in the past with root permission given via magisk.
After upgrading to Android 10, ES File Explorer can no longer modify the /system/priv-app giving me an error message saying "operation failed."
What alternatives do I have if I like to modify /system/priv-app?
The output of running set -x; exec >>/sdcard/mount.log 2>&1; id; mount -o rw,remount /system; mount | grep 'on /system '; touch /system/test_file; cat /sys/block/dm-*/dm/name:
uid=0(root) gid=0(root) groups=0(root) context=u:r:magisk:s0
mount: '/system' not in /proc/mounts
touch: '/system/test_file': Read-only file system
vendor-verity
It turns out:
On some system-as-root (SAR) devices you can no longer mount the system partition as read-write and doing systemless modifications through Magisk is the only way. If you do want to do actual edits of the system, and if it is even possible (from Android 10 it's actually impossible on some devices), you need to mount the root directory, /, as rw and not /system. You can also try editing the partition mirrors that Magisk keeps in /sbin/.magisk/mirror.
So all I needed to do is to:
mount -o rw,remount /