Previously, magic mount creates its own mirror devices and mount
mirror mount points. With these mirror mount points, magic mount
can get the original files and directory trees. However, some
devices use overlayfs to modify some mount points, and thus after
magic mount, the overlayed files are missing because the mirror
mount points do not contain the overlayed files. To address this
issue and make magic mount more compatible, this patch refactors
how magic mount works.
The new workflows are as follows:
1. make MAGISKTMP a private mount point so that we can create the
private mount points there
2. for mirror mount points, we instead of creating our own mirror
devices and mount the mirror mount points, we "copy" the
original mount points by recursively mounting /
3. to prevent magic mount affecting the mirror mount points, we
recursively set the mirror mount points private
4. to trace the mount points we created for reverting mounts, we
again make the mirror mount points shared, and by this way we
create a new peer group for each mirror mount points
5. as for tracing the newly created tmpfs mount point by magic
mount, we create a dedicated tmpfs mount point for them, namely
worker mount point, and obviously, it is shared as in a newly
created peer group for tracing
6. when reverting mount points by magic mount, we can then trace
the peer group id and unmount the mount points whose peer group
ids are created by us
The advantages are as follows:
1. it is more compatible, (e.g., with overlayfs, fix#2359)
2. it can mount more partitions for which previous implementation
cannot create mirror mount points (fix#3338)
