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Refactor magic mount to support overlayfs

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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)
This commit is contained in:
LoveSy
2022-12-27 04:30:12 +08:00
committed by John Wu
parent 97ed1b16d0
commit af01a36296
3 changed files with 52 additions and 109 deletions
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docs/guides.md
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@@ -120,7 +120,7 @@ All files you want to replace/inject should be placed in this folder. This folde
If you place a file named `.replace` in any of the folders, instead of merging its contents, that folder will directly replace the one in the real system. This can be very handy for swapping out an entire folder.
If you want to replace files in `/vendor`, `/product`, or `/system_ext`, please place them under `system/vendor`, `system/product`, and `system/system_ext` respectively. Magisk will transparently handle whether these partitions are in a separate partition or not.
If you want to replace files in `/vendor`, `/vendor_dlkm`, `/product`, `/system_ext`, `/system_dlkm`, `/odm`, or `/odm_dlkm`, please place them under `system/vendor`, `system/product`, and `system/system_ext` respectively. Magisk will transparently handle whether these partitions are in a separate partition or not.
#### Zygisk