In the current implementation, Magisk will either have to recreate
all early mount implementation (for legacy SAR and rootfs devices) or
delegate early mount to first stage init (for 2SI devices) to access
required partitions for loading sepolicy. It then has to recreate the
split sepolicy loading implementation in-house, apply patches, then
dump the compiled + patched policies into monolithic format somewhere.
Finally, it patches the original init to force it to load the sepolicy
file we just created.
With the increasing complexity involved in early mount and split
sepolicy (there is even APEX module involved in the future!),
it is about time to rethink Magisk's sepolicy strategy as rebuilding
init's functionality is not scalable and easy to maintain.
In this commit, instead of building sepolicy ourselves, we mock
selinuxfs with FIFO files connected to a pre-init daemon, waiting
for the actual init process to directly write the sepolicy file into
MagiskInit. We then patch the file and load it into the kernel. Some
FIFO tricks has to be used to hijack the original init process's
control flow and prevent race conditions, details are directly in the
comments in code.
At the moment, only system-as-root (read-only root) support is added.
Support for legacy rootfs devices will come with a follow up commit.
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