Integrating a Custom Daemon into AOSP: A Step-by-Step Guide

As Android developers, we often encounter the need to extend AOSP functionalities with custom background services. Recently, I implemented a custom daemon in Android with full integration into the AOSP build system, SELinux policies, and init.rc configuration. Here's a comprehensive guide with commands and details to help you replicate the process.

?? Steps to Create a Custom Daemon in AOSP

1?? Write the Daemon Code

Start by writing the C/C++ code for your daemon. Here's an example of a basic daemon that logs messages:

custom_daemon.c:

#include <stdio.h>
#include <unistd.h>

int main() {
    while (1) {
        printf("Custom daemon is running...\n");
        sleep(10);  // Replace with your daemon logic
    }
    return 0;
}        

2?? Integrate the Daemon into AOSP Build System

• Place your daemon code in the AOSP tree, e.g., system/core/custom_daemon/.
• Create a build script (Android.bp):

Android.bp:

cc_binary {
    name: "custom_daemon",
    srcs: ["custom_daemon.c"],
    shared_libs: ["liblog"],  // Add required libraries
    init_rc: ["custom_daemon.rc"],  // Link init.rc configuration
}        

3?? Configure init.rc

Create an init.rc file to define your service and add it to the daemon directory:

custom_daemon.rc:

service custom_daemon /system/bin/custom_daemon
    class main
    user system
    group system
    oneshot        

??? Add SELinux Policy

4?? Define SELinux Rules

• Create a policy file custom_daemon.te:

custom_daemon.te:

type custom_daemon, domain;
type custom_daemon_exec, exec_type, file_type;

# Allow init to start the daemon
init_daemon_domain(custom_daemon)

# Grant specific permissions
allow custom_daemon self:capability { sys_nice };
allow custom_daemon self:netlink_socket { create bind sendto };        

5?? Update File Contexts

Map the daemon binary to the correct SELinux type in file_contexts:

file_contexts:

/system/bin/custom_daemon   u:object_r:custom_daemon_exec:s0        

?? Build and Flash AOSP

6?? Rebuild AOSP

Run the following commands to build AOSP with your daemon:

source build/envsetup.sh
lunch <device_target>
make -j$(nproc)        

7?? Flash the Image to the Device

Once the build is complete, flash the updated image:

adb reboot bootloader
fastboot flashall
fastboot reboot        

?? Verify the Daemon

8?? Check if Daemon is Running

After the device boots, verify that the daemon is running:

ps | grep custom_daemon        

9?? Validate SELinux Policies

Check for any SELinux denials using dmesg:

dmesg | grep avc        

?? Key Takeaways

• Modularity: Integrate daemons cleanly with AOSP build and runtime systems.
• Security: Use SELinux to ensure isolation and enforce least privilege.
• Verification: Thoroughly test your daemon in both normal and edge-case scenarios.

This hands-on project highlighted the intricacies of AOSP development and the importance of secure, scalable system design. If you're working on similar challenges or exploring AOSP internals, feel free to connect or share your insights!

#Android #AOSP #AAOS #DaemonDevelopment #SELinux #EmbeddedSystems #Linux #SoftwareEngineering