Use strace

Strace enables you to see the system calls a process makes and what those system calls return.

Build strace

To build strace, run the following:

mmma -j6 external/strace

Attach to a running process

The simplest and most common use case for strace is to attach it to a running process, which you can do with:

adb shell strace -f -p PID

The -f flag tells strace to attach to all the threads in the process, plus any new threads spawned later.

A typical process makes a lot of system calls, so you'll want to review the strace man page to learn how to collect only data you're actually interested in.

Use on an app

To use strace on an app:

  1. Set up the device so that you can run strace. You need to be root, disable SELinux, and restart the runtime to remove the seccomp filter that will otherwise prevent strace from running:
    adb root
    adb shell setenforce 0
    adb shell stop
    adb shell start
  2. Set up a world-writable directory for strace logs, because strace will be running under the app's uid:
    adb shell mkdir -m 777 /data/local/tmp/strace
  3. Choose the process to trace and launch it:
    adb shell setprop '"logwrapper strace -f -o /data/local/tmp/strace/"'
  4. Launch the process normally.

Use on the zygote

To use strace on the zygote, fix the relevant init.rc zygote line (requires adb shell setenforce 0):

cd system/core/
patch -p1 <<EOF
--- a/rootdir/init.zygote32.rc
+++ b/rootdir/init.zygote32.rc
@@ -1,4 +1,4 @@
-service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
+service zygote /system/bin/strace -o /data/local/tmp/zygote.strace /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server
     class main
     socket zygote stream 660 root system
     onrestart write /sys/android_power/request_state wake

Get strace logs during Android boot

To get strace logs during Android boot, make the following changes:

  • Since the process name changes from zygote to strace, the given service may fail to start due to the missing SELinux file_context for strace. The solution is to add a new line for strace in system/sepolicy/private/file_contexts and copy the original file context over. Example:
    /dev/socket/zygote      u:object_r:zygote_socket:s0
    + /system/bin/strace u:object_r:zygote_socket:s0
  • Add kernel or bootconfig parameter, then boot the device in SELinux permissive mode. You can do this by adding androidboot.selinux=permissiveto BOARD_KERNEL_CMDLINE, or to BOARD_BOOTCONFIG in Android 12 with kernel version 5.10 or greater. (This variable becomes read-only in build/core/Makefile but is always available under /device/*/BoardConfig.)

    Example for the Pixel (sailfish) device in /device/google/marlin/sailfish/
    - BOARD_KERNEL_CMDLINE := ....  androidboot.hardware=sailfish ...
    +BOARD_KERNEL_CMDLINE := ....  androidboot.hardware=sailfish ...  androidboot.selinux=permissive
    After making the change, build and flash the boot image and the device will boot in permissive mode.