libddnatv-hook.so is an LD_PRELOAD library that lets the glibc rootfs run directly on Android: it maps the guest's absolute paths into the rootfs (plus the mounts proot used to provide, from DDNATV_BINDS), starts every program through the rootfs's ld.so, and answers the system calls Android's seccomp policy kills a process for (SIGSYS handler). There is no tracer. The app side is runtime/NativeGuest.kt; the setting is Performance > "Run without proot (experimental)".

The hook is derived from the Apache-2.0 libcortex-hook of ssfuisu/cortex (see LICENSE), renamed (cortex -> ddnatv, same length: the code compares environment names by length) and extended with:

DDNATV_BINDS mounts, DDNATV_PRELOAD (what /etc/ld.so.preload does for proot), and mapping of guest paths inside LD_PRELOAD;
POSIX shm_open/shm_unlink through /dev/shm;
starting Wine without its static preloader, which opens files with raw system calls.

Build: tools/nativeguest/build.sh out.so with an aarch64 glibc cross compiler.

Testing under Android's seccomp filter

Some devices (eng builds, setenforce 0 ROMs) run apps with no seccomp filter, so a hook that passes there can still die with SIGSYS on a stock phone (GUEST_EXIT 159). seccomp-sim.c applies a filter that answers the calls Android blocks with SIGSYS, then runs the program:

aarch64-linux-gnu-gcc -static -O2 -o seccomp-sim tools/nativeguest/seccomp-sim.c
# copy it into the rootfs, then name it for every native launch:
echo /usr/local/bin/seccomp-sim > <rootfs>/etc/droiddeck/seccomp-sim

grep Seccomp: /proc/<pid>/status should then show 2 for the session's processes. Empty the file to turn it off. seccomp-sim -a NR,NR PROGRAM leaves the listed calls allowed, to find which one kills a program.