“unable to inject library into process without libc” when attach to child process that origin=exec

从 examples/child_gating.py 开始，复现 Android 中 origin 从 fork 变为 exec 的 child-added 序列。沿着 frida/core.py 以及 frida_tools/application.py 中报告的调用点跟踪 attach 路径，对比成功的 fork attach 与失败的 exec attach。当报告的 exec 情况得到解决，或明确确定其属于不受支持的行为并由适当的回归检查覆盖时，即可视为完成。

this is my environment:
genymotion: 3.2.1 + Samsung Galaxy S7 6.0-API 23,also test on Google Nexus 5X 8.0-API 26
frida-server: frida-server-15.1.17-android-x86,also tested frida-server-15.1.14-android-x86 and frida-server-15.0.0-android-x86
script: https://github.com/frida/frida-python/blob/master/examples/child_gating.py
I only changed the argv to myappid
def _start(self):
argv = [myappid]

and I got these outputs:

✔ spawn(argv=['myappid'])
✔ attach(pid=4486)
✔ enable_child_gating()
✔ create_script()
✔ resume(pid=4486)
⚡ child_added: Child(pid=4602, parent_pid=4486, origin=fork)
✔ attach(pid=4602)
✔ enable_child_gating()
✔ create_script()
✔ resume(pid=4602)
⚡ child_removed: Child(pid=4602, parent_pid=4486, origin=fork)
⚡ child_added: Child(pid=4602, parent_pid=4602, origin=exec, ......)
✔ attach(pid=4602)
Exception in thread Thread-1:
Traceback (most recent call last):
File "/home/nothing/miniconda3/lib/python3.7/threading.py", line 926, in _bootstrap_inner
self.run()
File "/home/nothing/miniconda3/lib/python3.7/threading.py", line 870, in run
self._target(*self._args, **self._kwargs)
File "/home/nothing/miniconda3/lib/python3.7/site-packages/frida_tools/application.py", line 765, in _run
work()
File "lavpn.usa_spawn_gating.py", line 20, in
self._device.on("child-added", lambda child: self._reactor.schedule(lambda: self._on_child_added(child)))
File "lavpn.usa_spawn_gating.py", line 61, in _on_child_added
self._instrument(child.pid, child)
File "lavpn.usa_spawn_gating.py", line 45, in _instrument
session = self._device.attach(pid)
File "/home/nothing/miniconda3/lib/python3.7/site-packages/frida/core.py", line 26, in wrapper
return f(*args, **kwargs)
File "/home/nothing/miniconda3/lib/python3.7/site-packages/frida/core.py", line 165, in attach
return Session(self._impl.attach(self._pid_of(target), *args, **kwargs))
frida.NotSupportedError: unable to inject library into process without libc

We can see that when attach the child process which origin is fork,it's ok.
but when attach the same process which origin changed to exec,the error occurs.
I searched and find https://github.com/frida/frida/issues/936,but no lucky.
I think maybe because I used genymotion emulator, but why first attach is ok?