Null pointer dereference (Android + JNI)

[nadrolinux]

Hello,
At first, it's not a high priority issue in my opinion, but I found that Scapix may crash at:

// source/scapix/link/java/detail/api/ref.h
static jobject new_ref(jobject h) noexcept { return detail::env()->NewGlobalRef(h); }

Probably 'scope::local' and 'scope::weak' variants are also vulnerable to this issue.

When crash occurred I saw that 'jobject h' was nullptr, so 'NewLocalRef' caused abort with 'null pointer dereference' info. This issue may appear eg. when you store lambda with captured std::function to std::function (Scapix function stored here) and next you copy this std::function on another thread. I think that it may be relatated to detached JNI environment on another thread. I don't know if Scapix do some check like this one:
if (pVM->GetEnv((void**)&mEnvironment, JNI_VERSION_1_6) == JNI_EDETACHED && pVM->AttachCurrentThread(&mEnvironment, nullptr) == JNI_OK)
but if not it may be source of problem. However this issue may be relative easy solved by just store std::shared_ptr with std::function, so copy operate just on std::pointer instead of std::function, thats why I think that priority of this issue isn't high.

Best regards,
Patryk

[Boris-Rasin]

As a workaround, you can call once in each thread created from C++:

scapix::link::java::attach_current_thread();

For convenience, I can also add generic version of this function (which will do nothing with other bridge languages):

scapix::init_thread();

I can also add optional automatic thread attachment:

target_compile_definitions(scapix PUBLIC SCAPIX_JNI_AUTO_ATTACH_THREAD)

I want to make it optional because it would add slight runtime overhead on each JNI call.

What do you think?

[nadrolinux]

Hi,
I think that 3rd option with optional auto attachments is the best, because of automatic nature, but I agree that marked it as optional is better choice, because in this way we can still rely on options 1 and/or 2 if auto performance will be a problem. Please just remember that for option no. 2 you should also add 'scapix::exit_thread' or similar with internal 'scapix::link::java::detach_current_thread()' call.
Thanks for a quick response and good solutions as always.
Patryk

[Boris-Rasin]

Commit: 0256e3f
Documentation: https://www.scapix.com/java_link/threads/

[Boris-Rasin]

Released in cmodule v1.0.35:
https://github.com/scapix-com/cmodule/releases

[nadrolinux]

Great, thanks a lot.