Seemingly many ptracer issues

[theIDinside]

From what I gather recording (and therefore emulating) ptracers is difficult. This will be a meta bug that also links to my repo that contains the various different programs that fail under an RR recording.

https://github.com/theIDinside/rr-waitpid-bug now contains a couple of different bugs:

• RR waitpid bug not seen during non-recording #3658
• Signalfd + waitpid + multithreading #3898
• Can't configure ptrace in non-main thread #3929

All these have some multi-threaded nature to them.

[khuey]

Is there a motivation for these? Just having a perfect reimplementation of ptrace is not really a goal. Supporting recording e.g. gdb or something on the other hand is a reasonable goal.

[theIDinside]

As far as I know, gdb doesn't use signalfd nor have a dedicated thread to do waitpid operations on, though I don't see why they wouldn't want it. The reason why I am using it is because it makes multi process debugging just a lot easier, since a "main thread" can be concerned with actual debugger work, instead of being ground to a halt when heavier applications like browsers for instance spam wait statuses to the ptracer.

So is it of any real world use yet? No, so I can definitely accept it not being a priority for rr (but it would of course be important to me :P).

[theIDinside]

Is there a motivation for these? Just having a perfect reimplementation of ptrace is not really a goal. Supporting recording e.g. gdb or something on the other hand is a reasonable goal.

@khuey - I found an acceptable workaround in my case. I can use signalfd and just integrate the waitpid directly on the maintread because of it without offloading it to a separate thread, in my case it's a tiny bit less efficient, but it's more than acceptable.

In the debugger I'm writing I use signalfd + waitpid on a separate thread (but ptrace commands issued on the thread that actually created new processes, because this is required for TRACEME). I think we can close this entirely since a full compat with ptrace is probably not really what RR aims for any how.