Isn't this a way to introduce bugs? For example:
- injected DLL can change some process-related settings and break main program
- injected DLL can call non-reenterable function from a system library simultaneously with main program
- injected DLL can change some process-related settings and break main program
- injected DLL can call non-reenterable function from a system library simultaneously with main program
(And as far as the terminology, these aren't "bugs" since they aren't defects in the software.)
Linux can do this too with ptrace. And mac has something similar I'm sure, you gotta be able to debug.
Now, the target process can implement countermeasures against you, that's what anti-debug is, but it's impossible in the general case to defend yourself against a debugger with the same privilege level as you. (it is an arms race though, so sometimes it's easiest to resort to kernel-mode anti-anti-debug techniques even against pure user-mode anti-debug. If you do that you need to disable KPP, and you can't disable KPP in the supported way because the supported way is to attach a kernel debugger to the kernel, which will make it obvious that someone is debugging!)