Messages in this thread | | | Date | Thu, 08 Jun 2017 16:08:54 +0200 | From | Ilja Nevolin <> | Subject | Circular debugging using ptrace results in deadlock due to race condition? |
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Hi guys,
As part of my master's thesis I am facing a challenging problem. I am trying to let two processes be each other's debuggers using the ptrace syscall. However, my proof-of-concept implementation always results in a deadlock state (both processes get stuck in 't+' state as shown by 'ps aux').
Here is my code, it's pretty simple: https://pastebin.com/A1iBA3nh
I have compiled and run this on an ARMv7 developer board with kernel version 3.0.35 (Linaro 13.08). The output of the above code is this:
A waiting to continue... B attachTo: 0 B setOptions: 0 B setVarData: 0 B setVarData: 0 B cont: 0 B waiting to continue... B waiting to continue... A attachTo: 0
As you can see it never reaches the "finished" printf code, and gets stuck as soon as the other process attempts to attach to the debugger.
I have done a similar experiment for 3 processes, such that each one attempts to attach to the other in a circular fashion: A -> B -> C -> A The result in this case was exactly the same. However, here I was able to detect a race condition, because sometimes the code executed properly without getting stuck in a deadlock (but it's hard to reproduce).
If you wish, you can test this by using a lightweight debugger I've developed and three console terminals. Here's the code: https://pastebin.com/fPJb8ZNb
Once you've compiled the above code, you simply run the binary on each console and enter the PID of another process to establish a 3-way circle.
I am far from an expert on the kernel, but I did have a look at the ARM specific kernel implementation which left me puzzled. I couldn't find where/how/why this code does not work. Now I'm wondering if it's possible at all to make this work without a deadlock occurring? Does anyone have any experience with this, or can provide some clues/feedback?
Thank you greatly for your time, attention and effort! Ilya Nevolin ilja.nevolin@ugent.be
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