lkml.org 
[lkml]   [2017]   [Jun]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectCircular debugging using ptrace results in deadlock due to race condition?

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


\
 
 \ /
  Last update: 2017-06-12 01:30    [W:0.027 / U:0.256 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site