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I read and write but never execute. A cs major in cool pajamas. Alias rachejazz OR h3ck

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30 August 2019 Back to Menu

GNU toolchain on InterProcess Communication(IPC)

by Divya

Run, break, study, re-run, dump. The 3G from GNU tool chain is here to make your life easier.

While gorging down your favorite snack, what tells your hands to stop before you die from overeating? Well thanks to our operating systems, we never need to check over our process queue to ensure everything is working smooth.

So does this box thingy in our hands also needs a check over it’s workflow?

Just a screenshot of my dramatic phone’s log file.

What stops them from dying from overeating? While diagnosing disorders in us doesn’t require anatomizing ourselves, in the IT world there are tools that can be used to troubleshoot or “Look What’s Inside?” any operating system and how process communication work.

*enter stage The 3G. Here’s a short bio for each:

gcc : the GNU project C and C++ compiler. Involves preprocessing, compilation, assembly and linking.
gdb : the GNU Debugger. Allows you to see what happens “inside” a program when it executes.
gcov : the GNU coverage testing tool. Helps you to discover untested parts of your code, helps you to analyze the number of times every line of your code is executed.

To know more about each of them, open your terminal, type man followed by any of them.

To investigate their business and their roles in systems development, we will

now create a tiny subsystem of 2 processes communicating with each other.

I will summon the powers of IPC and SIGNALs to help me make them talk.

A brief little map of how I established communication:

Read more about process communication and signal handling.

These are the neighbours we’ll be studying today. One annoys, while the other tries to calm things down and provides the user with 3 options:

The nextdoor neighbour annoys me. What should I do to him? Kidnap, kill or let him go?

Neighbour 1(annoying infinite looped process disturbing neighbour 2):

neighbour1.c View neighbour1.c on gist.github.com

Neighbour 2(resolves by sending signals to neighbour 1):

neighbour2.c View neighbour2.c on gist.github.com

Compile both the processes using gcc. Although, we will use different flags for each.
For process 1, we will analyze code coverage using gcov. Compile using:

$gcc -Wall -fprofile-arcs -ftest-coverage neighboour1.c -o neighbour1

For process 2, we will experiment and try changing the user input data in the course of execution process using gdb.(Wait. Is that possible?) Compile using:

$gcc -g neighbour2.c -o neighbour2

Alright! Now that our mini ecosystem is created, we employ the 3G to analyse what’s going on. Execute them to make them talk.

You can either execute them in 2 different windows, or use the following to send neighbour1 to background and execute them simultaneously:

$./neighbour1 & ./neighbour2

Both of them should work. Now, what will a normal user do to a process annoying him? Kill? Kidnap? Let’s see:

user sending kidnap or kill instruction.

So we see process 2 ended up killing process 1. Although just before getting killed neighbour 1 dumped his execution logs. Let’s analyze that. Remember gcov? Next step:

$gcov neighbour1

File 'neighbour1.c'
Lines executed:55.10% of 49
Creating 'neighbour1.c.gcov'

$cat neighbour1.c.gcov

What you now see is the number of times each line of code is executed/checked. Here’s a snippet for you to relate:


Owkay..time for repentance. Now it’s time to employ gdb . Did you notice that the user opted to spare the life of his fellow neighbour?

Can’t he be let go once? A second chance? Why so cruel?

Suppose the user has the evil intention to always kidnap or kill or send corrupted data. So…can you check on the input data and change it while it gets executed? Of course input data is received, there shouldn’t be any way you can change values of variables while they’re still inside your process memory? However gdb can do it for you! Here’s how:

Code injection with gdb to change value of variables:

Cool. So… neighbour1 is about to get killed. Can we save him?

Welps! Using compile code we were successful to change the value of the input data. Guess who just got saved?

Sent SIGUSR2 signal to neighbour. Neighbour 8623 is spared

So there you go! Today we learnt how to analyze inter-process communication(and even change the messages) with the help of The 3G. Legends say that 3G is always around you. They’re just a few commands away. Give them a call whenever you need ❤.
If you’re interested in further reading, these are some nice starting points:

The Heisenberg Debugging Technology
The GNU C library