The understanding and usage of bitmap

[Lingjun Zhou]

Here I will express some initial understanding(may be not all right) of bitmap in afl and also come up with a question. Hope to get your opinions. Thank you!

As we all know, afl will provide us a code coverage information which is displayed in screen for every input. After I stop fuzzing process, I notice that there is a new file called fuzz_bitmap in results directory. When I try to open it, I find that it's not readable file for human.Then I use hexdump command to show it,the results like below. In this stage, I used to think that every bit in bitmap represents a code block and it will be set as 1 if certain code block is executed given an input. But it seems to be an misunderstanding later.

After I searched more information about it, I found that there is also a tool called afl-showmap that can generate a readable bitmap file for human. I used command shown in picture below and found it was not run as expected.

But it still generated a readable output file like below. In this stage, I think what bitmap can provide us is a tuple<code block id, execution times of certain code block given an input>.

At last, I found an reasonable explanation in a paper. In this stage, I think that every byte in the bitmap represents a count of edge between two code blocks. However, there is a doubt hit on me now. Is it really works for XOR operation to identify a unique path between two code blocks?

The content showed above is what I've learnt about bitmap in different stages.

In the end, I want to ask another question about bitmap. Is it possible for me to get every input and corresponding  feedback from afl ? 

Because I can only get some useful input files in /results/queue directory and one fuzz_bitmap file in /results directory now.

Thanks in advance! 

[Lingjun Zhou]

Is there anyone who has an idea about it ?

Hope to get your replies 

[Jules Irenge]

I am on the boat you were three years ago. I am trying to understand AFL bitmap.

[longl bright]

I'm trting to understand AFL bitmap too.