Rookie mistake python loops and variable scope

While writing some simple python code I stumbled upon a rookie mistake which I appear to still be making so writing it down here to remind me.

Scope of variables is important in loops especially when using them to increment.

I was writing some code to work out if the product of two 3 digit numbers is a palindrome (not very elegant algorithm just brute forces it). Here is my original algorithm

a=100
b=100

numbers=[]

while a < 1000:
  while b < 1000:
    product=a*b
    print a, b, product
    if (product == int(str(product)[::-1])):
        print "palindrome"
        numbers.append(product)
    b+=1
  a+=1

From the output I noticed something strange the outer loop only went round once even though I thought I had incremented a after the inner loop.

Then it struck me I had defined b outside my while loop so it would not get reset upon next iteration of the outer loop and therefore the inner loop would not run again (and print something out). Therefore it looked like I was only looping once on the outer loop.
This could be proven by adding "print a" before incrementing a on the last line.

Moving "b=100" inside the outer loop solves the problem.

Working code looks like so

a=100
numbers=[]
while a < 1000:
  b=100
  while b < 1000:
    product=a*b
    print a, b, product
    if (product == int(str(product)[::-1])):
        print "palindrome"
        numbers.append(product)
    b+=1
  a+=1

Another alternative is to change the while loops to for loops.

Easter egg in gnome-shell

I have just found and easter egg in gnome-shell.

In gnome, move the cursor up to the top left corner as if you were going to open an application, then in the search box type "free the fish"

Wanda

Clicking on the icon or hitting return will bring up a dialog box containing a random fortune.

Drag and Drop in a webpage

I saw a cool feature on a website allowing you to drag and drop a file onto the page and it will be uploaded.

I wondered how this was done and after some experimentation I believe I understand how to do this. I should note that I was working with firefox on Linux and I have not tested this code in other browsers on other platforms.

I created a simple blank html page with a line explaining to drag and drop a file onto the page.

Then a little javascript to handle the dragging and dropping a file.

In my script I need to get the whole document (or the body element) which can be done with
document.documentElement

Then I need to add some event listeners to this to listen for the drag and drop events (in particular the "drop" event). I created some functions to be called when these events are triggered.
To prove a file has been dropped all I did was popup an alert with the name of the file dragged and dropped.
My test code only handled the first file dropped, but it should not be too difficult to add a loop to process the rest.

So my basic javascript would look like this

function noop(e) {
    e.stopPropagation();
    e.preventDefault();
    return false;
}

var drop = document.documentElement;

drop.addEventListener("dragover", noop, false);
drop.addEventListener("dragexit", noop, false);
drop.addEventListener("drop", function(e) {
     e.stopPropagation();
     e.preventDefault();
     //only get the first file, add loop for multiple file handling
     var files = e.dataTransfer.files;
     var file = files[0];
     alert("file '"+file.name+"' dragged and dropped");
     return false;
},false);

This was not successful and kept giving me an error in firefox, this was probably due to trying to do things before the document was ready. Therefore I used jQuery to ensure the document was ready, now my code was wrapped like so

$(document).ready(function() {
...
});

However, this still did not seem to be working. I found some old bugs which suggested that firefox had problems with drag and drop from anything other than nautilus, but I was using nautilus so it should work.
Then it struck me I was dragging items from the recent files window in nautilus, could these be references to the files? I opened the actual directory where the test files were and drag and drop worked perfectly.

I did not do anything with the file dropped, but is should not be difficult to add upload code to pass to a server side script for processing.

Reversing Shellcode

I was looking at some shellcode and there are lots of articles on how to compose shellcode but not that many on how to get it back to assembly for analysis.

The easiest way appears to be to dump the shellcode to a file,  this can be done with a perl one liner e.g.

 perl -e 'print "\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e"' > shellcode

 Then using ndisasm from nasm to convert this shellcode back to assembly mnemonics. The shellcode I was looking at was for 32bit architecture therefore I can use the following

ndisasm -b 32 shellcode

Which gives me
00000000  682F2F7368        push dword 0x68732f2f
00000005  682F62696E        push dword 0x6e69622f

It should be noted that this is a nonsense example as the shellcode only pushes "/bin//sh" onto the stack and does not do anything with it, however it shows how to use the tools.

bash history tricks

I came across an interesting blog entry about maintaining a persistent history of commands across multiple bash sessions using some features of bash I was unaware of.

First there was a variable PROMPT_COMMAND which will run the command stored in the variable before presenting the new prompt.

Then there was the BASH_REMATCH variables, which store substring patterns when used with [[ ... ]] e.g.

test_string="Random string of numbers : 1234"
[[
   $test_string =~ (.*)\:\ +([0-9]+)
]]

echo ${BASH_REMATCH[0]}
echo ${BASH_REMATCH[1]}
echo ${BASH_REMATCH[2]}

This snippet will output the following
Random string of numbers : 1234
Random string of numbers
1234

I have simplified the example from Eli's blog as I am not using the same format for history output , so my code in .bashrc looks like

log_bash_history()
{
  [[
    $(history 1) =~ ^\ *[0-9]+\ +(.*)$
  ]]
  local command="${BASH_REMATCH[1]}"
  if [ "$command" != "$HISTORY_LAST" ]
  then
    echo "$command" >> ~/.persistent_history
    export HISTORY_LAST="$command"
  fi
}
export PROMPT_COMMAND=log_bash_history

gdb not stepping into break point set on strcpy

I was messing around with gdb and some test programs and I set a breakpoint on a call to strcpy, when I ran the program it stepped right over this breakpoint.

Initially I had assumed that glibc had been stripped but this was not the case. Further investigation lead me onto this discussion, where it was suggested to compile with the gcc option -fno-builtin.

This appeared to solve my issue and now when running this code in gdb it does step into the breakpoint set for strcpy.

launching applications upon login to Gnome3

I wanted to run a simple program after logging into a graphical session (Gnome3).

There is a simple graphical utility startup applications (gnome-session-properties), this can be run directly but does not by default show up in search.
To have it show up in all applications or search edit the file /usr/share/applications/gnome-session-properties.desktop and change the line NoDisplay to false e.g.

NoDisplay=false

Adding a new startup application will create a new desktop file in ~/.config/autostart. Therefore to create a new startup application a new file could be created in this directory, for format of the file just copy an existing file and modify as necessary.
A simple entry is below

[Desktop Entry]
Type=Application
Exec=<command>
Hidden=false
X-GNOME-Autostart-enabled=true
Name[en_US]=<name>
Name=<name>
Comment[en_US]=<comment>
Comment=<comment>