LoginARM Assembly for Hackers, Part 2: Leveraging GDB to Understand the ADD Instruction
Welcome back, cyberwarriors!
In a previous article, we explored some of the ARM assembler commands. Today, we will delve into the practical application of the ADD instruction. By leveraging the power of the GNU Debugger (GDB), we will explore how to analyze and manipulate this instruction to gain deeper insights into ARM architecture.
Prepare an Environment
Before starting to learn assembly, we should prepare an environment. About possible ways to do so, you can check out this article. I’ll be using a Raspberry Pi with 32-bit Raspbian OS.
To check if your system is running a 32-bit userland, run:
raspberrypi> getconf LONG_BIT
Next, check what architecture your binaries are:
raspberrypi> file /bin/bash
In the case above, you can see a pretty common issue on modern Raspberry Pis: Raspbian OS is 32-bit, but uses a 64-bit kernel. This is an optimal installation, because you get 32-bit compatibility for all your applications and libraries, and better hardware support from a 64-bit kernel.
ADD Instruction
This instruction adds an immediate value to a register value and writes the result to the destination register.
The syntax is as follows:
ADD{S}{<c>}{<q>} {<Rd>,} <Rn>, #<const>Where
S – if presented, the instruction updates the flags. We’ll talk about flags later;
<Rd> – destinations register;
<Rn> – first operand;
<const> – the immediate value to be added to the value obtained from <Rn>;
<c> and <q> – are optional assembler fields.
Let’s move on to the practical stage and write the code. I’ll create a file instructions.s and open it with Vim.
The beginning of the file is as usual – declare “_start” value globally. I’ve explained this step in more detail in the following article. Also, I’ll add a comment with the add instruction syntax for ease of learning.
First of all, we need to have a register (<Rn>) that will be added to our constant value (#<const>). We’re going to set up a general-purpose register with the mov instruction.
As you might already remember from my previous article, general-purpose registers are r0-r12.
To set up a general-purpose register with a value of our choice, we can use the following command:
mov r0, #7
Where
mov – instruction to copy the value to the register;
r0 – destination register, where we’re going to store a temporary value;
#7 – pound sign signifies that the following value is constant. For this example, I’ve used number 7; you can choose any you want.
After that, we’re good to go with our add instruction.
add r1, r0, #3
Where
r1 is the destination register where we’re going to store the sum of 7 + 3
r0 – our first operand with value 7.
#3 – constant value that will be added to r0. I’ve used value 3.
At this point, let’s assemble this code and see in gdb (GNU Debugger) what is happening.
To assemble, I’ll be using a GCC:
gcc -g -nostdlib -static -o instructions instructions.s
Where
-g – Include debugging information
-nostdlib – Don’t link with standard library (since we’re not using it)
-static – Create a static executable
Now, we can open the executable with GDB, but before that, I’ll install GEF (GDB Enhanced Features), which provides automatic register monitoring, color-code output, and more.
To install GEF, run:
raspberrypi> bash -c "$(curl -fsSL https://gef.blah.cat/sh)"
Now, let’s run GDB:
gdb ./instructions
First of all, I’m going to disable displaced stepping to avoid some possible errors in GDB.
(gdb) set displaced-stepping off
After that, we can set a breakpoint at the _start label so execution stops there:
(gdb) break _start
Run our program:
(gdb) run
Here we can see that the program started execution but stopped in _start because of the breakpoint.
Let’s check the value of all registers:
(gdb) info registers
They are empty at this point. Let’s step through one assembly instruction:
(gdb) stepi
And check the value of only register r0 and r1
(gdb) info registers r0 r1
And here we can see that register r0 already stores the value 0x7 or 7 in decimal.
If we step through the next assembly instruction and check the register value again with the same commands, we can see the value of the r1 register.
Value of r1 is 0xa or 10 in decimal, just like we programmed.
Summary
In this article, we take a look at the ADD instruction in ARM assembly language. We walk through assembling the code with GCC and using GDB (GNU Debugger) to monitor execution and inspect register values, demonstrating how the results reflect the programmed additions. Understanding such low-level behavior is essential in exploit development, where manipulating register values and controlling program flow—such as redirecting execution or crafting return-oriented programming (ROP) chains—depends on precise knowledge of how instructions like ADD affect the system state.
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