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Debugging, Disassembly & Reversing in Linux for x64 Architecture

GDB Disassembly Output—No Optimization

Explore how to use GDB to disassemble and debug a simple arithmetic program compiled without optimization. Learn to set breakpoints, disable address randomization, and analyze assembly output to better understand program execution and memory changes within the GDB environment.

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Arithmetic program

Let’s rewrite our arithmetic program in way that works in both C and C++. Corresponding assembly language instructions are written in the comments:

C++
int a, b;
int main(int argc, char* argv[])
{
   // Assign a value to variable a and b
   a = 1;        // movl $1, a
   b = 1;        // movl $1, b
   // Add b and a and store in variable b
   b = b + a;    // mov a, %eax
                 // mov b, %edx
                 // add %edx, %eax
                 // mov %eax, b
   // Increment variable a by one
   ++a;          // mov a, %eax
                 // add $1, %eax
                 // mov %eax, a
   // Multiply a with b and store into b
   b = b * a;    // mov b, %edx
                 // mov a, %eax
                 // imul %edx, %eax
                 // mov %eax, b
                 // results: (a) = 2 and (b) = 4
   // Return 0 means it executed successfully
   return 0;
}

Disassembly in no-optimization mode

It’s easier to track the progress of our program during debugging if we can recompile without optimization. The steps to initialize the GDB environment are below:

Note: You can practice all the commands in the coding playground provided at the end of the lesson.

Let’s compile and link the program in no optimization mode (default):

gcc
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