Playing with ptrace, Part II

In Part II of his series on ptrace, Pradeep tackles the more advanced topics of setting breakpoints and injecting code into running processes.


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Code injection doubts

sanmk's picture

Hi Pradeep,
Nice article...but I did not completely understand the code injection part.

The example you have explained inserts the code for printing "hello world" into a running process.
1. I did not exactly understand why you did the jump forward and backward steps.
Can you please elaborate on that?

2. I wrote a normal C program to print hello world:

int main()
printf("hello world\n");
return 0;

I generated the byte code for this program using gdb. I replaced the contents of
char insertcode[] array with this new bytecode and ran the program.
As you might have guessed, it didn't work . What is the difference between your and my implementation?

I carried out this experiment so as to be able to inject code without having to learn assembly language programming. How do I inject the code of normal C program, without having to use assembly coding?

hello.c didn't work for me (amd64)

Anonymous's picture

I coded it into:
void main()
"jmp forward\n"
"popq %rsi\n"
"movl $4, %eax\n"
"movl $2, %ebx\n"
"movl %esi, %ecx\n"
"movl $12, %edx\n"
"int $0x80\n"
"call backward\n"
".string \"Hello World\\n\"\n"


the instruction set you are

code_ninja's picture

the instruction set you are using is specific to intel's architecture, amd's architecture may differ and these instruction set will not run on amd. check out amd's manual for its instruction set

amd64 is not an x86

Anonymous's picture

he did say all sample code is for x86 only