Implement a Simple Debugger with ptrace

Topic Overview

ptrace: The Foundation of All Debuggers

ptrace (process trace) is the Linux system call that underpins GDB, strace, and every other debugger on Linux. It allows one process to observe and control the execution of another process.

Key ptrace Operations

• PTRACE_TRACEME: Child declares it should be traced by its parent
• PTRACE_PEEKDATA / PTRACE_POKEDATA: Read/write child memory
• PTRACE_SINGLESTEP: Execute exactly one instruction
• PTRACE_CONT: Continue execution
• PTRACE_GETREGS / PTRACE_SETREGS: Read/write registers

Setting breakpoints with INT3:

1. Save original byte with PTRACE_PEEKDATA
2. Replace with 0xCC (INT3) using PTRACE_POKEDATA
3. On hit, restore original byte, single-step, re-insert 0xCC

For This Exercise

You will implement a simple debugger using ptrace that forks a child, sets a breakpoint, and single-steps. Building a minimal debugger from scratch teaches you exactly what GDB does under the hood when you set a breakpoint or step through code.

Debugger Event Loop

The parent calls waitpid after each PTRACE_CONT or PTRACE_SINGLESTEP. When the child stops on SIGTRAP (breakpoint hit), the parent reads registers with PTRACE_GETREGS, inspects state, and decides whether to continue or step. GDB wraps this loop with user-friendly commands, but the ptrace syscall sequence is identical. Security policies like YAMA ptrace_scope restrict which processes can be traced on hardened systems.

Your Task

A debugger sets a breakpoint by writing INT3 (0xCC, one byte) over an instruction. When the CPU executes it, the trap leaves RIP one byte past the breakpoint. The debugger reports the stop, restores the original byte, and rewinds RIP by 1 so that the instruction runs.

Simulate this for a program whose N instructions sit at base, base + 0x10, base + 0x20, ... and run once each, in order. A breakpoint is hit when its address is one of those instruction addresses; an address that appears twice is still one breakpoint. N can be as large as 10^9, so decide from the breakpoint addresses rather than by stepping through every instruction.

Input

BASE N (BASE in hex), then B (0 to 8), then B breakpoint addresses in hex.

Output

For each breakpoint hit, in address order, two lines:

cCopy
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then

cCopy
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Addresses are lowercase hex without leading zeros.