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How does a debugger set a breakpoint?
Software Breakpoint This is accomplished by injecting a breakpoint instruction or when that is not supported by inserting an instruction that causes a fault that halts the core. With software breakpoints, a debugger can essentially expose an “unlimited” number of breakpoints.
How are debuggers implemented?
The simplified answer is: When you put a break-point into the program, the debugger replaces your code at that point with a int3 instruction which is a software interrupt. As an effect the program is suspended and the debugger is called.
How do software breakpoints work?
Thankfully, there are also software breakpoints. A software breakpoint is typically an instruction that temporarily replaces an instruction in RAM that is either an illegal instruction and causes a fault or is designed to cause the application to break. When the CPU reaches this instruction, it halts execution.
How is a debug trap implemented by a processor?
When such an instruction is executed, the CPU treats it like an interrupt – stops its normal flow of execution, saves its state and jumps to a handler routine. Such “traps” allow many of the wonders of modern OSes (task scheduling, virtual memory, memory protection, debugging) to be implemented efficiently.
How do you implement a breakpoint?
To set a breakpoint in source code, click in the far left margin next to a line of code. You can also select the line and press F9, select Debug > Toggle Breakpoint, or right-click and select Breakpoint > Insert breakpoint. The breakpoint appears as a red dot in the left margin.
How is ptrace implemented?
The ptrace system call allows the parent process to inspect the attached child. For example, in Linux, strace (which is implemented with the ptrace system call) can inspect the system calls invoked by the child process. When the attached child process invokes a system call, the ptracing parent process can be notified.
Is system call a trap?
System calls on the Intel architecture involve a special trap instruction. This instruction both modifies the flow of control and the privilege state of the processor. The table is a sequence of addresses, each address a function for handling the various interrupts and exceptions, including the trap.
What is the difference between an interrupt and a trap?
The main difference between trap and interrupt is that trap is triggered by a user program to invoke OS functionality while interrupt is triggered by a hardware device to allow the processor to execute the corresponding interrupt handler routine.
How do breakpoints work in a debugger?
Breakpoints are one of the two main pillars of debugging – the other being able to inspect values in the debugged process’s memory. We’ve already seen a preview of the other pillar in part 1 of the series, but breakpoints still remain mysterious. By the end of this article, they won’t be.
How are breakpoints implemented in the x86 architecture?
To implement breakpoints on the x86 architecture, software interrupts (also known as “traps”) are used. Before we get deep into the details, I want to explain the concept of interrupts and traps in general.
Why is one byte important in a debugger?
(This one byte form is valuable because it can be used to replace the first byte of any instruction with a breakpoint, including other one byte instructions, without over-writing other code). The part in parens is important, but it’s still too early to explain it.
How does a breakpoint in a CPU work?
This circuitry notices an activation of the interrupt and makes the CPU stop its current execution, save its state, and jump to a predefined address where a handler routine for the interrupt is located. When the handler finishes its work, the CPU resumes execution from where it stopped.