Project 7: Linux Syscall ABI Tracer

A tracer that logs syscall numbers and argument registers at runtime (conceptually similar to strace but narrower).

Quick Reference

Attribute Value
Difficulty Level 4
Time Estimate 2 weeks
Main Programming Language Python or C (Alternatives: Rust, Go)
Alternative Programming Languages Rust, Go
Coolness Level Level 4
Business Potential 1
Prerequisites System Interface - Privilege, Exceptions, and Syscalls, System Interface - Privilege, Exceptions, and Syscalls
Key Topics System Interface - Privilege, Exceptions, and Syscalls, System Interface - Privilege, Exceptions, and Syscalls

1. Learning Objectives

By completing this project, you will:

  1. Explain why linux syscall abi tracer reveals key x86-64 behaviors.
  2. Build a deterministic tool with clear, inspectable output.
  3. Validate correctness against a golden reference output.
  4. Connect the tool output to ABI and architecture rules.
  5. Syscall ABIs expose the exact boundary between user code and kernel services.

2. All Theory Needed (Per-Concept Breakdown)

System Interface - Privilege, Exceptions, and Syscalls

Fundamentals User-mode code does not directly control the hardware. It runs at a lower privilege level and must use system calls to request services. The CPU provides hardware mechanisms for privilege transitions, exceptions, and interrupts. The OS configures these mechanisms, defining how user code enters the kernel and how the kernel returns. In x86-64, the syscall instruction is a primary gateway to the kernel in 64-bit mode. Exception and interrupt handling is governed by descriptor tables and privilege checks. Understanding these boundaries is essential for debugging crashes, interpreting signals, and writing assembly that interacts with the OS. (Sources: Intel SDM, System V AMD64 ABI)

Deep Dive Privilege levels on x86-64 are often summarized as rings, with ring 0 for the kernel and ring 3 for user code. Although the architecture supports four rings, most modern OSes use two. The transition from user to kernel is tightly controlled; user code cannot just jump into kernel space. Instead, it uses system call instructions (such as syscall) that trigger a controlled transition. The CPU saves certain state, switches to a privileged stack, and transfers control to a kernel entry point configured by the OS. The kernel then validates arguments, performs the requested service, and returns to user mode, restoring state.

Exceptions and interrupts are different but related. Exceptions are synchronous events triggered by the current instruction (for example, divide-by-zero, invalid opcode, or page fault). Interrupts are asynchronous events triggered by hardware or timers. Both use the interrupt descriptor table (IDT) to locate handlers. The CPU pushes an error code or context onto the stack and changes privilege levels if required. This is why an exception can appear to “teleport” control flow; it is a hardware-driven branch with strict rules. For assembly programmers, this means that any instruction can potentially trigger an exception, and that the OS may deliver a signal or exception to the process.

System call ABIs define which registers carry system call numbers and arguments. On Linux x86-64, the system call number is placed in a designated register and arguments are passed in a specific register order. Certain registers are clobbered by the syscall instruction itself. This is defined by the ABI and the kernel conventions. If you do not respect those rules, the kernel may interpret your arguments incorrectly, leading to crashes or undefined behavior. On Windows, the user-mode system call interface is not stable in the same way; documented system calls are wrapped by higher-level APIs. The assembly-level calling convention is still defined, but its details are more platform-specific.

Signals and exceptions are often observed through user-space tools. For example, a segmentation fault is the result of a page fault exception that the OS translates into a signal. Understanding which instruction caused the fault and why is a core assembly skill. That analysis requires knowledge of instruction side effects, address translation, and privilege transitions.

In short, the system interface is the boundary between your assembly code and the OS. It is also the boundary between defined behavior and crashes. By mastering the rules of syscalls and exceptions, you gain the ability to reason about crashes, inspect kernel interactions, and build low-level tools such as tracers and sandboxes.

How this fits on projects

  • Projects 7 and 8 focus on syscall conventions and exception/interrupt flow.
  • Projects 9 and 10 touch privilege when analyzing loaders and relocation behavior.

Definitions & key terms

  • Privilege level: CPU execution ring (user vs kernel).
  • System call: Controlled transition to kernel services.
  • Exception: Synchronous fault triggered by an instruction.
  • Interrupt: Asynchronous event handled by the CPU/OS.
  • IDT: Interrupt Descriptor Table (maps vectors to handlers).

Mental model diagram

USER MODE (Ring 3)
   |
   | syscall / exception
   v
KERNEL MODE (Ring 0)
   |
   | return-from-syscall / iret
   v
USER MODE (Ring 3)

How it works

  1. User code issues a syscall instruction.
  2. CPU switches privilege and jumps to kernel entry.
  3. Kernel validates and executes the requested service.
  4. Kernel returns to user mode, restoring registers.

Invariants and failure modes:

  • Invariant: User code cannot jump directly into kernel space.
  • Failure: Incorrect syscall argument registers yield wrong behavior.
  • Invariant: Exceptions transfer control via IDT.
  • Failure: Misconfigured IDT or invalid instruction causes crash.

Minimal concrete example (pseudo-assembly, not real code)

# PSEUDOCODE ONLY
SYSCALL_NUM = OPEN_FILE
ARG1 = PTR_PATH
ARG2 = FLAGS
ARG3 = MODE
SYSCALL
RET = RESULT_REG

Common misconceptions

  • “Syscalls are just function calls.” They are privilege transitions.
  • “Exceptions only happen on errors.” They can be used for control flow.
  • “Windows syscalls are stable.” They are not part of the public ABI.

Check-your-understanding questions

  1. What is the difference between an exception and an interrupt?
  2. Why are syscalls a controlled entry to the kernel?
  3. Which registers are clobbered by a syscall on Linux?

Check-your-understanding answers

  1. Exceptions are synchronous; interrupts are asynchronous.
  2. To enforce security and isolation between user and kernel.
  3. The ABI defines specific clobbers; they must be saved if needed.

Real-world applications

  • Writing syscall tracers
  • Debugging crashes and segmentation faults
  • Building sandboxes and seccomp-like policies

Where you will apply it Projects 7, 8, 9

References

  • Intel 64 and IA-32 Architectures Software Developer’s Manual (Intel)
  • System V AMD64 ABI Draft 0.99.7
  • “The Linux Programming Interface” by Michael Kerrisk - Ch. 3, 4

Key insights The OS boundary is the most important boundary you will ever cross in assembly.

Summary Syscalls and exceptions define how user code interacts with the kernel, and misunderstanding them leads to crashes.

Homework/Exercises to practice the concept

  • Map a high-level API call to the system call boundary it ultimately uses.
  • Draw a timeline of events for a page fault leading to a signal.

Solutions to the homework/exercises

  • Identify the system call number and arguments used by the API.
  • Show fault, kernel handler, signal dispatch, and user handler.

    System Interface - Privilege, Exceptions, and Syscalls

Fundamentals User-mode code does not directly control the hardware. It runs at a lower privilege level and must use system calls to request services. The CPU provides hardware mechanisms for privilege transitions, exceptions, and interrupts. The OS configures these mechanisms, defining how user code enters the kernel and how the kernel returns. In x86-64, the syscall instruction is a primary gateway to the kernel in 64-bit mode. Exception and interrupt handling is governed by descriptor tables and privilege checks. Understanding these boundaries is essential for debugging crashes, interpreting signals, and writing assembly that interacts with the OS. (Sources: Intel SDM, System V AMD64 ABI)

Deep Dive Privilege levels on x86-64 are often summarized as rings, with ring 0 for the kernel and ring 3 for user code. Although the architecture supports four rings, most modern OSes use two. The transition from user to kernel is tightly controlled; user code cannot just jump into kernel space. Instead, it uses system call instructions (such as syscall) that trigger a controlled transition. The CPU saves certain state, switches to a privileged stack, and transfers control to a kernel entry point configured by the OS. The kernel then validates arguments, performs the requested service, and returns to user mode, restoring state.

Exceptions and interrupts are different but related. Exceptions are synchronous events triggered by the current instruction (for example, divide-by-zero, invalid opcode, or page fault). Interrupts are asynchronous events triggered by hardware or timers. Both use the interrupt descriptor table (IDT) to locate handlers. The CPU pushes an error code or context onto the stack and changes privilege levels if required. This is why an exception can appear to “teleport” control flow; it is a hardware-driven branch with strict rules. For assembly programmers, this means that any instruction can potentially trigger an exception, and that the OS may deliver a signal or exception to the process.

System call ABIs define which registers carry system call numbers and arguments. On Linux x86-64, the system call number is placed in a designated register and arguments are passed in a specific register order. Certain registers are clobbered by the syscall instruction itself. This is defined by the ABI and the kernel conventions. If you do not respect those rules, the kernel may interpret your arguments incorrectly, leading to crashes or undefined behavior. On Windows, the user-mode system call interface is not stable in the same way; documented system calls are wrapped by higher-level APIs. The assembly-level calling convention is still defined, but its details are more platform-specific.

Signals and exceptions are often observed through user-space tools. For example, a segmentation fault is the result of a page fault exception that the OS translates into a signal. Understanding which instruction caused the fault and why is a core assembly skill. That analysis requires knowledge of instruction side effects, address translation, and privilege transitions.

In short, the system interface is the boundary between your assembly code and the OS. It is also the boundary between defined behavior and crashes. By mastering the rules of syscalls and exceptions, you gain the ability to reason about crashes, inspect kernel interactions, and build low-level tools such as tracers and sandboxes.

How this fits on projects

  • Projects 7 and 8 focus on syscall conventions and exception/interrupt flow.
  • Projects 9 and 10 touch privilege when analyzing loaders and relocation behavior.

Definitions & key terms

  • Privilege level: CPU execution ring (user vs kernel).
  • System call: Controlled transition to kernel services.
  • Exception: Synchronous fault triggered by an instruction.
  • Interrupt: Asynchronous event handled by the CPU/OS.
  • IDT: Interrupt Descriptor Table (maps vectors to handlers).

Mental model diagram

USER MODE (Ring 3)
   |
   | syscall / exception
   v
KERNEL MODE (Ring 0)
   |
   | return-from-syscall / iret
   v
USER MODE (Ring 3)

How it works

  1. User code issues a syscall instruction.
  2. CPU switches privilege and jumps to kernel entry.
  3. Kernel validates and executes the requested service.
  4. Kernel returns to user mode, restoring registers.

Invariants and failure modes:

  • Invariant: User code cannot jump directly into kernel space.
  • Failure: Incorrect syscall argument registers yield wrong behavior.
  • Invariant: Exceptions transfer control via IDT.
  • Failure: Misconfigured IDT or invalid instruction causes crash.

Minimal concrete example (pseudo-assembly, not real code)

# PSEUDOCODE ONLY
SYSCALL_NUM = OPEN_FILE
ARG1 = PTR_PATH
ARG2 = FLAGS
ARG3 = MODE
SYSCALL
RET = RESULT_REG

Common misconceptions

  • “Syscalls are just function calls.” They are privilege transitions.
  • “Exceptions only happen on errors.” They can be used for control flow.
  • “Windows syscalls are stable.” They are not part of the public ABI.

Check-your-understanding questions

  1. What is the difference between an exception and an interrupt?
  2. Why are syscalls a controlled entry to the kernel?
  3. Which registers are clobbered by a syscall on Linux?

Check-your-understanding answers

  1. Exceptions are synchronous; interrupts are asynchronous.
  2. To enforce security and isolation between user and kernel.
  3. The ABI defines specific clobbers; they must be saved if needed.

Real-world applications

  • Writing syscall tracers
  • Debugging crashes and segmentation faults
  • Building sandboxes and seccomp-like policies

Where you will apply it Projects 7, 8, 9

References

  • Intel 64 and IA-32 Architectures Software Developer’s Manual (Intel)
  • System V AMD64 ABI Draft 0.99.7
  • “The Linux Programming Interface” by Michael Kerrisk - Ch. 3, 4

Key insights The OS boundary is the most important boundary you will ever cross in assembly.

Summary Syscalls and exceptions define how user code interacts with the kernel, and misunderstanding them leads to crashes.

Homework/Exercises to practice the concept

  • Map a high-level API call to the system call boundary it ultimately uses.
  • Draw a timeline of events for a page fault leading to a signal.

Solutions to the homework/exercises

  • Identify the system call number and arguments used by the API.
  • Show fault, kernel handler, signal dispatch, and user handler.

3. Project Specification

3.1 What You Will Build

A tracer that logs syscall numbers and argument registers at runtime (conceptually similar to strace but narrower).

Why this teaches x86-64: Syscall ABIs expose the exact boundary between user code and kernel services.

Included:

  • Deterministic CLI output for a fixed input
  • Clear mapping between inputs and architectural meaning
  • A small test suite with edge cases

Excluded:

  • Full compiler or full disassembler coverage
  • Production-grade UI or packaging

3.2 Functional Requirements

  1. Deterministic Output: Same input yields identical output.
  2. Architecture-Aware: Output references ABI/ISA rules where relevant.
  3. Validation Mode: Provide a compare mode against a golden output.

3.3 Non-Functional Requirements

  • Performance: Fast enough for small inputs and interactive use.
  • Reliability: Handles malformed inputs with clear errors.
  • Usability: Outputs are readable and documented.

3.4 Example Usage / Output

$ x64syscall-trace ./demo_app

SYSCALL 0x01
  ARG1=0x0000000000402000
  ARG2=0x0000000000000012
  ARG3=0x0000000000000001
  RESULT=0x0000000000000012

SYSCALL 0x02
  ARG1=0x0000000000403000
  ARG2=0x0000000000000000
  ARG3=0x0000000000000000
  RESULT=0x0000000000000003

3.5 Data Formats / Schemas / Protocols

  • Input format: line-oriented text or hex bytes (documented in README)
  • Output format: stable, human-readable report with labeled fields

3.6 Edge Cases

  • Empty input or missing fields
  • Invalid numeric values or malformed hex
  • Inputs that exercise maximum/minimum bounds

3.7 Real World Outcome

This section is your golden reference. Match it exactly.

3.7.1 How to Run (Copy/Paste)

  • Build: (if needed) make or equivalent
  • Run: P07-linux-syscall-abi-tracer with sample input
  • Working directory: project root

3.7.2 Golden Path Demo (Deterministic)

Run with the provided demo input and confirm output matches the transcript.

3.7.3 If CLI: exact terminal transcript

$ x64syscall-trace ./demo_app

SYSCALL 0x01
  ARG1=0x0000000000402000
  ARG2=0x0000000000000012
  ARG3=0x0000000000000001
  RESULT=0x0000000000000012

SYSCALL 0x02
  ARG1=0x0000000000403000
  ARG2=0x0000000000000000
  ARG3=0x0000000000000000
  RESULT=0x0000000000000003

4. Solution Architecture

4.1 High-Level Design

INPUT -> PARSER -> MODEL -> RENDERER -> REPORT

4.2 Key Components

Component Responsibility Key Decisions
Parser Turn input into structured records Strict vs permissive parsing
Model Apply ISA/ABI rules Deterministic state transitions
Renderer Produce readable output Stable formatting

4.4 Data Structures (No Full Code)

  • Record: holds one instruction/event with decoded fields
  • State: represents register/flag or address state
  • Report: list of formatted output lines

4.4 Algorithm Overview

Key Algorithm: Parse and Evaluate

  1. Parse input into records.
  2. Apply rules to update state.
  3. Render the state and summary output.

Complexity Analysis:

  • Time: O(n) over input records
  • Space: O(n) for report output

5. Implementation Guide

5.1 Development Environment Setup

# Ensure basic tools are installed
# build-essential or clang, plus objdump/readelf if needed

5.2 Project Structure

project-root/
├── src/
│   ├── main.*
│   ├── parser.*
│   └── model.*
├── tests/
│   └── test_cases.*
└── README.md

5.3 The Core Question You’re Answering

What exactly crosses the boundary when user code asks the kernel for help?

5.4 Concepts You Must Understand First

  1. Syscall ABI
    • Which registers carry syscall number and arguments?
    • Book Reference: “The Linux Programming Interface” - Ch. 3-4
  2. Privilege transitions
    • Why do syscalls change CPU mode?
    • Book Reference: “Operating Systems: Three Easy Pieces” - Ch. 10

5.5 Questions to Guide Your Design

  1. Tracing model
    • Will you trace at entry, exit, or both?
    • How will you map syscall numbers to names?
  2. Safety
    • How will you avoid corrupting the target process state?
    • How will you handle errors and signals?

5.6 Thinking Exercise

Syscall Boundary

Sketch the register state at syscall entry and exit. Explain which registers are preserved and which are clobbered.

Questions to answer:

  • Which registers are safe to use after syscall?
  • How would you detect an error return?

5.7 The Interview Questions They’ll Ask

  1. “Which registers carry Linux syscall arguments on x86-64?”
  2. “What does syscall clobber?”
  3. “How does a syscall differ from a function call?”
  4. “How would you trace syscalls without ptrace?”
  5. “Why is syscall ABI different from the C ABI?”

5.8 Hints in Layers

Hint 1: Starting Point Use ptrace or a similar mechanism to inspect registers at syscall entry/exit.

Hint 2: Next Level Maintain a map of syscall numbers to names for readability.

Hint 3: Technical Details Log only the argument registers defined by the ABI and ignore the rest.

Hint 4: Tools/Debugging Compare your output with a standard strace run.

5.9 Books That Will Help

Topic Book Chapter
Syscalls “The Linux Programming Interface” Ch. 3-4
OS boundary “Operating Systems: Three Easy Pieces” Ch. 10

5.10 Implementation Phases

Phase 1: Foundation (2-3 days)

Goals:

  • Parse input format
  • Produce a minimal output Tasks:
    1. Define input grammar and example files.
    2. Implement a minimal parser and renderer. Checkpoint: Golden output matches a small input.

Phase 2: Core Functionality (1 week)

Goals:

  • Implement full rule set
  • Add validation and errors Tasks:
    1. Implement rule engine for core cases.
    2. Add error handling for invalid inputs. Checkpoint: All core tests pass.

Phase 3: Polish & Edge Cases (2-3 days)

Goals:

  • Add edge-case coverage
  • Improve output readability Tasks:
    1. Add edge-case tests.
    2. Refine output formatting and summary. Checkpoint: Output matches golden transcript for all cases.

5.11 Key Implementation Decisions

Decision Options Recommendation Rationale
Input format Text, JSON Text Easiest to audit and diff
Output format Plain text, JSON Plain text Matches CLI tooling

6. Testing Strategy

6.1 Test Categories

Category Purpose Examples
Unit Tests Validate parsing and rule application Valid/invalid inputs
Integration Tests End-to-end output comparison Golden transcripts
Edge Case Tests Stress unusual inputs Empty input, max values

6.2 Critical Test Cases

  1. Minimal Input: One record, verify output.
  2. Boundary Values: Largest/smallest values.
  3. Malformed Input: Ensure clean error messages.

6.3 Test Data

INPUT: sample_min.txt
EXPECTED: matches golden transcript

7. Common Pitfalls & Debugging

7.1 Frequent Mistakes

Pitfall Symptom Solution
Wrong assumptions Output mismatches Re-read ABI/ISA rules
Off-by-one parsing Missing fields Add explicit length checks
Ambiguous output Hard to verify Add labels and separators

Project-specific pitfalls

Problem 1: “Arguments are garbage”

  • Why: Using the wrong register mapping for syscall ABI.
  • Fix: Follow the ABI register order strictly.
  • Quick test: Trace a known syscall like write and compare output.

7.2 Debugging Strategies

  • Golden diffing: Use diff to compare outputs line by line.
  • State logging: Print intermediate state after each step.

7.3 Performance Traps

  • Avoid over-optimizing; correctness and determinism matter most.

8. Extensions & Challenges

8.1 Beginner Extensions

  • Add a new input case and golden output
  • Add a summary line with counts

8.2 Intermediate Extensions

  • Add JSON output mode
  • Add validation warnings for suspicious inputs

8.3 Advanced Extensions

  • Support additional ABI or instruction variants
  • Integrate with a real binary to collect inputs

9. Real-World Connections

9.1 Industry Applications

  • Profilers and tracers: Use similar decoding and state models.
  • Security analysis: Use precise ABI knowledge to interpret crashes.
  • objdump: reference tool for binary inspection.
  • llvm-objdump: LLVM-based disassembly and inspection.

9.3 Interview Relevance

  • ABI and calling conventions are common systems interview topics.
  • Explaining decoding and linking demonstrates low-level fluency.

10. Resources

10.1 Essential Reading

  • Intel 64 and IA-32 Architectures Software Developer’s Manual - ISA reference
  • System V AMD64 ABI Draft 0.99.7 - calling convention rules

10.2 Video Resources

  • Vendor and university lectures on x86-64 and ABIs (search official channels)