examples: Add chmodsnoop example for tracing chmod() syscalls

[MSScaleXiaohei]

Pull Request Description

Summary

This PR adds a new example chmodsnoop to the BCC project, demonstrating how to trace chmod() system calls using BCC and eBPF.

Changes

• examples/tracing/chmodsnoop.py: Main example script that traces chmod() syscalls
• examples/tracing/chmodsnoop_example.txt: Example usage documentation

Features

The example demonstrates:

• Tracing system calls using kprobes/kretprobes
• Extracting string arguments from user space
• Filtering by PID (-p option)
• Showing only failed calls (-x option)
• Including timestamps (-T option)
• Formatting file permissions (mode) in octal format

Example Usage

# Trace all chmod() syscalls
sudo ./chmodsnoop

# Include timestamps
sudo ./chmodsnoop -T

# Only show failed calls
sudo ./chmodsnoop -x

# Filter by PID
sudo ./chmodsnoop -p 181

Testing

The code follows the same pattern as other tracing examples (e.g., statsnoop, opensnoop) and has been checked for:

• ✅ Code style compliance
• ✅ No linter errors
• ✅ Follows project conventions
• ✅ Proper documentation

Type of Change

• New example (examples directory)
• Bug fix
• Performance improvement
• Documentation update

Checklist

• Code follows project style guidelines
• Code is properly commented
• Example documentation included
• No breaking changes
• Tested locally (basic syntax check)

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Note: This is a simple example for the examples directory, focusing on demonstrating BCC capabilities rather than production use.