feat: Implement BPF struct_ops example with custom kernel module and user-space loader

src/features/struct_ops/README.md

@@ -0,0 +1,74 @@
+# BPF struct_ops Example with Custom Kernel Module
+
+This example demonstrates BPF struct_ops functionality using a custom kernel module that defines struct_ops operations triggered via a proc file write.
+
+## Overview
+
+struct_ops allows BPF programs to implement kernel subsystem operations dynamically. This example includes:
+
+1. **Kernel Module** (`module/hello.c`) - Defines `bpf_testmod_ops` struct_ops with three callbacks
+2. **BPF Program** (`struct_ops.bpf.c`) - Implements the struct_ops callbacks in BPF
+3. **User-space Loader** (`struct_ops.c`) - Loads the BPF program and triggers callbacks via `/proc/bpf_testmod_trigger`
+
+## Building and Running
+
+### 1. Build the kernel module:
+```bash
+cd module
+make
+cd ..
+```
+
+### 2. Load the kernel module:
+```bash
+sudo insmod module/hello.ko
+```
+
+### 3. Build the BPF program:
+```bash
+make
+```
+
+### 4. Run the example:
+```bash
+sudo ./struct_ops
+```
+
+### 5. Check kernel logs:
+```bash
+sudo dmesg -w
+```
+
+You should see output like:
+```
+bpf_testmod loaded with struct_ops support
+bpf_testmod_ops registered
+Calling struct_ops callbacks:
+BPF test_1 called!
+test_1() returned: 42
+BPF test_2 called: 10 + 20 = 30
+test_2(10, 20) returned: 30
+BPF test_3 called with buffer length 21
+First char: H
+test_3() called with buffer
+```
+
+### 6. Clean up:
+```bash
+sudo rmmod hello
+make clean
+```
+
+## How It Works
+
+1. The kernel module registers a custom struct_ops type `bpf_testmod_ops`
+2. It creates `/proc/bpf_testmod_trigger` - writing to this file triggers the callbacks
+3. The BPF program implements the three callbacks: `test_1`, `test_2`, and `test_3`
+4. The user-space program loads the BPF program and periodically writes to the proc file
+5. Each write triggers all registered callbacks, demonstrating BPF struct_ops in action
+
+## Troubleshooting
+
+- If you get "Failed to attach struct_ops", make sure the kernel module is loaded
+- Check `dmesg` for any error messages from the kernel module or BPF verifier
+- Ensure your kernel has CONFIG_BPF_SYSCALL=y and supports struct_ops