Add eBPF syscall-write-trace example

ebpf/README.md

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+## wolfSSL eBPF Examples
+
+This directory contains **eBPF-based observability examples** demonstrating how Linux eBPF can be used to monitor system calls, user-space wolfSSL operations, network activity, and TLS behavior **without modifying application code**.
+
+These examples are designed for:
+
+* **debugging**
+* **education**
+* **performance tracing**
+* **understanding TLS internals**
+* **research and experimentation**
+
+All examples require:
+
+* Linux kernel with eBPF support (4.19+ recommended)
+* clang/LLVM (for compiling `.bpf.c` programs)
+* libbpf and libelf
+* root privileges
+* A recent installation of wolfSSL if running TLS examples

ebpf/syscall-write-trace/.gitignore

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+# Binaries
+client-tcp
+server-tcp
+write_tracer
+*.o
+*.bpf.o
+*.log
+
+# Editor files
+*~
+*.swp
+.DS_Store
+

ebpf/syscall-write-trace/Makefile

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+# Makefile for syscall-write-trace example
+
+CC = gcc
+CLANG = clang
+
+CFLAGS = -O2 -g -Wall
+BPF_CFLAGS = -O2 -g -target bpf -D__TARGET_ARCH_$(ARCH)
+
+# Detect architecture for correct include path
+ARCH := $(shell uname -m)
+ifeq ($(ARCH),x86_64)
+    ARCH_DIR = x86_64-linux-gnu
+else ifeq ($(ARCH),aarch64)
+    ARCH_DIR = aarch64-linux-gnu
+else
+    ARCH_DIR = x86_64-linux-gnu
+endif
+
+BPF_INCLUDES = -I/usr/include -I/usr/include/$(ARCH_DIR)
+LIBBPF_LIBS = -lbpf -lelf -lz
+
+TARGETS = write_tracer write_tracer.bpf.o client-tcp server-tcp
+
+.PHONY: all clean help
+
+all: $(TARGETS)
+
+write_tracer.bpf.o: write_tracer.bpf.c
+	$(CLANG) $(BPF_CFLAGS) $(BPF_INCLUDES) -c $< -o $@
+
+write_tracer: write_tracer.c write_tracer.bpf.o
+	$(CC) $(CFLAGS) write_tracer.c -lelf -lz -lbpf -o write_tracer
+
+client-tcp: client-tcp.c
+	$(CC) $(CFLAGS) client-tcp.c -o client-tcp
+
+server-tcp: server-tcp.c
+	$(CC) $(CFLAGS) server-tcp.c -o server-tcp
+
+clean:
+	rm -f *.o write_tracer client-tcp server-tcp
+
+help:
+	@echo "Targets:"
+	@echo "  all       - Build syscall tracer and TCP demo apps"
+	@echo "  clean     - Remove binaries"
+	@echo ""
+	@echo "Instructions:"
+	@echo "  1. sudo ./write_tracer"
+	@echo "  2. ./server-tcp"
+	@echo "  3. ./client-tcp 127.0.0.1"

ebpf/syscall-write-trace/README.md

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+# syscall-write-trace
+
+### eBPF Example: Tracing Plaintext at the `write()` Syscall Boundary
+
+This example demonstrates how to use **Linux eBPF** to intercept the `write()` system call and extract plaintext data **before the kernel performs any buffering, encryption, or processing**.
+It uses a simple TCP client/server pair to generate predictable network writes and an eBPF tracepoint program to observe them.
+
+This example is part of the **wolfSSL eBPF observability suite**, designed to help developers understand how plaintext flows through the system and how eBPF can be used to debug, monitor, or study application behavior without modifying application code.
+
+---
+
+# 📌 **Problem**
+
+When debugging network applications, especially TLS applications, developers often want to inspect:
+
+* What plaintext is being written
+* What data is being sent to the network
+* Whether buffers contain what we expect
+* Whether the application or kernel is modifying data
+* Whether the problem is at the app layer, kernel layer, or crypto layer
+
+However, once an application calls:
+
+```
+write(fd, buffer, count)
+```
+
+the kernel:
+
+* does **not** expose the plaintext
+* may buffer or coalesce writes
+* may encrypt data (TLS offload, QUIC, etc.)
+* hides memory from tools
+* provides no visibility into the user buffer
+
+Traditional debugging tools (tcpdump, Wireshark, strace) **cannot see the plaintext** before encryption or kernel processing.
+
+This creates a visibility gap.
+
+---
+
+# 🎯 **Solution**
+
+We attach an **eBPF tracepoint** to:
+
+```
+tracepoint/syscalls/sys_enter_write
+```
+
+This gives us:
+
+* access to the syscall arguments
+* the calling process’s PID/TID
+* the file descriptor
+* the byte count
+* the raw user pointer to the data
+* ability to read the plaintext with `bpf_probe_read_user()`
+
+The eBPF program:
+
+1. Filters events only from a target process (`client-tcp`)
+2. Copies up to 255 bytes of user-space buffer safely
+3. Sends them to user-space via a perf buffer
+4. The userspace loader prints ASCII and hex output
+
+This provides **perfect visibility** into the plaintext leaving the application.
+
+---
+
+# 🧩 **Architecture**
+
+```
+   TCP Client App (user space)
+         |
+         | 1. call write(fd, buf, count)
+         v
+ ┌──────────────────────────────┐
+ │ tracepoint: sys_enter_write  │  ← eBPF hook runs BEFORE write executes
+ │ eBPF program:                │
+ │  - filters process name       │
+ │  - reads buffer from user mem │
+ │  - emits event via perf buf   │
+ └───────────────┬──────────────┘
+                 |
+                 | 2. event (plaintext)
+                 v
+        Userspace Loader (write_tracer)
+        --------------------------------
+        - loads BPF program
+        - attaches tracepoint
+        - opens perf buffer
+        - prints plaintext
+                 |
+                 | 3. human-readable output
+                 v
+              Terminal
+```
+
+---
+
+# 🔍 **Detailed Walkthrough**
+
+## 1. The Sample Applications
+
+### `server-tcp.c`
+
+A simple TCP echo server on port 11111:
+
+* waits for a connection
+* receives a message
+* prints it
+* echoes back a canned response
+* loops
+
+### `client-tcp.c`
+
+A matching TCP client:
+
+* prompts user for input
+* writes it to the server
+* prints server response
+
+These programs provide **predictable write() calls** for tracing.
+
+---
+
+## 2. The eBPF Program: `write_tracer.bpf.c`
+
+Hooks:
+
+```
+tracepoint/syscalls/sys_enter_write
+```
+
+Key details:
+
+### ✔ Event Filtering
+
+Checks the process name (`comm`) to avoid tracing all processes.
+
+### ✔ Safe Memory Access
+
+Uses:
+
+```
+bpf_probe_read_user()
+```
+
+to copy user memory safely, limited to 255 bytes (verifier-friendly bound).
+
+### ✔ Perf Buffer Emission
+
+Writes events to a ring buffer consumed by user-space.
+
+### ✔ Struct of event data
+
+Contains:
+
+* PID, TID
+* FD
+* count
+* process name
+* captured data
+
+---
+
+## 3. The Userspace Loader: `write_tracer.c`
+
+It:
+
+1. Raises RLIMIT_MEMLOCK
+2. Loads `write_tracer.bpf.o`
+3. Attaches the tracepoint
+4. Opens perf buffer
+5. Pretty-prints events:
+
+```
+=== WRITE SYSCALL INTERCEPTED ===
+Process: client-tcp (PID: 1234)
+FD: 3
+Count: 13 bytes
+Data: "hello world!"
+Hex: 68 65 6c ...
+```
+
+This gives full plaintext visibility.
+
+---
+
+# 🚀 **How to Build**
+
+Dependencies (Ubuntu):
+
+```bash
+sudo apt install clang llvm libbpf-dev libelf-dev zlib1g-dev build-essential
+```
+
+Then:
+
+```bash
+make
+```
+
+This compiles:
+
+* TCP client/server
+* eBPF program (`write_tracer.bpf.o`)
+* userspace loader (`write_tracer`)
+
+---
+
+# ▶️ **How to Run**
+
+### 1. Terminal #1 — Start the tracer
+
+```bash
+sudo ./write_tracer
+```
+
+### 2. Terminal #2 — Start the TCP server
+
+```bash
+./server-tcp
+```
+
+### 3. Terminal #3 — Run client and type message
+
+```bash
+./client-tcp 127.0.0.1
+```
+
+Tracer output:
+
+```
+=== WRITE SYSCALL INTERCEPTED ===
+Process: client-tcp
+File Descriptor: 3
+Write Count: 13 bytes
+Data (first 13 bytes): hello world!
+```
+
+---
+
+# 🎁 **Benefits of This Example**
+
+### ✔ Shows how to read plaintext before kernel/network/TLS processing
+
+### ✔ Demonstrates safe buffer access in eBPF
+
+### ✔ Demonstrates filtering (PID, process name)
+
+### ✔ Teaches core eBPF concepts: tracepoints, perf buffers, verifier constraints
+
+### ✔ Foundation for more advanced examples (TLS plaintext, handshake tracing)
+
+### ✔ Helps wolfSSL developers debug TLS behavior
+
+### ✔ Useful for application developers integrating wolfSSL
+
+---
+
+# ⚙️ **Nitty-Gritty Details**
+
+### 1. Why tracepoint instead of kprobe?
+
+Tracepoints are:
+
+* stable
+* argument offsets fixed
+* preferred for syscall entry tracing
+
+Allows verifier to analyze program more easily.
+
+### 2. Why use process name filtering?
+
+Without it, the tracer prints:
+
+* output from bash
+* systemd
+* everything reading/writing
+
+Filtering avoids noise.
+
+### 3. Why limit buffer to 255 bytes?
+
+Verifier restrictions require fixed bounded copy sizes.
+A 255-byte buffer is safe and sufficient for demos.
+
+### 4. Why use perf buffer instead of ringbuf?
+
+Perf buffer is more compatible with older kernels (e.g., Ubuntu LTS).
+Perfect for examples.
+
+### 5. Why use simple TCP client/server?
+
+Consistent, predictable write() calls make tracing easy to demo.