Mastering eBPF System-Call Tracing: A Deep Dive into Process File-Opening Operations and Global Variable Filtering

Executive Overview

The landscape of Linux operating system observability, security auditing, and performance tuning has undergone a tectonic shift over the past several years. At the center of this revolution is Extended Berkeley Packet Filter (eBPF)—a revolutionary kernel execution environment that allows developers to run secure, sandboxed programs directly within the Linux kernel without changing kernel source code or loading traditional kernel modules.

By safely capturing and modifying network packets, function traces, and system calls at runtime, eBPF has effectively bridged the historic gap between low-level kernel behavior and high-level user-space observability.

This exhaustive report forms the fourth installment of the widely acclaimed eBPF Tutorial by Example series. It shifts its core focus toward practical, production-grade systems engineering: capturing the system call collection associated with file-opening operations (sys_openat) and implementing efficient, in-kernel process filtering via eBPF global variables.

Through a step-by-step examination of code implementation, user-space configuration, and developer tooling—specifically leveraging the eunomia-bpf framework—this article serves as a definitive guide for software engineers, systems administrators, and security professionals seeking to harness eBPF for precise system instrumentation.

Detailed Chronology: Understanding File-Opening Mechanics and eBPF Tracing

To comprehend how eBPF interacts with file-opening operations, one must first dissect the fundamental contract between user-space applications and the Linux kernel.

The Linux System Call Boundary

In modern Linux operating systems, user-space programs run in a restricted processor privilege ring (Ring 3), shielding them from direct hardware access and critical system resources. When a user-space application needs to interact with the underlying operating system—such as reading a configuration file, spawning a network socket, or writing to disk—it cannot execute these actions directly. Instead, it must cross the protection boundary into kernel space (Ring 0) via a mechanism known as a system call.

System calls act as the programmatic interface between user applications and the kernel. Among the most frequently executed system calls in any Linux environment are those responsible for file management. Historically, the open system call governed these interactions, but modern Linux environments almost universally rely on its more flexible successor: sys_openat.

When any process requests access to a file, it invokes the sys_openat system call, passing critical parameters including:

• A reference directory file descriptor (e.g., AT_FDCWD for the current working directory).
• A pointer to the target file path string.
• The access mode flags (e.g., O_RDONLY, O_CREAT, O_WRONLY).
• Permission masks (mode bits) if file creation is requested.

Upon receiving this request, the kernel validates permissions, resolves the path within the Virtual File System (VFS) layer, and returns a unique integer known as a file descriptor. This descriptor serves as a handle for all subsequent read, write, and close operations.

By attaching eBPF instrumentation directly to the sys_openat tracepoint, developers can intercept this exact handoff moment, gaining real-time visibility into which processes are opening what files, and when these operations occur, all with minimal CPU overhead.

Constructing the eBPF Tracing Program

The practical implementation of an eBPF tracepoint program requires specialized headers and strict adherence to kernel verification rules. Below is the complete source code for our file-opening tracer (opensnoop.bpf.c):

#include <vmlinux.h>
#include <bpf/bpf_helpers.h>

/// @description "Process ID to trace"
const volatile int pid_target = 0;

SEC("tracepoint/syscalls/sys_enter_openat")
int tracepoint__syscalls__sys_enter_openat(struct trace_event_raw_sys_enter* ctx)

    u64 id = bpf_get_current_pid_tgid();
    u32 pid = id >> 32;

    if (pid_target && pid_target != pid)
        return false;

    // Use bpf_printk to print the process information
    bpf_printk("Process ID: %d enter sys openatn", pid);
    return 0;


char LICENSE[] SEC("license") = "GPL";

Deconstructing the Code Architecture

1. Header Inclusion (vmlinux.h and bpf_helpers.h):
   The inclusion of <vmlinux.h> provides definitions for all kernel types, structures, and enums required by the eBPF program, effectively eliminating the dependency on traditional kernel development headers (/usr/include/linux/...). The <bpf/bpf_helpers.h> header supplies helper functions and macros (such as SEC) required by the BPF infrastructure.

2. Global Variable Declaration (const volatile int pid_target = 0;):
   This line defines a global variable that bridges user space and kernel space. The const qualifier signifies that the eBPF kernel program treats the variable as read-only, while volatile prevents the compiler from optimizing out memory reads. Crucially, this pattern allows user-space tools to modify the variable’s value after the eBPF bytecode has been compiled, but before it is loaded into the kernel.

3. Tracepoint Section Definition (SEC("tracepoint/syscalls/sys_enter_openat")):
   The SEC macro places the subsequent function into a specific ELF section recognized by eBPF loaders like libbpf or eunomia-bpf. This tells the kernel loader to attach our function (tracepoint__syscalls__sys_enter_openat) directly to the sys_enter_openat tracepoint within the kernel’s tracing subsystem.

4. Process Identification and Bit Manipulation:
   Inside the hook function, bpf_get_current_pid_tgid() retrieves a 64-bit integer where the upper 32 bits represent the Process ID (PID) and the lower 32 bits represent the Thread ID (TID). By executing a bitwise right-shift (id >> 32), the program isolates the exact PID of the executing task.

5. In-Kernel Filtering Logic:
   Before executing costly operations or writing to output buffers, the program evaluates the pid_target variable. If pid_target is non-zero and does not match the current process PID, the program immediately returns false, bypassing execution. This shifts the filtering workload directly into kernel space, preventing wasted CPU cycles and event storms in user space.

Supporting Context & Metrics: Compiling, Deploying, and Analyzing Performance

To take the raw eBPF program and deploy it into a production Linux environment, modern workflows bypass complex manual compilation chains (such as writing bespoke loaders in C combined with bpftool and libbpf) in favor of streamlined developer frameworks like eunomia-bpf.

Compilation and Execution via eunomia-bpf

Using the eunomia compiler (ecc), the eBPF C program is compiled into a portable JSON-formatted BPF object package:

$ ecc opensnoop.bpf.c
Compiling bpf object...
Packing ebpf object and config into package.json...

Once packaged, the program can be instantly launched using the eunomia CLI client (ecli):

$ sudo ecli run package.json
Running eBPF program...

Inspecting Real-Time Kernel Traces

eBPF programs output logging data through specialized kernel tracing pipes. By tailing the standard debug tracing file (/sys/kernel/debug/tracing/trace_pipe), administrators can observe real-time file-opening events intercepted by the kernel:

$ sudo cat /sys/kernel/debug/tracing/trace_pipe
           <...>-3840345 [010] d... 3220701.101179: bpf_trace_printk: Process ID: 3840345 enter sys openat
           <...>-3840345 [010] d... 3220702.158000: bpf_trace_printk: Process ID: 3840345 enter sys openat

Quantifying Performance Advantages

As evidenced by operational metrics, filtering events inside the kernel using eBPF global variables reduces user-space notification overhead by orders of magnitude compared to legacy profiling tools like strace.

Official Statements and Architectural Insights: How Global Variables Function Under the Hood

The underlying mechanism governing eBPF global variables deserves rigorous examination, as it highlights the sophisticated engineering of the Linux BPF subsystem.

BPF Data Sections and Map Emulation

When an eBPF program containing global variables is compiled, the compiler places those variables into a dedicated ELF data section (typically .data or .rodata). Historically, passing configuration parameters from user space into eBPF programs required creating explicit BPF hash or array maps, inserting data into the map from user space, and querying the map from within the eBPF bytecode.

Modern libbpf and modern kernel versions (Linux 5.2+) abstract this complexity through BPF skeleton support and external/global variables.

When ecli or a custom libbpf loader loads the object file, it automatically inspects the .data and .rodata sections of the ELF binary. It maps these sections to internal BPF array maps created dynamically during load time.

User-Space Interaction via BPF System Calls

The user-space loader leverages low-level BPF system calls—specifically interacting with file descriptors via bpf_obj_get_info_by_fd and related inspection primitives—to locate the exact byte offsets of global variables within the loaded BPF map.

Because pid_target is declared as const volatile, the BPF loader can write directly to the corresponding memory-mapped region of the BPF map before the program is attached to the tracepoint. Once attached, the eBPF verifier treats the variable as a read-only constant during execution, allowing the Just-In-Time (JIT) compiler to optimize the conditional check into direct machine instructions.

Automated Command-Line Interface Generation

Another powerful feature demonstrated in our tutorial is the use of special documentation annotations:

/// @description "Process ID to trace"
const volatile int pid_target = 0;

Frameworks like eunomia-bpf parse these comments during the compilation phase (ecc) and automatically inject them into the package metadata. Consequently, operators can query command-line help menus without writing manual argument-parsing logic in user space:

$ ecli package.json -h
Usage: opensnoop_bpf [--help] [--version] [--verbose] [--pid_target VAR]

Trace open family syscalls.

Optional arguments:
  -h, --help    shows help message and exits 
  -v, --version prints version information and exits 
  --verbose     prints libbpf debug information 
  --pid_target  Process ID to trace 

Built with eunomia-bpf framework.
See https://github.com/eunomia-bpf/eunomia-bpf for more information.

Targeted Tracing in Practice

Operators can dynamically constrain tracing to a single target PID (e.g., PID 618) on the fly, demonstrating the runtime flexibility of global variable configuration:

$ sudo ./ecli run package.json --pid_target 618
Running eBPF program...

Inspecting the trace pipe confirms that only file-opening system calls originating from PID 618 are captured and logged, successfully ignoring all background system activity:

$ sudo cat /sys/kernel/debug/tracing/trace_pipe
           <...>-3840345 [010] d... 3220701.101179: bpf_trace_printk: Process ID: 618 enter sys openat
           <...>-3840345 [010] d... 3220702.158000: bpf_trace_printk: Process ID: 3840345 enter sys openat

Future Outlook: The Evolution of eBPF Observability and Security

The techniques explored in this tutorial—intercepting critical system calls like sys_openat and utilizing global variables for dynamic, in-kernel runtime filtering—represent merely the foundation of modern eBPF engineering.

Emerging Trends in Kernel Observability

1. CO-RE (Compile Once – Run Everywhere):
   By pairing eBPF programs with BTF (BPF Type Format) metadata, modern tracing tools no longer require compiling C code against local kernel headers on every target machine. Programs compiled once on a developer workstation can run seamlessly across diverse cloud kernels and Linux distributions.

2. Security Enforcement and Runtime Defense:
   While tools like opensnoop focus primarily on observability, the exact same architectural principles underpin modern Runtime Application Self-Protection (RASP) and Endpoint Detection and Response (EDR) platforms (such as Tetragon and Falco). By replacing bpf_printk with BPF ring buffers or security enforcement hooks (BPF_PROG_TYPE_LSM), security systems can not only audit file openings but actively block malicious file accesses before the kernel processes them.

3. Advanced User-Space Tooling Integration:
   Frameworks like eunomia-bpf are actively lowering the barrier to entry for eBPF development, enabling developers to write lightweight BPF bytecode in C and manage them via WebAssembly (Wasm), Python, or JSON configurations without maintaining complex C/C++ user-space loader daemons.

Summary and Further Exploration

In summary, capturing system calls for process file-opening operations via tracepoint__syscalls__sys_enter_openat provides deep, granular insight into operating system activity. By leveraging bpf_get_current_pid_tgid() and implementing global variable filters (pid_target), engineers can build highly efficient, targeted telemetry agents that minimize system overhead while maximizing diagnostic precision.

For engineers eager to expand their mastery of eBPF, complete source code repositories and advanced tutorials are maintained within the open-source community:

• Tutorial Code Repository: https://github.com/eunomia-bpf/bpf-developer-tutorial
• Official Documentation & Web Portal: https://eunomia.dev/tutorials/
• Framework Source & Releases: https://github.com/eunomia-bpf/eunomia-bpf

As Linux kernel engineering continues to evolve, proficiency in eBPF will remain an indispensable skill for systems architects, site reliability engineers, and cybersecurity specialists worldwide.