python-bpf/docs/user-guide/helpers.md

Helper Functions and Utilities

PythonBPF provides helper functions and utilities for BPF programs and userspace code.

BPF Helper Functions

BPF helper functions are kernel-provided functions that BPF programs can call to interact with the system. PythonBPF exposes these through the pythonbpf.helper module.

from pythonbpf.helper import pid, ktime, comm

Process and Task Information

pid()

Get the current process ID.

from pythonbpf.helper import pid

@bpf
@section("tracepoint/syscalls/sys_enter_open")
def trace_open(ctx: c_void_p) -> c_int64:
    process_id = pid()
    print(f"Process {process_id} opened a file")
    return c_int64(0)

Returns: c_int32 - The process ID of the current task

comm()

Get the current process command name (up to 16 characters).

from pythonbpf.helper import comm

@bpf
@section("tracepoint/syscalls/sys_enter_execve")
def trace_exec(ctx: c_void_p) -> c_int64:
    process_name = comm()
    print(f"Executing: {process_name}")
    return c_int64(0)

Returns: str(16) - The command name of the current task

Note: The returned string is limited to 16 characters (TASK_COMM_LEN).

uid()

Get the current user ID.

from pythonbpf.helper import uid

@bpf
@section("tracepoint/syscalls/sys_enter_open")
def trace_open(ctx: c_void_p) -> c_int64:
    user_id = uid()
    if user_id == 0:
        print("Root user opened a file")
    return c_int64(0)

Returns: c_int32 - The user ID of the current task

Time and Timing

ktime()

Get the current kernel time in nanoseconds since system boot.

from pythonbpf.helper import ktime

@bpf
@section("tracepoint/syscalls/sys_enter_read")
def measure_latency(ctx: c_void_p) -> c_int64:
    start_time = ktime()
    # Store for later comparison
    return c_int64(0)

Returns: c_int64 - Current time in nanoseconds

Use cases:

• Measuring latency
• Timestamping events
• Rate limiting
• Timeout detection

CPU Information

smp_processor_id()

Get the ID of the CPU on which the BPF program is running.

from pythonbpf.helper import smp_processor_id

@bpf
@section("tracepoint/sched/sched_switch")
def track_cpu(ctx: c_void_p) -> c_int64:
    cpu = smp_processor_id()
    print(f"Running on CPU {cpu}")
    return c_int64(0)

Returns: c_int32 - The current CPU ID

Use cases:

• Per-CPU statistics
• Load balancing analysis
• CPU affinity tracking

Memory Operations

probe_read()

Safely read data from kernel memory.

from pythonbpf.helper import probe_read

@bpf
def read_kernel_data(ctx: c_void_p) -> c_int64:
    dst = c_uint64(0)
    size = 8
    src = c_void_p(...)  # kernel address
    
    result = probe_read(dst, size, src)
    if result == 0:
        print(f"Read value: {dst}")
    return c_int64(0)

Parameters:

• dst - Destination buffer
• size - Number of bytes to read
• src - Source kernel address

Returns: c_int64 - 0 on success, negative on error

Safety: This function performs bounds checking and prevents invalid memory access.

probe_read_str()

Safely read a null-terminated string from kernel memory.

from pythonbpf.helper import probe_read_str

@bpf
def read_filename(ctx: c_void_p) -> c_int64:
    filename = str(256)
    src = c_void_p(...)  # pointer to filename in kernel
    
    result = probe_read_str(filename, src)
    if result > 0:
        print(f"Filename: {filename}")
    return c_int64(0)

Parameters:

• dst - Destination buffer (string)
• src - Source kernel address

Returns: c_int64 - Length of string on success, negative on error

deref()

Dereference a pointer safely.

from pythonbpf.helper import deref

@bpf
def access_pointer(ctx: c_void_p) -> c_int64:
    ptr = c_void_p(...)
    value = deref(ptr)
    print(f"Value at pointer: {value}")
    return c_int64(0)

Parameters:

• ptr - Pointer to dereference

Returns: The dereferenced value or 0 if null

Random Numbers

random()

Generate a pseudo-random 32-bit number.

from pythonbpf.helper import random

@bpf
@section("tracepoint/syscalls/sys_enter_open")
def sample_events(ctx: c_void_p) -> c_int64:
    # Sample 1% of events
    if (random() % 100) == 0:
        print("Sampled event")
    return c_int64(0)

Returns: c_int32 - A pseudo-random number

Use cases:

• Event sampling
• Load shedding
• A/B testing
• Randomized algorithms

Network Helpers

skb_store_bytes()

Store bytes into a socket buffer (for network programs).

from pythonbpf.helper import skb_store_bytes

@bpf
@section("classifier")
def modify_packet(ctx: c_void_p) -> c_int32:
    offset = 14  # Skip Ethernet header
    data = b"\x00\x01\x02\x03"
    size = len(data)
    
    result = skb_store_bytes(offset, data, size)
    return c_int32(0)

Parameters:

• offset - Offset in the socket buffer
• from_buf - Data to write
• size - Number of bytes to write
• flags - Optional flags

Returns: c_int64 - 0 on success, negative on error

Userspace Utilities

PythonBPF provides utilities for working with BPF programs from Python userspace code.

trace_pipe()

Read and display output from the kernel trace pipe.

from pythonbpf import trace_pipe

# After loading and attaching BPF programs
trace_pipe()

Description:

The trace_pipe() function reads from /sys/kernel/tracing/trace_pipe and displays BPF program output to stdout. This is the output from print() statements in BPF programs.

Usage:

from pythonbpf import bpf, section, bpfglobal, BPF, trace_pipe
from ctypes import c_void_p, c_int64

@bpf
@section("tracepoint/syscalls/sys_enter_execve")
def trace_exec(ctx: c_void_p) -> c_int64:
    print("Process started")  # This goes to trace_pipe
    return c_int64(0)

@bpf
@bpfglobal
def LICENSE() -> str:
    return "GPL"

b = BPF()
b.load_and_attach()
trace_pipe()  # Display BPF output

Behavior:

• Blocks until Ctrl+C is pressed
• Displays output in real-time
• Shows task name, PID, CPU, timestamp, and message
• Automatically handles trace pipe access errors

Requirements:

• Root or sudo access
• Accessible /sys/kernel/tracing/trace_pipe

trace_fields()

Parse one line from the trace pipe into structured fields.

from pythonbpf import trace_fields

# Read and parse trace output
task, pid, cpu, flags, ts, msg = trace_fields()
print(f"Task: {task}, PID: {pid}, CPU: {cpu}, Time: {ts}, Message: {msg}")

Returns: Tuple of (task, pid, cpu, flags, timestamp, message)

• task - String: Task/process name (up to 16 chars)
• pid - Integer: Process ID
• cpu - Integer: CPU number
• flags - Bytes: Trace flags
• timestamp - Float: Timestamp in seconds
• message - String: The actual trace message

Description:

The trace_fields() function reads one line from the trace pipe and parses it into individual fields. This is useful when you need programmatic access to trace data rather than just displaying it.

Usage:

from pythonbpf import bpf, section, bpfglobal, BPF, trace_fields
from ctypes import c_void_p, c_int64

@bpf
@section("tracepoint/syscalls/sys_enter_execve")
def trace_exec(ctx: c_void_p) -> c_int64:
    print(f"PID:{pid()}")
    return c_int64(0)

@bpf
@bpfglobal
def LICENSE() -> str:
    return "GPL"

b = BPF()
b.load_and_attach()

# Process trace events
try:
    while True:
        task, pid, cpu, flags, ts, msg = trace_fields()
        print(f"[{ts:.6f}] {task}({pid}) on CPU{cpu}: {msg}")
except KeyboardInterrupt:
    print("Stopped")

Error Handling:

• Raises ValueError if line cannot be parsed
• Skips lines about lost events
• Blocks waiting for next line

Helper Function Examples

Example 1: Latency Measurement

from pythonbpf import bpf, map, section, bpfglobal, BPF, trace_pipe
from pythonbpf.maps import HashMap
from pythonbpf.helper import pid, ktime
from ctypes import c_void_p, c_int64, c_uint32, c_uint64

@bpf
@map
def start_times() -> HashMap:
    return HashMap(key=c_uint32, value=c_uint64, max_entries=4096)

@bpf
@section("tracepoint/syscalls/sys_enter_read")
def read_start(ctx: c_void_p) -> c_int64:
    process_id = pid()
    start = ktime()
    start_times.update(process_id, start)
    return c_int64(0)

@bpf
@section("tracepoint/syscalls/sys_exit_read")
def read_end(ctx: c_void_p) -> c_int64:
    process_id = pid()
    start = start_times.lookup(process_id)
    
    if start:
        latency = ktime() - start
        print(f"Read latency: {latency} ns")
        start_times.delete(process_id)
    
    return c_int64(0)

@bpf
@bpfglobal
def LICENSE() -> str:
    return "GPL"

b = BPF()
b.load_and_attach()
trace_pipe()

Example 2: Process Tracking

from pythonbpf import bpf, section, bpfglobal, BPF, trace_pipe
from pythonbpf.helper import pid, comm, uid
from ctypes import c_void_p, c_int64

@bpf
@section("tracepoint/syscalls/sys_enter_execve")
def track_exec(ctx: c_void_p) -> c_int64:
    process_id = pid()
    process_name = comm()
    user_id = uid()
    
    print(f"User {user_id} started {process_name} (PID: {process_id})")
    return c_int64(0)

@bpf
@bpfglobal
def LICENSE() -> str:
    return "GPL"

b = BPF()
b.load_and_attach()
trace_pipe()

Example 3: CPU Load Monitoring

from pythonbpf import bpf, map, section, bpfglobal, BPF
from pythonbpf.maps import HashMap
from pythonbpf.helper import smp_processor_id
from ctypes import c_void_p, c_int64, c_uint32, c_uint64

@bpf
@map
def cpu_counts() -> HashMap:
    return HashMap(key=c_uint32, value=c_uint64, max_entries=256)

@bpf
@section("tracepoint/sched/sched_switch")
def count_switches(ctx: c_void_p) -> c_int64:
    cpu = smp_processor_id()
    count = cpu_counts.lookup(cpu)
    
    if count:
        cpu_counts.update(cpu, count + 1)
    else:
        cpu_counts.update(cpu, c_uint64(1))
    
    return c_int64(0)

@bpf
@bpfglobal
def LICENSE() -> str:
    return "GPL"

b = BPF()
b.load_and_attach()

import time
time.sleep(5)

# Read results
from pylibbpf import BpfMap
map_obj = BpfMap(b, cpu_counts)
for cpu, count in map_obj.items():
    print(f"CPU {cpu}: {count} context switches")

Example 4: Event Sampling

from pythonbpf import bpf, section, bpfglobal, BPF, trace_pipe
from pythonbpf.helper import random, pid, comm
from ctypes import c_void_p, c_int64

@bpf
@section("tracepoint/syscalls/sys_enter_open")
def sample_opens(ctx: c_void_p) -> c_int64:
    # Sample 5% of events
    if (random() % 100) < 5:
        process_id = pid()
        process_name = comm()
        print(f"Sampled: {process_name} ({process_id}) opening file")
    
    return c_int64(0)

@bpf
@bpfglobal
def LICENSE() -> str:
    return "GPL"

b = BPF()
b.load_and_attach()
trace_pipe()

Best Practices

1. Use appropriate helpers - Choose the right helper for your use case
2. Handle errors - Check return values from helpers like probe_read()
3. Minimize overhead - Helper calls have cost; use judiciously
4. Sample when appropriate - Use random() for high-frequency events
5. Clean up resources - Delete map entries when done

Common Patterns

Store-and-Compare Pattern

# Store a value
key = pid()
value = ktime()
my_map.update(key, value)

# Later: compare
stored = my_map.lookup(key)
if stored:
    difference = ktime() - stored

Filtering Pattern

# Filter by user
user_id = uid()
if user_id == 0:  # Only root
    # Process event
    pass

Sampling Pattern

# Sample 1 in N events
if (random() % N) == 0:
    # Process sampled event
    pass

Troubleshooting

Helper Not Available

If a helper function doesn't work:

• Check your kernel version (some helpers are newer)
• Verify the helper is available with bpftool feature
• Ensure your LICENSE is GPL-compatible

Trace Pipe Access Denied

If trace_pipe() fails:

• Run with sudo/root
• Check /sys/kernel/tracing/ is accessible
• Verify tracing is enabled in kernel config

probe_read Failures

If probe_read() returns errors:

• Ensure the source address is valid kernel memory
• Check that the size is reasonable
• Verify you're not reading from restricted areas

Next Steps

• Explore {doc}maps for data storage with helpers
• Learn about {doc}compilation to understand helper implementation
• See {doc}decorators for marking BPF functions