hakurei/internal/uevent/uevent.go

// Package uevent provides userspace client for consuming events from a
// NETLINK_KOBJECT_UEVENT socket, as well as helpers for supplementing
// events received from the kernel.
package uevent

import (
	"context"
	"errors"
	"strconv"
	"sync/atomic"
	"syscall"

	"hakurei.app/internal/netlink"
)

type (
	// Recoverable is satisfied by errors that are safe to recover from.
	Recoverable interface{ recoverable() }
	// Nontrivial is satisfied by errors preferring a JSON encoding.
	Nontrivial interface{ nontrivial() }

	// NeedsColdboot is satisfied by errors indicating divergence of local state
	// from the kernel, usually from lost uevent data.
	NeedsColdboot interface {
		Recoverable
		coldboot()
	}
)

const (
	exclConsume = iota

	_exclLen
)

// Conn represents a NETLINK_KOBJECT_UEVENT socket.
type Conn struct {
	conn *netlink.Conn

	// Whether currently between a call to enterExcl and exitExcl.
	excl [_exclLen]atomic.Bool
}

// enterExcl must be called entering a critical section that interacts with conn.
func (c *Conn) enterExcl(k int) error {
	if !c.excl[k].CompareAndSwap(false, true) {
		return syscall.EAGAIN
	}
	return nil
}

// exitExcl must be called exiting a critical section that interacts with conn.
func (c *Conn) exitExcl(k int) { c.excl[k].Store(false) }

// Close closes the underlying socket.
func (c *Conn) Close() error { return c.conn.Close() }

// Dial returns the address of a newly connected [Conn].
func Dial(rcvbuf int64) (*Conn, error) {
	// kernel group is hard coded in lib/kobject_uevent.c, undocumented
	c, err := netlink.Dial(syscall.NETLINK_KOBJECT_UEVENT, 1, rcvbuf)
	if err != nil {
		return nil, err
	}
	return &Conn{conn: c}, err
}

var (
	// ErrBadSocket is returned by [Conn.Consume] for a reply from a
	// syscall.Sockaddr with unexpected concrete type.
	ErrBadSocket = errors.New("unexpected socket address")
)

// ReceiveBufferError indicates one or more [Message] being lost due to the
// socket receive buffer filling up. This is usually caused by epoll waking the
// receiving program up too late.
type ReceiveBufferError struct{ _ [0]*ReceiveBufferError }

var _ NeedsColdboot = ReceiveBufferError{}

func (ReceiveBufferError) recoverable()    {}
func (ReceiveBufferError) coldboot()       {}
func (ReceiveBufferError) Unwrap() error   { return syscall.ENOBUFS }
func (e ReceiveBufferError) Error() string { return syscall.ENOBUFS.Error() }

// BadPortError is returned by [Conn.Consume] upon receiving a message that did
// not come from the kernel.
type BadPortError syscall.SockaddrNetlink

var _ Recoverable = new(BadPortError)

func (*BadPortError) recoverable() {}
func (e *BadPortError) Error() string {
	return "unexpected message from port id " + strconv.Itoa(int(e.Pid)) +
		" on NETLINK_KOBJECT_UEVENT"
}

// receiveEvent receives a single event and returns the address of its [Message].
func (c *Conn) receiveEvent(ctx context.Context) (*Message, error) {
	data, _, from, err := c.conn.Recvmsg(ctx, 0)
	if err != nil {
		if errors.Is(err, syscall.ENOBUFS) {
			return nil, ReceiveBufferError{}
		}
		return nil, err
	}

	// lib/kobject_uevent.c:
	// set portid 0 to inform userspace message comes from kernel
	if v, ok := from.(*syscall.SockaddrNetlink); !ok {
		return nil, ErrBadSocket
	} else if v.Pid != 0 {
		return nil, (*BadPortError)(v)

	}

	var msg Message
	if err = msg.UnmarshalBinary(data); err != nil {
		return nil, err
	}
	return &msg, err
}

// Consume continuously receives and parses events from the kernel. It returns
// the first error it encounters.
//
// Callers must not restart event processing after a non-nil error that does not
// satisfy [Recoverable] is returned.
func (c *Conn) Consume(ctx context.Context, events chan<- *Message) error {
	if err := c.enterExcl(exclConsume); err != nil {
		return err
	}
	defer c.exitExcl(exclConsume)

	for {
		msg, err := c.receiveEvent(ctx)
		if err != nil {
			return err
		}
		events <- msg
	}
}