internal/uevent: optionally pass UUID during coldboot

This enables rejection of non-coldboot synthetic events.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/uevent/uevent.go

@@ -5,10 +5,14 @@ package uevent
 
 import (
 	"context"
+	"encoding"
+	"encoding/hex"
 	"errors"
+	"fmt"
 	"strconv"
 	"sync/atomic"
 	"syscall"
+	"unsafe"
 
 	"hakurei.app/internal/netlink"
 )
@@ -121,6 +125,117 @@ func (c *Conn) receiveEvent(ctx context.Context) (*Message, error) {
 	return &msg, err
 }
 
+// UUID represents the value of SYNTH_UUID.
+//
+// This is not a generic UUID implementation. Do not attempt to use it for
+// anything other than passing and interpreting the SYNTH_UUID environment
+// variable of a uevent.
+type UUID [16]byte
+
+const (
+	// SizeUUID is the fixed size of string representation of [UUID] according
+	// to Documentation/ABI/testing/sysfs-uevent.
+	SizeUUID = 4 + len(UUID{})*2
+
+	// UUIDSep is the separator byte of [UUID].
+	UUIDSep = '-'
+)
+
+var (
+	_ encoding.TextAppender    = new(UUID)
+	_ encoding.TextMarshaler   = new(UUID)
+	_ encoding.TextUnmarshaler = new(UUID)
+)
+
+// String formats uuid according to Documentation/ABI/testing/sysfs-uevent.
+func (uuid *UUID) String() string {
+	s := make([]byte, 0, SizeUUID)
+	s = hex.AppendEncode(s, uuid[:4])
+	s = append(s, UUIDSep)
+	s = hex.AppendEncode(s, uuid[4:6])
+	s = append(s, UUIDSep)
+	s = hex.AppendEncode(s, uuid[6:8])
+	s = append(s, UUIDSep)
+	s = hex.AppendEncode(s, uuid[8:10])
+	s = append(s, UUIDSep)
+	s = hex.AppendEncode(s, uuid[10:16])
+	return unsafe.String(unsafe.SliceData(s), len(s))
+}
+
+func (uuid *UUID) AppendText(data []byte) ([]byte, error) {
+	return append(data, uuid.String()...), nil
+}
+func (uuid *UUID) MarshalText() ([]byte, error) {
+	return uuid.AppendText(nil)
+}
+
+var (
+	// ErrAutoUUID is returned parsing a SYNTH_UUID generated by the kernel for
+	// a synthetic event without a UUID passed in.
+	ErrAutoUUID = errors.New("UUID is not passed in")
+)
+
+// UUIDSizeError describes an incorrectly sized string representation of [UUID].
+type UUIDSizeError int
+
+func (e UUIDSizeError) Error() string {
+	return "got " + strconv.Itoa(int(e)) + " bytes " +
+		"instead of " + strconv.Itoa(SizeUUID)
+}
+
+// UUIDSeparatorError is an invalid separator in a malformed string
+// representation of [UUID].
+type UUIDSeparatorError byte
+
+func (e UUIDSeparatorError) Error() string {
+	return fmt.Sprintf("invalid UUID separator: %#U", rune(e))
+}
+
+// UnmarshalText parses data according to Documentation/ABI/testing/sysfs-uevent.
+func (uuid *UUID) UnmarshalText(data []byte) (err error) {
+	if len(data) == 1 && data[0] == '0' {
+		return ErrAutoUUID
+	}
+	if len(data) != SizeUUID {
+		return UUIDSizeError(len(data))
+	}
+
+	if _, err = hex.Decode(uuid[:], data[:8]); err != nil {
+		return
+	}
+	if data[8] != UUIDSep {
+		return UUIDSeparatorError(data[8])
+	}
+	data = data[9:]
+
+	if _, err = hex.Decode(uuid[4:], data[:4]); err != nil {
+		return
+	}
+	if data[4] != UUIDSep {
+		return UUIDSeparatorError(data[4])
+	}
+	data = data[5:]
+
+	if _, err = hex.Decode(uuid[6:], data[:4]); err != nil {
+		return
+	}
+	if data[4] != UUIDSep {
+		return UUIDSeparatorError(data[4])
+	}
+	data = data[5:]
+
+	if _, err = hex.Decode(uuid[8:], data[:4]); err != nil {
+		return
+	}
+	if data[4] != UUIDSep {
+		return UUIDSeparatorError(data[4])
+	}
+	data = data[5:]
+
+	_, err = hex.Decode(uuid[10:], data)
+	return
+}
+
 // Consume continuously receives and parses events from the kernel and handles
 // [Recoverable] and [NeedsColdboot] errors via caller-supplied functions,
 // entering coldboot when required.
@@ -145,6 +260,7 @@ func (c *Conn) receiveEvent(ctx context.Context) (*Message, error) {
 func (c *Conn) Consume(
 	ctx context.Context,
 	sysfs string,
+	uuid *UUID,
 	events chan<- *Message,
 	coldboot bool,
 
@@ -193,7 +309,7 @@ coldboot:
 	ctxColdboot, cancelColdboot := context.WithCancel(ctx)
 	var coldbootErr error
 	go func() {
-		coldbootErr = Coldboot(ctxColdboot, sysfs, visited, handleWalkErr)
+		coldbootErr = Coldboot(ctxColdboot, sysfs, uuid, visited, handleWalkErr)
 		close(visited)
 	}()
 	for pathname := range visited {