fix(promtail): windows forward event crash (backport k239) (#16163)

Co-authored-by: William Dumont <[email protected]>

Author: loki-gh-app[bot]

clients/pkg/promtail/targets/windows/win_eventlog/event.go

@@ -33,28 +33,41 @@ package win_eventlog
 // More info on schema, if there will be need to add more:
 // https://docs.microsoft.com/en-us/windows/win32/wes/eventschema-elements
 type Event struct {
-	Source        Provider    `xml:"System>Provider"`
-	EventID       int         `xml:"System>EventID"`
-	Version       int         `xml:"System>Version"`
-	Level         int         `xml:"System>Level"`
-	Task          int         `xml:"System>Task"`
-	Opcode        int         `xml:"System>Opcode"`
-	Keywords      string      `xml:"System>Keywords"`
-	TimeCreated   TimeCreated `xml:"System>TimeCreated"`
-	EventRecordID int         `xml:"System>EventRecordID"`
-	Correlation   Correlation `xml:"System>Correlation"`
-	Execution     Execution   `xml:"System>Execution"`
-	Channel       string      `xml:"System>Channel"`
-	Computer      string      `xml:"System>Computer"`
-	Security      Security    `xml:"System>Security"`
-	UserData      UserData    `xml:"UserData"`
-	EventData     EventData   `xml:"EventData"`
+	Source        Provider       `xml:"System>Provider"`
+	EventID       int            `xml:"System>EventID"`
+	Version       int            `xml:"System>Version"`
+	Level         int            `xml:"System>Level"`
+	Task          int            `xml:"System>Task"`
+	Opcode        int            `xml:"System>Opcode"`
+	Keywords      string         `xml:"System>Keywords"`
+	TimeCreated   TimeCreated    `xml:"System>TimeCreated"`
+	EventRecordID int            `xml:"System>EventRecordID"`
+	Correlation   Correlation    `xml:"System>Correlation"`
+	Execution     Execution      `xml:"System>Execution"`
+	Channel       string         `xml:"System>Channel"`
+	Computer      string         `xml:"System>Computer"`
+	Security      Security       `xml:"System>Security"`
+	UserData      UserData       `xml:"UserData"`
+	EventData     EventData      `xml:"EventData"`
+	RenderingInfo *RenderingInfo `xml:"RenderingInfo"`
 	Message       string
 	LevelText     string
 	TaskText      string
 	OpcodeText    string
 }
 
+// RenderingInfo is provided for events forwarded by Windows Event Collector
+// see https://learn.microsoft.com/en-us/windows/win32/api/winevt/nf-winevt-evtformatmessage#parameters
+type RenderingInfo struct {
+	Message  string   `xml:"Message"`
+	Level    string   `xml:"Level"`
+	Task     string   `xml:"Task"`
+	Opcode   string   `xml:"Opcode"`
+	Channel  string   `xml:"Channel"`
+	Provider string   `xml:"Provider"`
+	Keywords []string `xml:"Keywords>Keyword"`
+}
+
 // UserData Application-provided XML data
 type UserData struct {
 	InnerXML []byte `xml:",innerxml"`

clients/pkg/promtail/targets/windows/win_eventlog/win_eventlog.go

@@ -483,6 +483,39 @@ func (w *EventFetcher) renderEvent(eventHandle EvtHandle, lang uint32) (Event, e
 		return event, nil
 	}
 
+	// Do resolve local messages the usual way, while using built-in information for events forwarded by WEC.
+	// This is a safety measure as the underlying Windows-internal EvtFormatMessage might segfault in cases
+	// where the publisher (i.e. the remote machine which forwared the event) is unavailable e.g. due to
+	// a reboot. See https://github.com/influxdata/telegraf/issues/12328 for the full story.
+	if event.RenderingInfo == nil {
+		return w.renderLocalMessage(event, eventHandle, lang)
+	}
+
+	// We got 'RenderInfo' elements, so try to apply them in the following function
+	return w.renderRemoteMessage(event)
+}
+
+func (w *EventFetcher) renderRemoteMessage(event Event) (Event, error) {
+	// Populating text values from RenderingInfo part of the XML
+	if len(event.RenderingInfo.Keywords) > 0 {
+		event.Keywords = strings.Join(event.RenderingInfo.Keywords, ",")
+	}
+	if event.RenderingInfo.Message != "" {
+		event.Message = event.RenderingInfo.Message
+	}
+	if event.RenderingInfo.Level != "" {
+		event.LevelText = event.RenderingInfo.Level
+	}
+	if event.RenderingInfo.Task != "" {
+		event.TaskText = event.RenderingInfo.Task
+	}
+	if event.RenderingInfo.Opcode != "" {
+		event.OpcodeText = event.RenderingInfo.Opcode
+	}
+	return event, nil
+}
+
+func (w *EventFetcher) renderLocalMessage(event Event, eventHandle EvtHandle, lang uint32) (Event, error) {
 	publisherHandle, err := openPublisherMetadata(0, event.Source.Name, lang)
 	if err != nil {
 		return event, nil
@@ -525,6 +558,11 @@ func formatEventString(
 		return "", err
 	}
 
+	// Handle empty elements
+	if bufferUsed < 1 {
+		return "", nil
+	}
+
 	bufferUsed *= 2
 	buffer := make([]byte, bufferUsed)
 	bufferUsed = 0