File xsa326-06.patch of Package xen.31431

From 5c18ae5ed96fd62f462b4be9a95022f143f6dee4 Mon Sep 17 00:00:00 2001
From: Juergen Gross <jgross@suse.com>
Date: Tue, 13 Sep 2022 07:35:08 +0200
Subject: tools/xenstore: don't buffer multiple identical watch events

A guest not reading its Xenstore response buffer fast enough might
pile up lots of Xenstore watch events buffered. Reduce the generated
load by dropping new events which already have an identical copy
pending.

The special events "@..." are excluded from that handling as there are
known use cases where the handler is relying on each event to be sent
individually.

This is part of XSA-326.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>

diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index 5f1733112a4f..0621023bca16 100644
--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -821,6 +821,7 @@ void send_reply(struct connection *conn, enum xsd_sockmsg_type type,
 	bdata->inhdr = true;
 	bdata->used = 0;
 	bdata->timeout_msec = 0;
+	bdata->watch_event = false;
 
 	if (len <= DEFAULT_BUFFER_SIZE)
 		bdata->buffer = bdata->default_buffer;
@@ -853,7 +854,7 @@ void send_reply(struct connection *conn, enum xsd_sockmsg_type type,
 void send_event(struct buffered_data *req, struct connection *conn,
 		const char *path, const char *token)
 {
-	struct buffered_data *bdata;
+	struct buffered_data *bdata, *bd;
 	unsigned int len;
 
 	len = strlen(path) + 1 + strlen(token) + 1;
@@ -875,12 +876,29 @@ void send_event(struct buffered_data *req, struct connection *conn,
 	bdata->hdr.msg.type = XS_WATCH_EVENT;
 	bdata->hdr.msg.len = len;
 
+	/*
+	 * Check whether an identical event is pending already.
+	 * Special events are excluded from that check.
+	 */
+	if (path[0] != '@') {
+		list_for_each_entry(bd, &conn->out_list, list) {
+			if (bd->watch_event && bd->hdr.msg.len == len &&
+			    !memcmp(bdata->buffer, bd->buffer, len)) {
+				trace("dropping duplicate watch %s %s for domain %u\n",
+				      path, token, conn->id);
+				talloc_free(bdata);
+				return;
+			}
+		}
+	}
+
 	if (timeout_watch_event_msec && domain_is_unprivileged(conn)) {
 		bdata->timeout_msec = get_now_msec() + timeout_watch_event_msec;
 		if (!conn->timeout_msec)
 			conn->timeout_msec = bdata->timeout_msec;
 	}
 
+	bdata->watch_event = true;
 	bdata->pend.req = req;
 	if (req)
 		req->pend.ref.event_cnt++;
diff --git a/tools/xenstore/xenstored_core.h b/tools/xenstore/xenstored_core.h
index edeaa96dd10b..1eb6131fc88d 100644
--- a/tools/xenstore/xenstored_core.h
+++ b/tools/xenstore/xenstored_core.h
@@ -51,6 +51,9 @@ struct buffered_data
 	/* Are we still doing the header? */
 	bool inhdr;
 
+	/* Is this a watch event? */
+	bool watch_event;
+
 	/* How far are we? */
 	unsigned int used;