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 54e6add1a157ed89d5c1bc8a441b7aa5e4ea8ffe..45feae313ae6f236cc2ecb584e8810bd35db1a74 100644 (file)
--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -912,6 +912,7 @@ 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;
@@ -944,7 +945,7 @@ 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;
@@ -966,12 +967,29 @@ 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 acb6b9fe2ac3c101d7d5f969b865d257effb8d9b..e1d47f88445f3cbf8f791e1f86b208307e830eb7 100644 (file)
--- a/tools/xenstore/xenstored_core.h
+++ b/tools/xenstore/xenstored_core.h
@@ -62,6 +62,9 @@ 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;