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>
(cherry picked from commit b5c0bdb96d33e18c324c13d8e33c08732d77eaa2)

diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index 5f1733112a4f23b60fbe24e259f0c9c0716af152..0621023bca162e759c76666e294bb2d53852d0df 100644 (file)
--- 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 edeaa96dd10ba06c6a1ff83bd1002640a22e4de8..1eb6131fc88d58262146c8bf66a07ffc2e58b3b3 100644 (file)
--- 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;