[xen staging] tools/xenstore: don't buffer multiple identical watch events

[patchbot@xxxxxxx]

commit b5c0bdb96d33e18c324c13d8e33c08732d77eaa2
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Tue Sep 13 07:35:08 2022 +0200
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Nov 1 13:05:44 2022 +0000

    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@xxxxxxxx>
    Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 tools/xenstore/xenstored_core.c | 20 +++++++++++++++++++-
 tools/xenstore/xenstored_core.h |  3 +++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index 54e6add1a1..45feae313a 100644
--- 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 acb6b9fe2a..e1d47f8844 100644
--- 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;
 
--
generated by git-patchbot for /home/xen/git/xen.git#staging