[PATCH 04/10] tools/xenstored: Limit the number of requests a connection can delay

[Julien Grall <julien@xxxxxxx>]

From: Julien Grall <jgrall@xxxxxxxxxx>

Currently, only liveupdate request can be delayed. The request can only
be performed by a privileged connection (e.g. dom0). So it is fine to
have no limits.

In a follow-up patch we will want to delay request for unprivileged
connection as well. So it is best to apply a limit.

For now and for simplicity, only a single request can be delayed
for a given unprivileged connection.

Take the opportunity to tweak the prototype and provide a way to
bypass the quota check. This would be useful when the function
is called from the restore code.

Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 tools/xenstore/xenstored_control.c |  2 +-
 tools/xenstore/xenstored_core.c    | 11 ++++++++++-
 tools/xenstore/xenstored_core.h    |  3 ++-
 3 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/tools/xenstore/xenstored_control.c 
b/tools/xenstore/xenstored_control.c
index 7acc2d134f9f..1c24d4869eab 100644
--- a/tools/xenstore/xenstored_control.c
+++ b/tools/xenstore/xenstored_control.c
@@ -737,7 +737,7 @@ static const char *lu_start(const void *ctx, struct 
connection *conn,
        lu_status->timeout = to;
        lu_status->started_at = time(NULL);
 
-       errno = delay_request(conn, conn->in, do_lu_start, NULL);
+       errno = delay_request(conn, conn->in, do_lu_start, NULL, false);
 
        return NULL;
 }
diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index 41b26d7094c8..51d210828922 100644
--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -279,10 +279,19 @@ static void call_delayed(struct connection *conn, struct 
delayed_request *req)
 }
 
 int delay_request(struct connection *conn, struct buffered_data *in,
-                 bool (*func)(struct delayed_request *), void *data)
+                 bool (*func)(struct delayed_request *), void *data,
+                 bool no_quota_check)
 {
        struct delayed_request *req;
 
+       /*
+        * Only allow one request can be delayed for an unprivileged
+        * connection.
+        */
+       if (!no_quota_check && domain_is_unprivileged(conn) &&
+           !list_empty(&conn->delayed))
+               return ENOSPC;
+
        req = talloc(in, struct delayed_request);
        if (!req)
                return ENOMEM;
diff --git a/tools/xenstore/xenstored_core.h b/tools/xenstore/xenstored_core.h
index 89ce155e755b..34839b34f6e9 100644
--- a/tools/xenstore/xenstored_core.h
+++ b/tools/xenstore/xenstored_core.h
@@ -213,7 +213,8 @@ char *get_parent(const void *ctx, const char *node);
 
 /* Delay a request. */
 int delay_request(struct connection *conn, struct buffered_data *in,
-                 bool (*func)(struct delayed_request *), void *data);
+                 bool (*func)(struct delayed_request *), void *data,
+                 bool no_quota_check);
 
 /* Tracing infrastructure. */
 void trace_create(const void *data, const char *type);
-- 
2.17.1