[PATCH] tools: xentoolcore_restrict_all: Do deregistration before close

[Ian Jackson <ian.jackson@eu.citrix.com>]

Closing the fd before unhooking it from the list runs the risk that a
concurrent thread calls xentoolcore_restrict_all will operate on the
old fd value, which might refer to a new fd by then.  So we need to do
it in the other order.

Sadly this weakens the guarantee provided by xentoolcore_restrict_all
slight, but not (I think) in a problematic way.  It would be possible
to implement the previous guarantee, but it would involve replacing
all of the close() calls in all of the individual osdep parts of all
of the individual libraries with calls to a new function which does
   dup2("/dev/null", thing->fd);
   pthread_mutex_lock(&handles_lock);
   thing->fd = -1;
   pthread_mutex_unlock(&handles_lock);
   close(fd);
which would be terribly tedious.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
---
 tools/libs/call/core.c                             | 4 ++--
 tools/libs/devicemodel/core.c                      | 4 ++--
 tools/libs/evtchn/core.c                           | 4 ++--
 tools/libs/foreignmemory/core.c                    | 4 ++--
 tools/libs/gnttab/gnttab_core.c                    | 4 ++--
 tools/libs/toolcore/include/xentoolcore.h          | 9 +++++++++
 tools/libs/toolcore/include/xentoolcore_internal.h | 6 ++++--
 tools/xenstore/xs.c                                | 4 ++--
 8 files changed, 25 insertions(+), 14 deletions(-)

diff --git a/tools/libs/call/core.c b/tools/libs/call/core.c
index b256fce..f3a3400 100644
--- a/tools/libs/call/core.c
+++ b/tools/libs/call/core.c
@@ -59,8 +59,8 @@ xencall_handle *xencall_open(xentoollog_logger *logger, unsigned open_flags)
     return xcall;
 
 err:
-    osdep_xencall_close(xcall);
     xentoolcore__deregister_active_handle(&xcall->tc_ah);
+    osdep_xencall_close(xcall);
     xtl_logger_destroy(xcall->logger_tofree);
     free(xcall);
     return NULL;
@@ -73,8 +73,8 @@ int xencall_close(xencall_handle *xcall)
     if ( !xcall )
         return 0;
 
-    rc = osdep_xencall_close(xcall);
     xentoolcore__deregister_active_handle(&xcall->tc_ah);
+    rc = osdep_xencall_close(xcall);
     buffer_release_cache(xcall);
     xtl_logger_destroy(xcall->logger_tofree);
     free(xcall);
diff --git a/tools/libs/devicemodel/core.c b/tools/libs/devicemodel/core.c
index b66d4f9..355b7de 100644
--- a/tools/libs/devicemodel/core.c
+++ b/tools/libs/devicemodel/core.c
@@ -68,8 +68,8 @@ xendevicemodel_handle *xendevicemodel_open(xentoollog_logger *logger,
 
 err:
     xtl_logger_destroy(dmod->logger_tofree);
-    xencall_close(dmod->xcall);
     xentoolcore__deregister_active_handle(&dmod->tc_ah);
+    xencall_close(dmod->xcall);
     free(dmod);
     return NULL;
 }
@@ -83,8 +83,8 @@ int xendevicemodel_close(xendevicemodel_handle *dmod)
 
     rc = osdep_xendevicemodel_close(dmod);
 
-    xencall_close(dmod->xcall);
     xentoolcore__deregister_active_handle(&dmod->tc_ah);
+    xencall_close(dmod->xcall);
     xtl_logger_destroy(dmod->logger_tofree);
     free(dmod);
     return rc;
diff --git a/tools/libs/evtchn/core.c b/tools/libs/evtchn/core.c
index 2dba58b..aff6ecf 100644
--- a/tools/libs/evtchn/core.c
+++ b/tools/libs/evtchn/core.c
@@ -55,8 +55,8 @@ xenevtchn_handle *xenevtchn_open(xentoollog_logger *logger, unsigned open_flags)
     return xce;
 
 err:
-    osdep_evtchn_close(xce);
     xentoolcore__deregister_active_handle(&xce->tc_ah);
+    osdep_evtchn_close(xce);
     xtl_logger_destroy(xce->logger_tofree);
     free(xce);
     return NULL;
@@ -69,8 +69,8 @@ int xenevtchn_close(xenevtchn_handle *xce)
     if ( !xce )
         return 0;
 
-    rc = osdep_evtchn_close(xce);
     xentoolcore__deregister_active_handle(&xce->tc_ah);
+    rc = osdep_evtchn_close(xce);
     xtl_logger_destroy(xce->logger_tofree);
     free(xce);
     return rc;
diff --git a/tools/libs/foreignmemory/core.c b/tools/libs/foreignmemory/core.c
index 79b24d2..7c8562a 100644
--- a/tools/libs/foreignmemory/core.c
+++ b/tools/libs/foreignmemory/core.c
@@ -57,8 +57,8 @@ xenforeignmemory_handle *xenforeignmemory_open(xentoollog_logger *logger,
     return fmem;
 
 err:
-    osdep_xenforeignmemory_close(fmem);
     xentoolcore__deregister_active_handle(&fmem->tc_ah);
+    osdep_xenforeignmemory_close(fmem);
     xtl_logger_destroy(fmem->logger_tofree);
     free(fmem);
     return NULL;
@@ -71,8 +71,8 @@ int xenforeignmemory_close(xenforeignmemory_handle *fmem)
     if ( !fmem )
         return 0;
 
-    rc = osdep_xenforeignmemory_close(fmem);
     xentoolcore__deregister_active_handle(&fmem->tc_ah);
+    rc = osdep_xenforeignmemory_close(fmem);
     xtl_logger_destroy(fmem->logger_tofree);
     free(fmem);
     return rc;
diff --git a/tools/libs/gnttab/gnttab_core.c b/tools/libs/gnttab/gnttab_core.c
index 5f761e5..98f1591 100644
--- a/tools/libs/gnttab/gnttab_core.c
+++ b/tools/libs/gnttab/gnttab_core.c
@@ -54,8 +54,8 @@ xengnttab_handle *xengnttab_open(xentoollog_logger *logger, unsigned open_flags)
     return xgt;
 
 err:
-    osdep_gnttab_close(xgt);
     xentoolcore__deregister_active_handle(&xgt->tc_ah);
+    osdep_gnttab_close(xgt);
     xtl_logger_destroy(xgt->logger_tofree);
     free(xgt);
     return NULL;
@@ -68,8 +68,8 @@ int xengnttab_close(xengnttab_handle *xgt)
     if ( !xgt )
         return 0;
 
-    rc = osdep_gnttab_close(xgt);
     xentoolcore__deregister_active_handle(&xgt->tc_ah);
+    rc = osdep_gnttab_close(xgt);
     xtl_logger_destroy(xgt->logger_tofree);
     free(xgt);
     return rc;
diff --git a/tools/libs/toolcore/include/xentoolcore.h b/tools/libs/toolcore/include/xentoolcore.h
index 8d28c2d..b3a3c93 100644
--- a/tools/libs/toolcore/include/xentoolcore.h
+++ b/tools/libs/toolcore/include/xentoolcore.h
@@ -39,6 +39,15 @@
  * fail (even though such a call is potentially meaningful).
  * (If called again with a different domid, it will necessarily fail.)
  *
+ * Note for multi-threaded programs: If xentoolcore_restrict_all is
+ * called concurrently with a function which /or closes Xen library
+ * handles (e.g.  libxl_ctx_free, xs_close), the restriction is only
+ * guaranteed to be effective after all of the closing functions have
+ * returned, even if that is later than the return from
+ * xentoolcore_restrict_all.  (Of course if xentoolcore_restrict_all
+ * it is called concurrently with opening functions, the new handles
+ * might or might not be restricted.)
+ *
  *  ====================================================================
  *  IMPORTANT - IMPLEMENTATION STATUS
  *
diff --git a/tools/libs/toolcore/include/xentoolcore_internal.h b/tools/libs/toolcore/include/xentoolcore_internal.h
index dbdb1dd..04f5848 100644
--- a/tools/libs/toolcore/include/xentoolcore_internal.h
+++ b/tools/libs/toolcore/include/xentoolcore_internal.h
@@ -48,8 +48,10 @@
  *     4. ONLY THEN actually open the relevant fd or whatever
  *
  *   III. during the "close handle" function
- *     1. FIRST close the relevant fd or whatever
- *     2. call xentoolcore__deregister_active_handle
+ *     1. FIRST call xentoolcore__deregister_active_handle
+ *     2. close the relevant fd or whatever
+ *
+ * [ III(b). Do the same as III for error exit from the open function. ]
  *
  *   IV. in the restrict_callback function
  *     * Arrange that the fd (or other handle) can no longer by used
diff --git a/tools/xenstore/xs.c b/tools/xenstore/xs.c
index 23f3f09..abffd9c 100644
--- a/tools/xenstore/xs.c
+++ b/tools/xenstore/xs.c
@@ -279,9 +279,9 @@ err:
 	saved_errno = errno;
 
 	if (h) {
+		xentoolcore__deregister_active_handle(&h->tc_ah);
 		if (h->fd >= 0)
 			close(h->fd);
-		xentoolcore__deregister_active_handle(&h->tc_ah);
 	}
 	free(h);
 
@@ -342,8 +342,8 @@ static void close_fds_free(struct xs_handle *h) {
 		close(h->watch_pipe[1]);
 	}
 
-        close(h->fd);
 	xentoolcore__deregister_active_handle(&h->tc_ah);
+        close(h->fd);
         
 	free(h);
 }
-- 
2.1.4


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