[PATCH v2] remote_daemon: Don't run virStateCleanup() if virStateReload() is still running

[Michal Privoznik]

When a SIGHUP is received a thread is spawned that runs
virStateReload(). However, if SIGINT is received while the former
thread is still running then we may get into problematic
situation: the cleanup code in main() sees drivers initialized
and thus calls virStateCleanup(). So now we have two threads, one
running virStateReload() the other virStateCleanup(). In this
situation it's very likely that a race condition occurs and
either of threads causes SIGSEGV.

To fix this, unmark drivers as initialized in the
virStateReload() thread for the time the function runs.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=2075837
Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
---

v2 of:

https://listman.redhat.com/archives/libvir-list/2022-April/230415.html

diff to v1:
- reworked how int is set (instead of inc/dec I'm using set(0)/set(1))
so that reload can be attempted again and again if previous attempt
failed.

 src/remote/remote_daemon.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/src/remote/remote_daemon.c b/src/remote/remote_daemon.c
index 26469e0d9f..b8ecc51758 100644
--- a/src/remote/remote_daemon.c
+++ b/src/remote/remote_daemon.c
@@ -77,7 +77,7 @@ virNetSASLContext *saslCtxt = NULL;
 virNetServerProgram *remoteProgram = NULL;
 virNetServerProgram *qemuProgram = NULL;
 
-volatile bool driversInitialized = false;
+volatile gint driversInitialized = 0;
 
 static void daemonErrorHandler(void *opaque G_GNUC_UNUSED,
                                virErrorPtr err G_GNUC_UNUSED)
@@ -453,8 +453,13 @@ static void daemonReloadHandlerThread(void *opaque G_GNUC_UNUSED)
     VIR_INFO("Reloading configuration on SIGHUP");
     virHookCall(VIR_HOOK_DRIVER_DAEMON, "-",
                 VIR_HOOK_DAEMON_OP_RELOAD, SIGHUP, "SIGHUP", NULL, NULL);
-    if (virStateReload() < 0)
+
+    if (virStateReload() < 0) {
         VIR_WARN("Error while reloading drivers");
+    }
+
+    /* Drivers are initialized again. */
+    g_atomic_int_set(&driversInitialized, 1);
 }
 
 static void daemonReloadHandler(virNetDaemon *dmn G_GNUC_UNUSED,
@@ -463,7 +468,7 @@ static void daemonReloadHandler(virNetDaemon *dmn G_GNUC_UNUSED,
 {
     virThread thr;
 
-    if (!driversInitialized) {
+    if (!g_atomic_int_compare_and_exchange(&driversInitialized, 1, 0)) {
         VIR_WARN("Drivers are not initialized, reload ignored");
         return;
     }
@@ -474,6 +479,10 @@ static void daemonReloadHandler(virNetDaemon *dmn G_GNUC_UNUSED,
          * Not much we can do on error here except log it.
          */
         VIR_ERROR(_("Failed to create thread to handle daemon restart"));
+
+        /* Drivers were initialized at the beginning, otherwise we wouldn't
+         * even get here. */
+        g_atomic_int_set(&driversInitialized, 1);
     }
 }
 
@@ -607,7 +616,7 @@ static void daemonRunStateInit(void *opaque)
         goto cleanup;
     }
 
-    driversInitialized = true;
+    g_atomic_int_set(&driversInitialized, 1);
 
     virNetDaemonSetShutdownCallbacks(dmn,
                                      virStateShutdownPrepare,
@@ -1212,10 +1221,9 @@ int main(int argc, char **argv) {
  cleanup:
     virNetlinkEventServiceStopAll();
 
-    if (driversInitialized) {
+    if (g_atomic_int_compare_and_exchange(&driversInitialized, 1, 0)) {
         /* NB: Possible issue with timing window between driversInitialized
          * setting if virNetlinkEventServerStart fails */
-        driversInitialized = false;
         virStateCleanup();
     }
 
-- 
2.35.1