filemon: fix watch IDs to avoid potential wraparound issues

Watch IDs are allocated from incrementing a int counter against
the QFileMonitor object. In very long life QEMU processes with
a huge amount of USB MTP activity creating & deleting directories
it is just about conceivable that the int counter can wrap
around. This would result in incorrect behaviour of the file
monitor watch APIs due to clashing watch IDs.

Instead of trying to detect this situation, this patch changes
the way watch IDs are allocated. It is turned into an int64_t
variable where the high 32 bits are set from the underlying
inotify "int" ID. This gives an ID that is guaranteed unique
for the directory as a whole, and we can rely on the kernel
to enforce this. QFileMonitor then sets the low 32 bits from
a per-directory counter.

The USB MTP device only sets watches on the directory as a
whole, not files within, so there is no risk of guest
triggered wrap around on the low 32 bits.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

1 files changed, 5 insertions, 5 deletions

diff --git a/hw/usb/dev-mtp.c b/hw/usb/dev-mtp.c
index 4dc1317..ebf210f 100644
--- a/hw/usb/dev-mtp.c
+++ b/hw/usb/dev-mtp.c
@@ -170,7 +170,7 @@ struct MTPObject {
     char         *path;
     struct stat  stat;
     /* file monitor watch id */
-    int          watchid;
+    int64_t      watchid;
     MTPObject    *parent;
     uint32_t     nchildren;
     QLIST_HEAD(, MTPObject) children;
@@ -498,7 +498,7 @@ static MTPObject *usb_mtp_object_lookup_name(MTPObject *parent,
     return NULL;
 }
 
-static MTPObject *usb_mtp_object_lookup_id(MTPState *s, int id)
+static MTPObject *usb_mtp_object_lookup_id(MTPState *s, int64_t id)
 {
     MTPObject *iter;
 
@@ -511,7 +511,7 @@ static MTPObject *usb_mtp_object_lookup_id(MTPState *s, int id)
     return NULL;
 }
 
-static void file_monitor_event(int id,
+static void file_monitor_event(int64_t id,
                                QFileMonitorEvent ev,
                                const char *name,
                                void *opaque)
@@ -625,8 +625,8 @@ static void usb_mtp_object_readdir(MTPState *s, MTPObject *o)
     }
 
     if (s->file_monitor) {
-        int id = qemu_file_monitor_add_watch(s->file_monitor, o->path, NULL,
-                                             file_monitor_event, s, &err);
+        int64_t id = qemu_file_monitor_add_watch(s->file_monitor, o->path, NULL,
+                                                 file_monitor_event, s, &err);
         if (id == -1) {
             error_report("usb-mtp: failed to add watch for %s: %s", o->path,
                          error_get_pretty(err));