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, 13 insertions, 12 deletions

diff --git a/util/filemonitor-inotify.c b/util/filemonitor-inotify.c
index 3eb29f8..b5f4b93 100644
--- a/util/filemonitor-inotify.c
+++ b/util/filemonitor-inotify.c
@@ -29,7 +29,6 @@
 
 struct QFileMonitor {
     int fd;
-    int nextid; /* watch ID counter */
     QemuMutex lock; /* protects dirs & idmap */
     GHashTable *dirs; /* dirname => QFileMonitorDir */
     GHashTable *idmap; /* inotify ID => dirname */
@@ -37,7 +36,7 @@ struct QFileMonitor {
 
 
 typedef struct {
-    int id; /* watch ID */
+    int64_t id; /* watch ID */
     char *filename; /* optional filter */
     QFileMonitorHandler cb;
     void *opaque;
@@ -46,7 +45,8 @@ typedef struct {
 
 typedef struct {
     char *path;
-    int id; /* inotify ID */
+    int inotify_id; /* inotify ID */
+    int next_file_id; /* file ID counter */
     GArray *watches; /* QFileMonitorWatch elements */
 } QFileMonitorDir;
 
@@ -126,7 +126,8 @@ static void qemu_file_monitor_watch(void *arg)
             g_assert_not_reached();
         }
 
-        trace_qemu_file_monitor_event(mon, dir->path, name, ev->mask, dir->id);
+        trace_qemu_file_monitor_event(mon, dir->path, name, ev->mask,
+                                      dir->inotify_id);
         for (i = 0; i < dir->watches->len; i++) {
             QFileMonitorWatch *watch = &g_array_index(dir->watches,
                                                       QFileMonitorWatch,
@@ -237,7 +238,7 @@ qemu_file_monitor_free(QFileMonitor *mon)
     g_idle_add((GSourceFunc)qemu_file_monitor_free_idle, mon);
 }
 
-int
+int64_t
 qemu_file_monitor_add_watch(QFileMonitor *mon,
                             const char *dirpath,
                             const char *filename,
@@ -247,7 +248,7 @@ qemu_file_monitor_add_watch(QFileMonitor *mon,
 {
     QFileMonitorDir *dir;
     QFileMonitorWatch watch;
-    int ret = -1;
+    int64_t ret = -1;
 
     qemu_mutex_lock(&mon->lock);
     dir = g_hash_table_lookup(mon->dirs, dirpath);
@@ -265,7 +266,7 @@ qemu_file_monitor_add_watch(QFileMonitor *mon,
 
         dir = g_new0(QFileMonitorDir, 1);
         dir->path = g_strdup(dirpath);
-        dir->id = rv;
+        dir->inotify_id = rv;
         dir->watches = g_array_new(FALSE, TRUE, sizeof(QFileMonitorWatch));
 
         g_hash_table_insert(mon->dirs, dir->path, dir);
@@ -276,7 +277,7 @@ qemu_file_monitor_add_watch(QFileMonitor *mon,
         }
     }
 
-    watch.id = mon->nextid++;
+    watch.id = (((int64_t)dir->inotify_id) << 32) | dir->next_file_id++;
     watch.filename = g_strdup(filename);
     watch.cb = cb;
     watch.opaque = opaque;
@@ -297,7 +298,7 @@ qemu_file_monitor_add_watch(QFileMonitor *mon,
 
 void qemu_file_monitor_remove_watch(QFileMonitor *mon,
                                     const char *dirpath,
-                                    int id)
+                                    int64_t id)
 {
     QFileMonitorDir *dir;
     gsize i;
@@ -322,10 +323,10 @@ void qemu_file_monitor_remove_watch(QFileMonitor *mon,
     }
 
     if (dir->watches->len == 0) {
-        inotify_rm_watch(mon->fd, dir->id);
-        trace_qemu_file_monitor_disable_watch(mon, dir->path, dir->id);
+        inotify_rm_watch(mon->fd, dir->inotify_id);
+        trace_qemu_file_monitor_disable_watch(mon, dir->path, dir->inotify_id);
 
-        g_hash_table_remove(mon->idmap, GINT_TO_POINTER(dir->id));
+        g_hash_table_remove(mon->idmap, GINT_TO_POINTER(dir->inotify_id));
         g_hash_table_remove(mon->dirs, dir->path);
 
         if (g_hash_table_size(mon->dirs) == 0) {