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, 72 insertions, 58 deletions

diff --git a/tests/test-util-filemonitor.c b/tests/test-util-filemonitor.c
index 71a7cf5..46e781c 100644
--- a/tests/test-util-filemonitor.c
+++ b/tests/test-util-filemonitor.c
@@ -43,12 +43,12 @@ typedef struct {
     int type;
     const char *filesrc;
     const char *filedst;
-    int watchid;
+    int64_t *watchid;
     int eventid;
 } QFileMonitorTestOp;
 
 typedef struct {
-    int id;
+    int64_t id;
     QFileMonitorEvent event;
     char *filename;
 } QFileMonitorTestRecord;
@@ -90,7 +90,7 @@ qemu_file_monitor_test_event_loop(void *opaque G_GNUC_UNUSED)
  * an ordered list of all events that it receives
  */
 static void
-qemu_file_monitor_test_handler(int id,
+qemu_file_monitor_test_handler(int64_t id,
                                QFileMonitorEvent event,
                                const char *filename,
                                void *opaque)
@@ -156,7 +156,7 @@ qemu_file_monitor_test_next_record(QFileMonitorTestData *data)
  */
 static bool
 qemu_file_monitor_test_expect(QFileMonitorTestData *data,
-                              int id,
+                              int64_t id,
                               QFileMonitorEvent event,
                               const char *filename)
 {
@@ -166,13 +166,14 @@ qemu_file_monitor_test_expect(QFileMonitorTestData *data,
     rec = qemu_file_monitor_test_next_record(data);
 
     if (!rec) {
-        g_printerr("Missing event watch id %d event %d file %s\n",
+        g_printerr("Missing event watch id %" PRIx64 " event %d file %s\n",
                    id, event, filename);
         return false;
     }
 
     if (id != rec->id) {
-        g_printerr("Expected watch id %d but got %d\n", id, rec->id);
+        g_printerr("Expected watch id %" PRIx64 " but got %" PRIx64 "\n",
+                   id, rec->id);
         goto cleanup;
     }
 
@@ -198,170 +199,176 @@ qemu_file_monitor_test_expect(QFileMonitorTestData *data,
 static void
 test_file_monitor_events(void)
 {
+    int64_t watch0 = 0;
+    int64_t watch1 = 0;
+    int64_t watch2 = 0;
+    int64_t watch3 = 0;
+    int64_t watch4 = 0;
+    int64_t watch5 = 0;
     QFileMonitorTestOp ops[] = {
         { .type = QFILE_MONITOR_TEST_OP_ADD_WATCH,
-          .filesrc = NULL, .watchid = 0 },
+          .filesrc = NULL, .watchid = &watch0 },
         { .type = QFILE_MONITOR_TEST_OP_ADD_WATCH,
-          .filesrc = "one.txt", .watchid = 1 },
+          .filesrc = "one.txt", .watchid = &watch1 },
         { .type = QFILE_MONITOR_TEST_OP_ADD_WATCH,
-          .filesrc = "two.txt", .watchid = 2 },
+          .filesrc = "two.txt", .watchid = &watch2 },
 
 
         { .type = QFILE_MONITOR_TEST_OP_CREATE,
           .filesrc = "one.txt", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "one.txt", .watchid = 0,
+          .filesrc = "one.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "one.txt", .watchid = 1,
+          .filesrc = "one.txt", .watchid = &watch1,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_CREATE,
           .filesrc = "two.txt", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 0,
+          .filesrc = "two.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 2,
+          .filesrc = "two.txt", .watchid = &watch2,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_CREATE,
           .filesrc = "three.txt", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "three.txt", .watchid = 0,
+          .filesrc = "three.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_UNLINK,
           .filesrc = "three.txt", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "three.txt", .watchid = 0,
+          .filesrc = "three.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_DELETED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_RENAME,
           .filesrc = "one.txt", .filedst = "two.txt" },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "one.txt", .watchid = 0,
+          .filesrc = "one.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_DELETED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "one.txt", .watchid = 1,
+          .filesrc = "one.txt", .watchid = &watch1,
           .eventid = QFILE_MONITOR_EVENT_DELETED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 0,
+          .filesrc = "two.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 2,
+          .filesrc = "two.txt", .watchid = &watch2,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_APPEND,
           .filesrc = "two.txt", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 0,
+          .filesrc = "two.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_MODIFIED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 2,
+          .filesrc = "two.txt", .watchid = &watch2,
           .eventid = QFILE_MONITOR_EVENT_MODIFIED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_TOUCH,
           .filesrc = "two.txt", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 0,
+          .filesrc = "two.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_ATTRIBUTES },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 2,
+          .filesrc = "two.txt", .watchid = &watch2,
           .eventid = QFILE_MONITOR_EVENT_ATTRIBUTES },
 
 
         { .type = QFILE_MONITOR_TEST_OP_DEL_WATCH,
-          .filesrc = "one.txt", .watchid = 1 },
+          .filesrc = "one.txt", .watchid = &watch1 },
         { .type = QFILE_MONITOR_TEST_OP_ADD_WATCH,
-          .filesrc = "one.txt", .watchid = 3 },
+          .filesrc = "one.txt", .watchid = &watch3 },
         { .type = QFILE_MONITOR_TEST_OP_CREATE,
           .filesrc = "one.txt", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "one.txt", .watchid = 0,
+          .filesrc = "one.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "one.txt", .watchid = 3,
+          .filesrc = "one.txt", .watchid = &watch3,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_DEL_WATCH,
-          .filesrc = "one.txt", .watchid = 3 },
+          .filesrc = "one.txt", .watchid = &watch3 },
         { .type = QFILE_MONITOR_TEST_OP_UNLINK,
           .filesrc = "one.txt", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "one.txt", .watchid = 0,
+          .filesrc = "one.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_DELETED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_MKDIR,
           .filesrc = "fish", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "fish", .watchid = 0,
+          .filesrc = "fish", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_ADD_WATCH,
-          .filesrc = "fish/", .watchid = 4 },
+          .filesrc = "fish/", .watchid = &watch4 },
         { .type = QFILE_MONITOR_TEST_OP_ADD_WATCH,
-          .filesrc = "fish/one.txt", .watchid = 5 },
+          .filesrc = "fish/one.txt", .watchid = &watch5 },
         { .type = QFILE_MONITOR_TEST_OP_CREATE,
           .filesrc = "fish/one.txt", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "one.txt", .watchid = 4,
+          .filesrc = "one.txt", .watchid = &watch4,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "one.txt", .watchid = 5,
+          .filesrc = "one.txt", .watchid = &watch5,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_DEL_WATCH,
-          .filesrc = "fish/one.txt", .watchid = 5 },
+          .filesrc = "fish/one.txt", .watchid = &watch5 },
         { .type = QFILE_MONITOR_TEST_OP_RENAME,
           .filesrc = "fish/one.txt", .filedst = "two.txt", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "one.txt", .watchid = 4,
+          .filesrc = "one.txt", .watchid = &watch4,
           .eventid = QFILE_MONITOR_EVENT_DELETED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 0,
+          .filesrc = "two.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 2,
+          .filesrc = "two.txt", .watchid = &watch2,
           .eventid = QFILE_MONITOR_EVENT_CREATED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_RMDIR,
           .filesrc = "fish", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "", .watchid = 4,
+          .filesrc = "", .watchid = &watch4,
           .eventid = QFILE_MONITOR_EVENT_IGNORED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "fish", .watchid = 0,
+          .filesrc = "fish", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_DELETED },
         { .type = QFILE_MONITOR_TEST_OP_DEL_WATCH,
-          .filesrc = "fish", .watchid = 4 },
+          .filesrc = "fish", .watchid = &watch4 },
 
 
         { .type = QFILE_MONITOR_TEST_OP_UNLINK,
           .filesrc = "two.txt", },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 0,
+          .filesrc = "two.txt", .watchid = &watch0,
           .eventid = QFILE_MONITOR_EVENT_DELETED },
         { .type = QFILE_MONITOR_TEST_OP_EVENT,
-          .filesrc = "two.txt", .watchid = 2,
+          .filesrc = "two.txt", .watchid = &watch2,
           .eventid = QFILE_MONITOR_EVENT_DELETED },
 
 
         { .type = QFILE_MONITOR_TEST_OP_DEL_WATCH,
-          .filesrc = "two.txt", .watchid = 2 },
+          .filesrc = "two.txt", .watchid = &watch2 },
         { .type = QFILE_MONITOR_TEST_OP_DEL_WATCH,
-          .filesrc = NULL, .watchid = 0 },
+          .filesrc = NULL, .watchid = &watch0 },
     };
     Error *local_err = NULL;
     GError *gerr = NULL;
@@ -374,6 +381,7 @@ test_file_monitor_events(void)
     char *pathsrc = NULL;
     char *pathdst = NULL;
     QFileMonitorTestData data;
+    GHashTable *ids = g_hash_table_new(g_int64_hash, g_int64_equal);
 
     qemu_mutex_init(&data.lock);
     data.records = NULL;
@@ -414,7 +422,6 @@ test_file_monitor_events(void)
     for (i = 0; i < G_N_ELEMENTS(ops); i++) {
         const QFileMonitorTestOp *op = &(ops[i]);
         int fd;
-        int watchid;
         struct utimbuf ubuf;
         char *watchdir;
         const char *watchfile;
@@ -427,8 +434,8 @@ test_file_monitor_events(void)
         switch (op->type) {
         case QFILE_MONITOR_TEST_OP_ADD_WATCH:
             if (debug) {
-                g_printerr("Add watch %s %s %d\n",
-                           dir, op->filesrc, op->watchid);
+                g_printerr("Add watch %s %s\n",
+                           dir, op->filesrc);
             }
             if (op->filesrc && strchr(op->filesrc, '/')) {
                 watchdir = g_strdup_printf("%s/%s", dir, op->filesrc);
@@ -442,7 +449,7 @@ test_file_monitor_events(void)
                 watchdir = g_strdup(dir);
                 watchfile = op->filesrc;
             }
-            watchid =
+            *op->watchid =
                 qemu_file_monitor_add_watch(mon,
                                             watchdir,
                                             watchfile,
@@ -450,20 +457,23 @@ test_file_monitor_events(void)
                                             &data,
                                             &local_err);
             g_free(watchdir);
-            if (watchid < 0) {
+            if (*op->watchid < 0) {
                 g_printerr("Unable to add watch %s",
                            error_get_pretty(local_err));
                 goto cleanup;
             }
-            if (watchid != op->watchid) {
-                g_printerr("Unexpected watch ID %d, wanted %d\n",
-                           watchid, op->watchid);
+            if (debug) {
+                g_printerr("Watch ID %" PRIx64 "\n", *op->watchid);
+            }
+            if (g_hash_table_contains(ids, op->watchid)) {
+                g_printerr("Watch ID %" PRIx64 "already exists", *op->watchid);
                 goto cleanup;
             }
+            g_hash_table_add(ids, op->watchid);
             break;
         case QFILE_MONITOR_TEST_OP_DEL_WATCH:
             if (debug) {
-                g_printerr("Del watch %s %d\n", dir, op->watchid);
+                g_printerr("Del watch %s %" PRIx64 "\n", dir, *op->watchid);
             }
             if (op->filesrc && strchr(op->filesrc, '/')) {
                 watchdir = g_strdup_printf("%s/%s", dir, op->filesrc);
@@ -472,18 +482,19 @@ test_file_monitor_events(void)
             } else {
                 watchdir = g_strdup(dir);
             }
+            g_hash_table_remove(ids, op->watchid);
             qemu_file_monitor_remove_watch(mon,
                                            watchdir,
-                                           op->watchid);
+                                           *op->watchid);
             g_free(watchdir);
             break;
         case QFILE_MONITOR_TEST_OP_EVENT:
             if (debug) {
-                g_printerr("Event id=%d event=%d file=%s\n",
-                           op->watchid, op->eventid, op->filesrc);
+                g_printerr("Event id=%" PRIx64 " event=%d file=%s\n",
+                           *op->watchid, op->eventid, op->filesrc);
             }
             if (!qemu_file_monitor_test_expect(
-                    &data, op->watchid, op->eventid, op->filesrc))
+                    &data, *op->watchid, op->eventid, op->filesrc))
                 goto cleanup;
             break;
         case QFILE_MONITOR_TEST_OP_CREATE:
@@ -596,6 +607,8 @@ test_file_monitor_events(void)
         pathsrc = pathdst = NULL;
     }
 
+    g_assert_cmpint(g_hash_table_size(ids), ==, 0);
+
     err = 0;
 
  cleanup:
@@ -647,6 +660,7 @@ test_file_monitor_events(void)
             abort();
         }
     }
+    g_hash_table_unref(ids);
     g_free(dir);
     g_assert(err == 0);
 }