tests: Pass in MigrateStart** into test_migrate_start()

test_migrate_start() will release the MigrateStart structure that passed
in, however that's not super clear to the caller because after the call
returned the pointer can still be referenced by the callers.  It can easily
be a source of use-after-free.

Let's pass in a double pointer of that, then we can safely clear the
pointer for the caller after the struct is released.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220301083925.33483-26-peterx@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  dgilbert: Fixup apply since I didn't take 24/25

tests/qtest/migration-test.c

@@ -495,7 +495,7 @@ static void migrate_start_destroy(MigrateStart *args)
 }
 
 static int test_migrate_start(QTestState **from, QTestState **to,
-                              const char *uri, MigrateStart *args)
+                              const char *uri, MigrateStart **pargs)
 {
     g_autofree gchar *arch_source = NULL;
     g_autofree gchar *arch_target = NULL;
@@ -507,6 +507,7 @@ static int test_migrate_start(QTestState **from, QTestState **to,
     g_autofree char *shmem_path = NULL;
     const char *arch = qtest_get_arch();
     const char *machine_opts = NULL;
+    MigrateStart *args = *pargs;
     const char *memory_size;
     int ret = 0;
 
@@ -621,6 +622,8 @@ static int test_migrate_start(QTestState **from, QTestState **to,
 
 out:
     migrate_start_destroy(args);
+    /* This tells the caller that this structure is gone */
+    *pargs = NULL;
     return ret;
 }
 
@@ -665,7 +668,7 @@ static int migrate_postcopy_prepare(QTestState **from_ptr,
     g_autofree char *uri = g_strdup_printf("unix:%s/migsocket", tmpfs);
     QTestState *from, *to;
 
-    if (test_migrate_start(&from, &to, uri, args)) {
+    if (test_migrate_start(&from, &to, uri, &args)) {
         return -1;
     }
 
@@ -788,7 +791,7 @@ static void test_baddest(void)
 
     args->hide_stderr = true;
 
-    if (test_migrate_start(&from, &to, "tcp:127.0.0.1:0", args)) {
+    if (test_migrate_start(&from, &to, "tcp:127.0.0.1:0", &args)) {
         return;
     }
     migrate_qmp(from, "tcp:127.0.0.1:0", "{}");
@@ -804,7 +807,7 @@ static void test_precopy_unix_common(bool dirty_ring)
 
     args->use_dirty_ring = dirty_ring;
 
-    if (test_migrate_start(&from, &to, uri, args)) {
+    if (test_migrate_start(&from, &to, uri, &args)) {
         return;
     }
 
@@ -892,7 +895,7 @@ static void test_xbzrle(const char *uri)
     MigrateStart *args = migrate_start_new();
     QTestState *from, *to;
 
-    if (test_migrate_start(&from, &to, uri, args)) {
+    if (test_migrate_start(&from, &to, uri, &args)) {
         return;
     }
 
@@ -946,7 +949,7 @@ static void test_precopy_tcp(void)
     g_autofree char *uri = NULL;
     QTestState *from, *to;
 
-    if (test_migrate_start(&from, &to, "tcp:127.0.0.1:0", args)) {
+    if (test_migrate_start(&from, &to, "tcp:127.0.0.1:0", &args)) {
         return;
     }
 
@@ -991,7 +994,7 @@ static void test_migrate_fd_proto(void)
     QDict *rsp;
     const char *error_desc;
 
-    if (test_migrate_start(&from, &to, "defer", args)) {
+    if (test_migrate_start(&from, &to, "defer", &args)) {
         return;
     }
 
@@ -1071,7 +1074,7 @@ static void do_test_validate_uuid(MigrateStart *args, bool should_fail)
     g_autofree char *uri = g_strdup_printf("unix:%s/migsocket", tmpfs);
     QTestState *from, *to;
 
-    if (test_migrate_start(&from, &to, uri, args)) {
+    if (test_migrate_start(&from, &to, uri, &args)) {
         return;
     }
 
@@ -1163,7 +1166,7 @@ static void test_migrate_auto_converge(void)
      */
     const int64_t expected_threshold = max_bandwidth * downtime_limit / 1000;
 
-    if (test_migrate_start(&from, &to, uri, args)) {
+    if (test_migrate_start(&from, &to, uri, &args)) {
         return;
     }
 
@@ -1232,7 +1235,7 @@ static void test_multifd_tcp(const char *method)
     QDict *rsp;
     g_autofree char *uri = NULL;
 
-    if (test_migrate_start(&from, &to, "defer", args)) {
+    if (test_migrate_start(&from, &to, "defer", &args)) {
         return;
     }
 
@@ -1318,7 +1321,7 @@ static void test_multifd_tcp_cancel(void)
 
     args->hide_stderr = true;
 
-    if (test_migrate_start(&from, &to, "defer", args)) {
+    if (test_migrate_start(&from, &to, "defer", &args)) {
         return;
     }
 
@@ -1357,7 +1360,7 @@ static void test_multifd_tcp_cancel(void)
     args = migrate_start_new();
     args->only_target = true;
 
-    if (test_migrate_start(&from, &to2, "defer", args)) {
+    if (test_migrate_start(&from, &to2, "defer", &args)) {
         return;
     }