fix VFIO_USER_DEVICE_GET_REGION_IO_FDS allocation (#785)

clang-tidy static analysis identified a zero-sized allocation in the
case that no ioregionfds had been configured. Fix this issue and add a
test for it.

Signed-off-by: John Levon <john.levon@nutanix.com>
Reviewed-by: Thanos Makatos <thanos.makatos@nutanix.com>
(cherry picked from commit 8d38e8c)

lib/libvfio-user.c

@@ -654,26 +654,31 @@ handle_device_get_region_io_fds(vfu_ctx_t *vfu_ctx, vfu_msg_t *msg)
                           sizeof(vfio_user_sub_region_ioeventfd_t);
 
     msg->out.nr_fds = 0;
-    if (req->argsz >= reply->argsz) {
+
+    if (max_sent_sub_regions > 0 && req->argsz >= reply->argsz) {
         msg->out.fds = calloc(sizeof(int),
                               max_sent_sub_regions + nr_shadow_reg);
         if (msg->out.fds == NULL) {
             return -1;
         }
 
         sub_reg = LIST_FIRST(&vfu_reg->subregions);
+
         for (i = 0; i < max_sent_sub_regions; i++) {
+            int fdi;
 
             ioefd = &reply->sub_regions[i].ioeventfd;
             ioefd->gpa_offset = sub_reg->gpa_offset;
             ioefd->size = sub_reg->size;
-            ioefd->fd_index = add_fd_index(msg->out.fds, &msg->out.nr_fds,
-                                        sub_reg->fd);
+            fdi = add_fd_index(msg->out.fds, &msg->out.nr_fds, sub_reg->fd);
+            ioefd->fd_index = fdi;
             if (sub_reg->shadow_fd == -1) {
                 ioefd->type = VFIO_USER_IO_FD_TYPE_IOEVENTFD;
             } else {
                 ioefd->type = VFIO_USER_IO_FD_TYPE_IOEVENTFD_SHADOW;
-                ioefd->shadow_mem_fd_index = add_fd_index(msg->out.fds, &msg->out.nr_fds, sub_reg->shadow_fd);
+                fdi = add_fd_index(msg->out.fds, &msg->out.nr_fds,
+                                   sub_reg->shadow_fd);
+                ioefd->shadow_mem_fd_index = fdi;
             }
             ioefd->flags = sub_reg->flags;
             ioefd->datamatch = sub_reg->datamatch;

test/py/test_shadow_ioeventfd.py

@@ -33,6 +33,34 @@
 import errno
 
 
+def test_shadow_ioeventfd_none():
+    """Test VFIO_USER_DEVICE_GET_REGION_IO_FDS when none are set up."""
+
+    # server setup
+    ctx = vfu_create_ctx(flags=LIBVFIO_USER_FLAG_ATTACH_NB)
+    assert ctx is not None
+    ret = vfu_setup_region(ctx, index=VFU_PCI_DEV_BAR0_REGION_IDX, size=0x1000,
+                           flags=VFU_REGION_FLAG_RW)
+    assert ret == 0
+
+    ret = vfu_realize_ctx(ctx)
+    assert ret == 0
+
+    # client
+    client = connect_client(ctx)
+    payload = vfio_user_region_io_fds_request(
+                argsz=len(vfio_user_region_io_fds_reply()) +
+                len(vfio_user_sub_region_ioeventfd()), flags=0,
+                index=VFU_PCI_DEV_BAR0_REGION_IDX, count=0)
+    newfds, ret = msg_fds(ctx, client.sock, VFIO_USER_DEVICE_GET_REGION_IO_FDS,
+                          payload, expect=0)
+    assert len(newfds) == 0
+    reply, ret = vfio_user_region_io_fds_reply.pop_from_buffer(ret)
+    assert reply.count == 0
+
+    vfu_destroy_ctx(ctx)
+
+
 def test_shadow_ioeventfd():
     """Configure a shadow ioeventfd, have the client trigger it, confirm that
     the server receives the notification and can see the value."""