virgl/gbm: exported read/writable buffers

With old DRM implementations, drmPrimeHandleToFD always returned a fd
that could be mapped read/write. However, new kernels require the
DRM_RDWR flag to be specified to create read/write mappings.

Try passing the DRM_RDWR flag but fall back to the old way if that
fails.

Signed-off-by: David Stevens <stevensd@chromium.org>
Acked-By: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>

src/virgl_gbm.c

@@ -400,10 +400,16 @@ int virgl_gbm_export_query(struct gbm_bo *bo, struct virgl_renderer_export_query
 
       if (i == query->out_num_fds) {
          if (query->in_export_fds) {
-            ret = drmPrimeHandleToFD(gbm_device_get_fd(gbm), handle, DRM_CLOEXEC,
+            ret = drmPrimeHandleToFD(gbm_device_get_fd(gbm), handle, DRM_CLOEXEC | DRM_RDWR,
                                      &query->out_fds[query->out_num_fds]);
-            if (ret)
-               goto err_close;
+            // Kernels with older DRM core versions block DRM_RDWR but give a
+            // read/write mapping anyway.
+            if (ret) {
+               ret = drmPrimeHandleToFD(gbm_device_get_fd(gbm), handle, DRM_CLOEXEC,
+                                        &query->out_fds[query->out_num_fds]);
+               if (ret)
+                 goto err_close;
+            }
          }
          query->out_num_fds++;
       }