[PATCH 2/2] drm/syncobj: flatten chains when importing sync_file into timeline point

[Julian Orth <ju.orth@gmail.com>]

A sync_file exported from a timeline point can contain a dma_fence_chain.
Importing such a sync_file into a timeline point with
DRM_SYNCOBJ_FD_TO_HANDLE_FLAGS_IMPORT_SYNC_FILE |
DRM_SYNCOBJ_FD_TO_HANDLE_FLAGS_TIMELINE passes the chain directly to
dma_fence_chain_init() and triggers

  WARNING: drivers/dma-buf/dma-fence-chain.c:286 at dma_fence_chain_init

because chain fences must not be wrapped in other chain fences.

Flatten the fence with dma_fence_unwrap_merge() before adding it to the
timeline, as drm_syncobj_transfer_to_timeline() already does.

Reproducer:

1. Export a sync_file from a point > 0 of a timeline syncobj with
   DRM_IOCTL_SYNCOBJ_HANDLE_TO_FD.
2. Import it into a point > 0 of another timeline syncobj with
   DRM_IOCTL_SYNCOBJ_FD_TO_HANDLE.

Fixes: c2d3a7300695 ("drm/syncobj: Extend EXPORT_SYNC_FILE for timeline syncobjs")
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Julian Orth <ju.orth@gmail.com>
---
 drivers/gpu/drm/drm_syncobj.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index 5ca5163d0..c545af77e 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -742,8 +742,18 @@ static int drm_syncobj_import_sync_file_fence(struct drm_file *file_private,
 	}
 
 	if (point) {
-		struct dma_fence_chain *chain = dma_fence_chain_alloc();
+		struct dma_fence_chain *chain;
+		struct dma_fence *tmp;
+
+		tmp = dma_fence_unwrap_merge(fence);
+		dma_fence_put(fence);
+		fence = tmp;
+		if (!fence) {
+			drm_syncobj_put(syncobj);
+			return -ENOMEM;
+		}
 
+		chain = dma_fence_chain_alloc();
 		if (!chain) {
 			ret = -ENOMEM;
 			goto out;
-- 
2.56.0