On 10/10/26 20:07, Julian Orth wrote:
> A sync_file exported from a timeline point can contain a dma_fence_chain.
No it can't, IIRC sync files are only supposed to contain raw fences or
dma_fence_array containers.
I think we never documented that and that's why this issue here leaked in.
I'm not sure if that doesn't break anywhere else as well, so I think we should
fix the exporting side instead.
Alternatively we need to audit everywhere where fences from syncfiles can be
imported somewhere else.
Regards,
Christian.
> 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 <[email protected]>
> ---
> 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;
