[PATCH] dma-fence: Fix dma_fence_timeline_name() to call get_timeline_name()

[PATCH] dma-fence: Fix dma_fence_timeline_name() to call get_timeline_name()

[Baineng Shou]

dma_fence_timeline_name() incorrectly invokes ops->get_driver_name()
instead of ops->get_timeline_name(), so every caller receives the
driver name where the timeline name was expected.

This is a copy-paste regression that has resurfaced twice. It was
originally introduced by commit 62918542b7bf ("dma-fence: Fix sparse
warnings due __rcu annotations") when adding the __rcu casts, fixed
by commit 033559473dd3 ("dma-fence: Fix safe access wrapper to call
timeline name method"), and then accidentally reintroduced by commit
e58b4dea9054 ("dma-buf/dma-fence: Add dma_fence_test_signaled_flag()")
when both wrappers were refactored to use the new helper.

Signed-off-by: Baineng Shou <shoubaineng@gmail.com>
---
 drivers/dma-buf/dma-fence.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index b3bfa6943a8e..5292d714419b 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -1202,7 +1202,7 @@ const char __rcu *dma_fence_timeline_name(struct dma_fence *fence)
 	/* RCU protection is required for safe access to returned string */
 	ops = rcu_dereference(fence->ops);
 	if (!dma_fence_test_signaled_flag(fence))
-		return (const char __rcu *)ops->get_driver_name(fence);
+		return (const char __rcu *)ops->get_timeline_name(fence);
 	else
 		return (const char __rcu *)"signaled-timeline";
 }
-- 
2.34.1

Re: [PATCH] dma-fence: Fix dma_fence_timeline_name() to call get_timeline_name()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [Critical] Potential NULL pointer dereference in `dma_fence_timeline_name()` and `dma_fence_driver_name()` due to a missing `ops` NULL check when the fence is concurrently signaled without a read memory barrier.
--

commit 118120d707b8593d71c08a5c22b1149fdc6d4229
Author: Baineng Shou <shoubaineng@gmail.com>

dma-fence: Fix dma_fence_timeline_name() to call get_timeline_name()

This patch fixes a copy-paste error where dma_fence_timeline_name()
incorrectly invoked get_driver_name() instead of get_timeline_name().

> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index b3bfa6943a8e1..5292d714419b1 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -1202,7 +1202,7 @@ const char __rcu *dma_fence_timeline_name(struct dma_fence *fence)
>  	/* RCU protection is required for safe access to returned string */
>  	ops = rcu_dereference(fence->ops);
>  	if (!dma_fence_test_signaled_flag(fence))
> -		return (const char __rcu *)ops->get_driver_name(fence);
> +		return (const char __rcu *)ops->get_timeline_name(fence);

[Severity: Critical]
This is a pre-existing issue, but is it possible for ops to be NULL here
due to memory reordering?

If dma_fence_signal_timestamp_locked() runs concurrently, it sets
DMA_FENCE_FLAG_SIGNALED_BIT (which includes a full memory barrier via
test_and_set_bit()) and then sets fence->ops = NULL via RCU_INIT_POINTER().

Since rcu_dereference() and test_bit() inside dma_fence_test_signaled_flag()
are independent memory loads without an intervening read memory barrier,
could a reader on weakly-ordered architectures experience reordering?

Reader CPU:
  1. Loads fence->flags -> 0 (sees not signaled)
  2. Loads fence->ops -> NULL (sees new value)
  3. !dma_fence_test_signaled_flag(fence) evaluates to true
  4. ops->get_timeline_name(fence) dereferences NULL

Would adding a NULL check for ops, or an smp_rmb() between the reads,
be necessary to prevent a potential kernel panic if this is reachable via
concurrent ioctls like SYNC_IOC_FILE_INFO?

Does dma_fence_driver_name() also share this same structural vulnerability
when dereferencing ops->get_driver_name?

>  	else
>  		return (const char __rcu *)"signaled-timeline";
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260625141227.38931-1-shoubaineng@gmail.com?part=1

Re: [PATCH] dma-fence: Fix dma_fence_timeline_name() to call get_timeline_name()

[Philipp Stanner]

+Cc André

On Thu, 2026-06-25 at 22:12 +0800, Baineng Shou wrote:
> dma_fence_timeline_name() incorrectly invokes ops->get_driver_name()
> instead of ops->get_timeline_name(), so every caller receives the
> driver name where the timeline name was expected.
> 
> This is a copy-paste regression that has resurfaced twice. It was
> originally introduced by commit 62918542b7bf ("dma-fence: Fix sparse
> warnings due __rcu annotations") when adding the __rcu casts, fixed
> by commit 033559473dd3 ("dma-fence: Fix safe access wrapper to call
> timeline name method"), and then accidentally reintroduced by commit
> e58b4dea9054 ("dma-buf/dma-fence: Add dma_fence_test_signaled_flag()")
> when both wrappers were refactored to use the new helper.
> 
> Signed-off-by: Baineng Shou <shoubaineng@gmail.com>

André has caught this a few days ago already:

https://lore.kernel.org/dri-devel/20260618-linux-drm_crtc_fix-v1-1-801f29c9853d@linaro.org/


To save my honor I want to state that e58b4dea9054 did not actually *do
this* change. But merging it must have caused it somehow, since that
commit was still based on the old bug. I guess somewhere in conflict
resolution somehow tho old code must have entered.


P.

> ---
>  drivers/dma-buf/dma-fence.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index b3bfa6943a8e..5292d714419b 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -1202,7 +1202,7 @@ const char __rcu *dma_fence_timeline_name(struct dma_fence *fence)
>  	/* RCU protection is required for safe access to returned string */
>  	ops = rcu_dereference(fence->ops);
>  	if (!dma_fence_test_signaled_flag(fence))
> -		return (const char __rcu *)ops->get_driver_name(fence);
> +		return (const char __rcu *)ops->get_timeline_name(fence);
>  	else
>  		return (const char __rcu *)"signaled-timeline";
>  }