Hi,
On Thu, 2026-06-18 at 17:56 +0200, Philipp Stanner wrote:
> +Cc Danilo
>
> On Thu, 2026-06-18 at 15:03 +0100, André Draszik wrote:
> > Since commit 541c8f2468b9 ("dma-buf: detach fence ops on signal v3"),
> > I'm seeing the BUG_ON() triggering in drm_crtc's fence_to_crtc() via
> > drm_crtc_fence_get_driver_name() regularly:
> >
> > Call trace:
> > panic+0x58/0x5c
> > die+0x160/0x178
> > bug_brk_handler+0x70/0xa4
> > call_el1_break_hook+0x3c/0x1a0
> > do_el1_brk64+0x24/0x74
> > el1_brk64+0x34/0x54
> > el1h_64_sync_handler+0x80/0xfc
> > el1h_64_sync+0x84/0x88
> > drm_crtc_fence_get_driver_name+0x60/0x68 (P)
> > sync_file_get_name+0x184/0x45c
> > sync_file_ioctl+0x404/0xf70
> > __arm64_sys_ioctl+0x124/0x1dc
> >
> > This looks to be caused by a code flow similar to the following:
> >
> > +++ snip +++
> > thread A thread B
> >
> > ioctl(SYNC_IOC_FILE_INFO)
> > sync_file_ioctl()
> > sync_file_get_name()
> > dma_fence_signal_timestamp_locked() dma_fence_driver_name()
> > ops = rcu_dereference(fence->ops)
> > if (!dma_fence_test_signaled_flag())
> > ops->get_driver_name(fence) i.e.
> > drm_crtc_fence_get_driver_name()
> > test_and_set_bit(SIGNALED)
> > RCU_INIT_POINTER(fence->ops, NULL)
> > drm_crtc_fence_get_driver_name()
> > BUG_ON(rcu_access_pointer(fence->ops)
> > != &drm_crtc_fence_ops)
>
> Now this looks like a very similar problem that I have recently been
> concerned with:
>
> https://lore.kernel.org/dri-devel/[email protected]/
>
> https://lore.kernel.org/dri-devel/[email protected]/
>
>
> I continue to believe because of bugs like this and the ones I have
> quoted in the threads above the robustness of the kernel could be
> greatly improved if we could get dma_fence fully synchronized with its
> lock.
On top of that, sashiko highlighted (via my other patch) that the existing
code is missing some memory barriers:
https://sashiko.dev/#/patchset/[email protected]?part=1
I believe Lock synchronization would resolve that (as would adding explicit
memory barriers).
[...]
> > diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> > index 63ead8ba6756..31c8636e7467 100644
> > --- a/drivers/gpu/drm/drm_crtc.c
> > +++ b/drivers/gpu/drm/drm_crtc.c
> > @@ -73,6 +73,9 @@
> > * &drm_mode_config_funcs.atomic_check.
> > */
> >
> > +#define fence_to_crtc(f) container_of((f)->extern_lock, \
> > + struct drm_crtc, fence_lock)
>
> I agree that macros should be avoided if possible.
No problem, I'll change that.
>
> > +
> > /**
> > * drm_crtc_from_index - find the registered CRTC at an index
> > * @dev: DRM device
> > @@ -154,14 +157,6 @@ static void drm_crtc_crc_fini(struct drm_crtc *crtc)
> > #endif
> > }
> >
> > -static const struct dma_fence_ops drm_crtc_fence_ops;
> > -
> > -static struct drm_crtc *fence_to_crtc(struct dma_fence *fence)
> > -{
> > - BUG_ON(rcu_access_pointer(fence->ops) != &drm_crtc_fence_ops);
>
> +1
>
> BUG_ON is more or less deprecated and should not be used anymore. There
> needs to be bombastic justification for shooting down the entire
> kernel.
Yes, I meant to mention that in my commit message as well.
Now, you might have seen that sashiko highlighted that the existing
BUG_ON() is masking a potential use-after-free during driver removal which
I believe to be a correct observation. It suggests two alternative ways to
resolve it:
https://sashiko.dev/#/patchset/[email protected]?part=1
> Does the CRTC or DRM device need to be kept alive for the RCU grace period,
> or should the fence hold a proper reference to prevent the use-after-free
> when get_driver_name() and get_timeline_name() access the freed CRTC
> structure?
Do you guys have any preference on that? It appears the use-after-free
should be resolved before merging the removal of the BUG_ON(), and I'd like
to progress on this.
Cheers,
Andre'
