Re: [PATCH] dma-buf: fix debugfs versus rcu and fence dumping

["Koenig, Christian" <Christian.Koenig@amd.com>]

Am 06.12.18 um 02:41 schrieb jglisse@redhat.com:
> From: Jérôme Glisse <jglisse@redhat.com>
>
> The debugfs take reference on fence without dropping them. Also the
> rcu section are not well balance. Fix all that ...
>
> Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: linux-media@vger.kernel.org
> Cc: dri-devel@lists.freedesktop.org
> Cc: linaro-mm-sig@lists.linaro.org
> Cc: Stéphane Marchesin <marcheu@chromium.org>
> Cc: stable@vger.kernel.org

Well NAK, you are now taking the RCU lock twice and dropping the RCU and 
still accessing fobj has a huge potential for accessing freed up memory.

The only correct thing I can see here is to grab a reference to the 
fence before printing any info on it,
Christian.

> ---
>   drivers/dma-buf/dma-buf.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 13884474d158..f6f4de42ac49 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -1051,24 +1051,31 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
>   			fobj = rcu_dereference(robj->fence);
>   			shared_count = fobj ? fobj->shared_count : 0;
>   			fence = rcu_dereference(robj->fence_excl);
> +			fence = dma_fence_get_rcu(fence);
>   			if (!read_seqcount_retry(&robj->seq, seq))
>   				break;
>   			rcu_read_unlock();
>   		}
> -
> -		if (fence)
> +		if (fence) {
>   			seq_printf(s, "\tExclusive fence: %s %s %ssignalled\n",
>   				   fence->ops->get_driver_name(fence),
>   				   fence->ops->get_timeline_name(fence),
>   				   dma_fence_is_signaled(fence) ? "" : "un");
> +			dma_fence_put(fence);
> +		}
> +
> +		rcu_read_lock();
>   		for (i = 0; i < shared_count; i++) {
>   			fence = rcu_dereference(fobj->shared[i]);
>   			if (!dma_fence_get_rcu(fence))
>   				continue;
> +			rcu_read_unlock();
>   			seq_printf(s, "\tShared fence: %s %s %ssignalled\n",
>   				   fence->ops->get_driver_name(fence),
>   				   fence->ops->get_timeline_name(fence),
>   				   dma_fence_is_signaled(fence) ? "" : "un");
> +			dma_fence_put(fence);
> +			rcu_read_lock();
>   		}
>   		rcu_read_unlock();
>