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SubjectRe: [PATCH] dma-buf: fix debugfs versus rcu and fence dumping
Date
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();
>

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