Am 06.12.18 um 16:21 schrieb Jerome Glisse:

On Thu, Dec 06, 2018 at 08:09:28AM +0000, Koenig, Christian wrote:

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Am 06.12.18 um 02:41 schrieb jglisse@redhat.com:

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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.

Hu ? That is exactly what i am doing, take reference under rcu, rcu_unlock print the fence info, drop the fence reference, rcu lock rinse and repeat ...

Note that the fobj in _existing_ code is access outside the rcu end that there is an rcu imbalance in that code ie a lonlely rcu_unlock after the for loop.

So that the existing code is broken.

No, the existing code is perfectly fine.

Please note the break in the loop before the rcu_unlock();

		if (!read_seqcount_retry(&robj->seq, seq))
			break; <- HERE!
		rcu_read_unlock();
	}

So your patch breaks that and take the RCU read lock twice.

Regards, Christian.

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---

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();