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SubjectRe: [PATCH] dma-buf: fix debugfs versus rcu and fence dumping
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Am 06.12.18 um 16:21 schrieb Jerome Glisse:
> On Thu, Dec 06, 2018 at 08:09:28AM +0000, Koenig, Christian wrote:
>> 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.
> 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.

>
>>> ---
>>> 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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