RE: [PATCH] dma-buf/reservation: shouldn't kfree staged when slot available

From: Liu, Monk
Date: Mon Mar 05 2018 - 02:55:37 EST


Hi Christian

You are right on that part of obj-staged is set to NULL in add_fence,
So my following question will be why we kfree(obj->staged) in reserve_shared() if staged is always NULL in that point ?

Thanks
/Monk

-----Original Message-----
From: Christian KÃnig [mailto:ckoenig.leichtzumerken@xxxxxxxxx]
Sent: 2018å2æ28æ 16:27
To: Liu, Monk <Monk.Liu@xxxxxxx>; dri-devel@xxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: [PATCH] dma-buf/reservation: shouldn't kfree staged when slot available

Am 28.02.2018 um 07:44 schrieb Monk Liu:
> under below scenario the obj->fence would refer to a wild pointer:
>
> 1,call reservation_object_reserved_shared
> 2,call reservation_object_add_shared_fence
> 3,call reservation_object_reserved_shared
> 4,call reservation_object_add_shared_fence
>
> in step 1, staged is allocated,
>
> in step 2, code path will go reservation_object_add_shared_replace()
> and obj->fence would be assigned as staged (through RCU_INIT_POINTER)
>
> in step 3, obj->staged will be freed(by simple kfree), which make
> obj->fence point to a wild pointer...


Well that explanation is still nonsense. See
reservation_object_add_shared_fence:
> ÂÂÂÂÂÂÂ obj->staged = NULL;

Among the first things reservation_object_add_shared_fence() does is it
sets obj->staged to NULL.

So step 3 will not free anything and we never have a wild pointer.

Regards,
Christian.

>
> in step 4, code path will go reservation_object_add_shared_inplace()
> and inside it the @fobj (which equals to @obj->staged, set by above steps)
> is already a wild pointer
>
> should remov the kfree on staged in reservation_object_reserve_shared()
>
> Change-Id: If7c01f1b4be3d3d8a81efa90216841f79ab1fc1c
> Signed-off-by: Monk Liu <Monk.Liu@xxxxxxx>
> ---
> drivers/dma-buf/reservation.c | 7 ++-----
> 1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
> index 375de41..b473ccc 100644
> --- a/drivers/dma-buf/reservation.c
> +++ b/drivers/dma-buf/reservation.c
> @@ -74,12 +74,9 @@ int reservation_object_reserve_shared(struct reservation_object *obj)
> old = reservation_object_get_list(obj);
>
> if (old && old->shared_max) {
> - if (old->shared_count < old->shared_max) {
> - /* perform an in-place update */
> - kfree(obj->staged);
> - obj->staged = NULL;
> + if (old->shared_count < old->shared_max)
> return 0;
> - } else
> + else
> max = old->shared_max * 2;
> } else
> max = 4;