linux-kernel - Re: [PATCH] rpmsg: virtio_rpmsg

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Date:   Mon, 12 Aug 2019 12:48:24 -0400
From:   "Andrew F. Davis" <afd@...com>
To:     Suman Anna <s-anna@...com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
CC:     <linux-remoteproc@...r.kernel.org>,
        Loic Pallardy <loic.pallardy@...com>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] rpmsg: virtio_rpmsg_bus: replace "%p" with "%pK"

On 8/12/19 12:39 PM, Suman Anna wrote:
> On 8/12/19 11:36 AM, Andrew F. Davis wrote:
>> On 8/12/19 12:28 PM, Suman Anna wrote:
>>> On 8/12/19 10:47 AM, Andrew F. Davis wrote:
>>>> On 10/23/18 9:19 PM, Suman Anna wrote:
>>>>> The virtio_rpmsg_bus driver uses the "%p" format-specifier for
>>>>> printing the vring buffer address. This prints only a hashed
>>>>> pointer even for previliged users. Use "%pK" instead so that
>>>>> the address can be printed during debug using kptr_restrict
>>>>> sysctl.
>>>>>
>>>>
>>>>
>>>> s/previliged/privileged
>>>
>>> Bjorn,
>>> Can you fix this up when applying.
>>>
>>>>
>>>> You describe what the code does, but not why you need this. %pK is used
>>>> for only about 1% of pointer printing, why do you want to leak this
>>>> address to userspace at all?
>>>
>>> Andrew,
>>> Default behavior of %pK is same as %p, but it does allow you to control
>>> the print. The reason is clearly mentioned in the last sentence in the
>>> patch description.
>>>
>>
>>
>> Let me rephrase then, why would you ever set 'kptr_restrict' to anything
>> other than 0, or why do you want to be able to leak this address to
>> userspace at all? If the answer is just because you can, then all 12,000
>> instances of %p in kernel could be converted for the same reason.
> 
> It is a dev_dbg statement, so it is already under dynamic debug control.
> We would only ever use it during debug.
> 


Most pointer printings are in debug statements..

I'm simply not seeing what this helps us do. The DMA address I can
understand, it may be given to a remote core so we may want to verify it
is the same on both sides, but the actual virtual kernel address is of
no value to us, a hash to track it across uses is just as good.

Andrew


> regards
> Suman
> 
>>
>> Andrew
>>
>>
>>> regards
>>> Suman
>>>
>>>>
>>>> Andrew
>>>>
>>>>
>>>>> Signed-off-by: Suman Anna <s-anna@...com>
>>>>> ---
>>>>>  drivers/rpmsg/virtio_rpmsg_bus.c | 2 +-
>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/rpmsg/virtio_rpmsg_bus.c b/drivers/rpmsg/virtio_rpmsg_bus.c
>>>>> index f29dee731026..1345f373a1a0 100644
>>>>> --- a/drivers/rpmsg/virtio_rpmsg_bus.c
>>>>> +++ b/drivers/rpmsg/virtio_rpmsg_bus.c
>>>>> @@ -950,7 +950,7 @@ static int rpmsg_probe(struct virtio_device *vdev)
>>>>>  		goto vqs_del;
>>>>>  	}
>>>>>  
>>>>> -	dev_dbg(&vdev->dev, "buffers: va %p, dma %pad\n",
>>>>> +	dev_dbg(&vdev->dev, "buffers: va %pK, dma %pad\n",
>>>>>  		bufs_va, &vrp->bufs_dma);
>>>>>  
>>>>>  	/* half of the buffers is dedicated for RX */
>>>>>
>>>
>