netdev - Re: [PATCH net 1/3] net: bcmgenet: fix off-by-one in incrementing read pointer

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Date:	Fri, 10 Oct 2014 10:17:11 -0700
From:	Petri Gynther <pgynther@...gle.com>
To:	Florian Fainelli <f.fainelli@...il.com>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
	jaedon.shin@...il.com
Subject: Re: [PATCH net 1/3] net: bcmgenet: fix off-by-one in incrementing
 read pointer

On Fri, Oct 10, 2014 at 9:41 AM, Florian Fainelli <f.fainelli@...il.com> wrote:
> On 10/09/2014 11:01 PM, Petri Gynther wrote:
>> Hi Florian,
>>
>> On Thu, Oct 9, 2014 at 6:06 PM, Florian Fainelli <f.fainelli@...il.com> wrote:
>>> Commit b629be5c8399d7c423b92135eb43a86c924d1cbc ("net: bcmgenet: check
>>> harder for out of memory conditions") moved the increment of the local
>>> read pointer *before* reading from the hardware descriptor using
>>> dmadesc_get_length_status(), which creates an off-by-one situation.
>>>
>>> Fix this by moving again the read_ptr increment after we have read the
>>> hardware descriptor to get both the control block and the read pointer
>>> back in sync.
>>>
>>> Fixes: b629be5c8399 ("net: bcmgenet: check harder for out of memory conditions")
>>> Reported-by: Jaedon Shin <jaedon.shin@...il.com>
>>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>>> ---
>>>  drivers/net/ethernet/broadcom/genet/bcmgenet.c | 6 +++---
>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.c b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>>> index fff2634b6f34..f1bcebcbba80 100644
>>> --- a/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>>> +++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
>>> @@ -1287,9 +1287,6 @@ static unsigned int bcmgenet_desc_rx(struct bcmgenet_priv *priv,
>>>
>>>                 rxpktprocessed++;
>>>
>>> -               priv->rx_read_ptr++;
>>> -               priv->rx_read_ptr &= (priv->num_rx_bds - 1);
>>> -
>>
>> Wouldn't it be better to move the three lines:
>> rxpktprocessed++;
>> priv->rx_read_ptr++;
>> priv->rx_read_ptr &= (priv->num_rx_bds - 1)
>>
>> as the last lines of the while-loop, after the CB refill?
>
> That's basically what Jaedon did in his first patch, I don't have strong
> objections in doing that if you think that makes it look clearer.
>

I feel that this would be cleaner approach. First do the work (or skip
to refill), then increment necessary variables, and go back to the
while-loop.

>>
>> -- Petri
>>
>>
>>>                 /* We do not have a backing SKB, so we do not have a
>>>                  * corresponding DMA mapping for this incoming packet since
>>>                  * bcmgenet_rx_refill always either has both skb and mapping or
>>> @@ -1332,6 +1329,9 @@ static unsigned int bcmgenet_desc_rx(struct bcmgenet_priv *priv,
>>>                           __func__, p_index, priv->rx_c_index,
>>>                           priv->rx_read_ptr, dma_length_status);
>>>
>>> +               priv->rx_read_ptr++;
>>> +               priv->rx_read_ptr &= (priv->num_rx_bds - 1);
>>> +
>>>                 if (unlikely(!(dma_flag & DMA_EOP) || !(dma_flag & DMA_SOP))) {
>>>                         netif_err(priv, rx_status, dev,
>>>                                   "dropping fragmented packet!\n");
>>> --
>>> 1.9.1
>>>
>
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