Re: [PATCH net] net: gro: set the last skb->next to NULL when it get merged

[Eric Dumazet]

On 10/27/21 1:07 AM, Jason Xing wrote:
> On Wed, Oct 27, 2021 at 3:23 PM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
>>
>> On Tue, Oct 26, 2021 at 9:19 PM <kerneljasonxing@xxxxxxxxx> wrote:
>>>
>>> From: Jason Xing <xingwanli@xxxxxxxxxxxx>
>>>
>>> Setting the @next of the last skb to NULL to prevent the panic in future
>>> when someone does something to the last of the gro list but its @next is
>>> invalid.
>>>
>>> For example, without the fix (commit: ece23711dd95), a panic could happen
>>> with the clsact loaded when skb is redirected and then validated in
>>> validate_xmit_skb_list() which could access the error addr of the @next
>>> of the last skb. Thus, "general protection fault" would appear after that.
>>>
>>> Signed-off-by: Jason Xing <xingwanli@xxxxxxxxxxxx>
>>> ---
>>>  net/core/skbuff.c | 1 +
>>>  1 file changed, 1 insertion(+)
>>>
>>> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
>>> index 2170bea..7b248f1 100644
>>> --- a/net/core/skbuff.c
>>> +++ b/net/core/skbuff.c
>>> @@ -4396,6 +4396,7 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
>>>                 skb_shinfo(p)->frag_list = skb;
>>>         else
>>>                 NAPI_GRO_CB(p)->last->next = skb;
>>> +       skb->next = NULL;
>>>         NAPI_GRO_CB(p)->last = skb;
>>
>> Besides, I'm a little bit confused that this operation inserts the
>> newest skb into the tail of the flow, so the tail of flow is the
>> newest, head oldest. The patch (commit: 600adc18) introduces the flush
>> of the oldest when the flow is full to lower the latency, but actually
>> it fetches the tail of the flow. Do I get something wrong here? I feel
> 
> I have to update this part. The commit 600adc18 evicts and flushes the
> oldest flow. But for the current kernel, when
> "napi->gro_hash[hash].count >= MAX_GRO_SKBS" happens, the
> gro_flush_oldest() flushes the oldest skb of one certain flow,
> actually it is the newest skb because it is at the end of the list.

GRO only keeps one skb per flow in the main hash/lru.

I think you are not understanding GRO correctly.