linux-kernel - Re: [PATCH] flow disector: check if arp

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Date:   Fri, 13 Jan 2017 18:25:16 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Alexander Duyck <aduyck@...antis.com>,
        Jiri Pirko <jiri@...lanox.com>,
        Tom Herbert <tom@...bertland.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Simon Horman <simon.horman@...ronome.com>,
        Hadar Hen Zion <hadarh@...lanox.com>,
        Gao Feng <fgao@...ai8.com>, Amir Vadai <amir@...ai.me>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] flow disector: check if arp_eth is null rather than arp

On 13/01/17 18:24, Eric Dumazet wrote:
> On Fri, 2017-01-13 at 13:34 +0000, Colin King wrote:
>> From: Colin Ian King <colin.king@...onical.com>
>>
>> arp is being checked instead of arp_eth to see if the call to
>> __skb_header_pointer failed. Fix this by checking arp_eth is
>> null instead of arp.
>>
>> CoverityScan CID#1396428 ("Logically dead code") on 2nd
>> arp comparison (which should be arp_eth instead).
>>
>> Fixes: commit 55733350e5e8b70c5 ("flow disector: ARP support")
>> Signed-off-by: Colin Ian King <colin.king@...onical.com>
>> ---
>>  net/core/flow_dissector.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
>> index e3dffc7..fec48e9 100644
>> --- a/net/core/flow_dissector.c
>> +++ b/net/core/flow_dissector.c
>> @@ -409,7 +409,7 @@ bool __skb_flow_dissect(const struct sk_buff *skb,
>>  					       sizeof(_arp_eth), data,
>>  					       hlen - sizeof(_arp),
>>  					       &_arp_eth);
>> -		if (!arp)
>> +		if (!arp_eth)
>>  			goto out_bad;
>>  
>>  		if (dissector_uses_key(flow_dissector,
> 
> It looks that we try very hard to add critical bugs in flow dissector.
> 
> This is embarrassing really.
> 
> I am questioning if the __skb_header_pointer() is correct
> 
> Why using hlen - sizeof(_arp) ?
> 
>                arp_eth = __skb_header_pointer(skb, nhoff + sizeof(_arp),
>                                               sizeof(_arp_eth), data,
>                                               hlen - sizeof(_arp),
>                                               &_arp_eth);
> 

Yep, the sizeof maybe dubious too, I overlooked that one; if somebody
can clarify that then I'll send a V2 if it needs fixing up too.

Colin