From: Hannes Frederic Sowa <hannes@stressinduktion•org>
To: Benjamin Poirier <bpoirier@suse•com>,
Daniel Borkmann <daniel@iogearbox•net>
Cc: netdev@vger•kernel.org, Eric Dumazet <eric.dumazet@gmail•com>,
Hideaki YOSHIFUJI <yoshfuji@linux-ipv6•org>
Subject: Re: [PATCH] mld, igmp: Fix reserved tailroom calculation
Date: Mon, 29 Feb 2016 16:43:08 +0100 [thread overview]
Message-ID: <56D4670C.40902@stressinduktion.org> (raw)
In-Reply-To: <20160229151920.GA5474@f1.synalogic.ca>
On 29.02.2016 16:19, Benjamin Poirier wrote:
> On 2016/02/29 15:57, Daniel Borkmann wrote:
> [...]
>>
>> [ cutting the IPv4 part off as diff is the same ]
>>
>>> diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
>>> index 5ee56d0..c157edc 100644
>>> --- a/net/ipv6/mcast.c
>>> +++ b/net/ipv6/mcast.c
>>> @@ -1574,9 +1574,9 @@ static struct sk_buff *mld_newpack(struct inet6_dev *idev, unsigned int mtu)
>>> return NULL;
>>>
>>> skb->priority = TC_PRIO_CONTROL;
>>> - skb->reserved_tailroom = skb_end_offset(skb) -
>>> - min(mtu, skb_end_offset(skb));
>>> skb_reserve(skb, hlen);
>>> + skb->reserved_tailroom = skb_tailroom(skb) -
>>> + min_t(int, mtu, skb_tailroom(skb) - tlen);
>>
>> Are you sure this is correct? Wouldn't that mean (assuming we allocated
>> enough space), that I could now fill a larger than MTU frame?
>
> Quoting back a part of the log:
>
>>> The maximum space available for ip headers and payload without
>>> fragmentation is min(mtu, data + extra). Therefore,
>>> reserved_tailroom
>>> = data + extra + tlen - min(mtu, data + extra)
>>> = skb_end_offset - hlen - min(mtu, skb_end_offset - hlen - tlen)
>>> = skb_tailroom - min(mtu, skb_tailroom - tlen) ; after skb_reserve(hlen)
>
> The min() takes care of the situation you describe, ie. if the allocated
> space is large, reserved_tailroom will be large enough that we do not
> use more space than the mtu.
>
> I tested the mld and igmp code with different driver parameters, mtu
> values, number of multicast address records and even allocation
> failures. If you think the formula is wrong, please provide a
> counter-example with hlen, tlen, mtu and size values.
I think the code is fine albeit I think we should remove the min macro
and just do something:
if (skb_tailroom(skb) > mtu)
skb->reserved_tailroom = skb_tailroom(skb) - mtu;
Does that make sense? I think it is much more readable.
Thanks,
Hannes
next prev parent reply other threads:[~2016-02-29 15:43 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-27 19:57 [PATCH] mld, igmp: Fix reserved tailroom calculation Benjamin Poirier
2016-02-29 14:57 ` Daniel Borkmann
2016-02-29 15:19 ` Benjamin Poirier
2016-02-29 15:38 ` Daniel Borkmann
2016-02-29 15:43 ` Hannes Frederic Sowa [this message]
2016-02-29 18:08 ` Benjamin Poirier
2016-02-29 18:28 ` Hannes Frederic Sowa
2016-02-29 23:03 ` [PATCH net v2] " Benjamin Poirier
2016-03-01 10:09 ` Hannes Frederic Sowa
2016-03-01 10:18 ` Daniel Borkmann
2016-03-01 16:00 ` Hannes Frederic Sowa
2016-03-03 20:42 ` David Miller
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