netdev - Re: [PATCH net] net: ipv6: ip6_output: alloc skb with tailroom

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Date:   Wed, 6 Jun 2018 14:09:20 -0400
From:   Alexander Aring <aring@...atatu.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, yoshfuji@...ux-ipv6.org,
        david.palma@...u.no, rabinarayans0828@...il.com, jhs@...atatu.com,
        stefan@....samsung.com, linux-wpan@...r.kernel.org,
        kernel@...atatu.com
Subject: Re: [PATCH net] net: ipv6: ip6_output: alloc skb with tailroom

Hi,

On Wed, Jun 06, 2018 at 01:53:39PM -0400, David Miller wrote:
> From: Alexander Aring <aring@...atatu.com>
> Date: Tue,  5 Jun 2018 18:04:04 -0400
> 
> > This patch adds care about tailroom length for allocate a skb from ipv6
> > level stack. In case of 6lowpan we had the problem the skb runs into a
> > skb_over_panic() in some special length cases. The root was there was no
> > tailroom allocated for the IEEE 802.15.4 checksum, although we had
> > the necessary tailroom specified inside the netdev structure.
> > 
> > Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=195059
> > Reported-by: David Palma <david.palma@...u.no>
> > Reported-by: Rabi Narayan Sahoo <rabinarayans0828@...il.com>
> > Signed-off-by: Alexander Aring <aring@...atatu.com>
> 
> needed_tailroom is an optimization to avoid SKB reallocations
> and adjustments, it is not a guarantee.
> 

okay, then you want to have this patch for net-next? As an optimization?

Of course, when it's open again.

> If you are seeing crashes, it means code is assuming something which
> is not to be assumed.
> 
> Whatever code is involved, it needs to check that the necessary
> tailroom is there and reallocate if necessary, rather than
> blindly pushing past the end of the SKB data.
> 

I see, I will add checks and reallocs (if necessary) in the underlaying
subsystem level.

Thanks for clarifying this.

- Alex