Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 7 Jul 2021 09:42:18 -0700
From:   Jakub Kicinski <kuba@kernel.org>
To:     David Ahern <dsahern@gmail.com>
Cc:     Vasily Averin <vvs@virtuozzo.com>,
        "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH IPV6 1/1] ipv6: allocate enough headroom in
 ip6_finish_output2()
Message-ID: <20210707094218.0e9b6ffc@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
In-Reply-To: <8996db63-5554-d3dc-cd36-94570ade6d18@gmail.com>
References: <cover.1625665132.git.vvs@virtuozzo.com>
        <3cb5a2e5-4e4c-728a-252d-4757b6c9612d@virtuozzo.com>
        <8996db63-5554-d3dc-cd36-94570ade6d18@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Wed, 7 Jul 2021 08:45:13 -0600 David Ahern wrote:
> On 7/7/21 8:04 AM, Vasily Averin wrote:
> > diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> > index ff4f9eb..e5af740 100644
> > --- a/net/ipv6/ip6_output.c
> > +++ b/net/ipv6/ip6_output.c
> > @@ -61,9 +61,24 @@ static int ip6_finish_output2(struct net *net, struct sock *sk, struct sk_buff *
> >  	struct dst_entry *dst = skb_dst(skb);
> >  	struct net_device *dev = dst->dev;
> >  	const struct in6_addr *nexthop;
> > +	unsigned int hh_len = LL_RESERVED_SPACE(dev);
> >  	struct neighbour *neigh;
> >  	int ret;
> >  
> > +	/* Be paranoid, rather than too clever. */
> > +	if (unlikely(skb_headroom(skb) < hh_len && dev->header_ops)) {
> > +		struct sk_buff *skb2;
> > +
> > +		skb2 = skb_realloc_headroom(skb, LL_RESERVED_SPACE(dev));  
> 
> why not use hh_len here?

Is there a reason for the new skb? Why not pskb_expand_head()?

> > +		if (!skb2) {
> > +			kfree_skb(skb);
> > +			return -ENOMEM;
> > +		}
> > +		if (skb->sk)
> > +			skb_set_owner_w(skb2, skb->sk);
> > +		consume_skb(skb);
> > +		skb = skb2;
> > +	}
> >  	if (ipv6_addr_is_multicast(&ipv6_hdr(skb)->daddr)) {
> >  		struct inet6_dev *idev = ip6_dst_idev(skb_dst(skb));