netdev - Re: [PATCHv3 net] bonding: fix ICMPv6 header handling when receiving IPv6 messages

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Date:   Fri, 18 Nov 2022 10:49:29 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     Jay Vosburgh <jay.vosburgh@...onical.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Toppins <jtoppins@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>,
        David Ahern <dsahern@...il.com>, Liang Li <liali@...hat.com>,
        David Ahern <dsahern@...nel.org>
Subject: Re: [PATCHv3 net] bonding: fix ICMPv6 header handling when receiving
 IPv6 messages

On Thu, Nov 17, 2022 at 12:04:11PM -0800, Jay Vosburgh wrote:
> Hangbin Liu <liuhangbin@...il.com> wrote:
> 
> >On Wed, Nov 16, 2022 at 08:29:58PM -0800, Jay Vosburgh wrote:
> >> > #if IS_ENABLED(CONFIG_IPV6)
> >> >-	} else if (is_ipv6) {
> >> >+	} else if (is_ipv6 && skb_header_pointer(skb, 0, sizeof(ip6_hdr), &ip6_hdr)) {
> >> > 		return bond_na_rcv(skb, bond, slave);
> >> > #endif
> >> > 	} else {
> >> >
> >> >What do you think?
> >> 
> >> 	I don't see how this solves the icmp6_hdr() / ipv6_hdr() problem
> >> in bond_na_rcv(); skb_header_pointer() doesn't do a pull, it just copies
> >> into the supplied struct (if necessary).
> >
> >Hmm... Maybe I didn't get what you and Eric means. If we can copy the
> >supplied buffer success, doesn't this make sure IPv6 header is in skb?
> 
> 	The header is in the skb, but it may not be in the linear part
> of the skb, i.e., the header is wholly or partially in a skb frag, not
> in the area covered by skb->data ... skb->tail.  The various *_hdr()
> macros only look in the linear area, not the frags, and don't check to
> see if the linear area contains the entire header.
> 
> 	skb_header_pointer() is smart enough to check, and if the
> requested data is entirely within the linear area, it returns a pointer
> to there; if not, it copies from the frags into the supplied struct and
> returns a pointer to that.  What it doesn't do is a pull (move data from
> a frag into the linear area), so merely calling skb_header_pointer()
> doesn't affect the layout of what's in the skb (which is the point,
> bonding uses it here to avoid changing the skb).
> 
> 	There may be better explanations out there, but
> 
> http://vger.kernel.org/~davem/skb_data.html
> 
> 	covers the basics.  Look for the references to "paged data."

Hi Jay,

Thanks a lot for your explanation. I thought you mean there may no IPv6
header in skb. Now I know the problem is using ipv6_hdr() for non-liner skb.
I will update the patch with Eric suggested.

Thanks
Hangbin