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Date:   Mon, 13 May 2019 09:07:48 -0600
From:   David Ahern <dsahern@...il.com>
To:     Sabrina Dubroca <sd@...asysnail.net>, netdev@...r.kernel.org
Cc:     Dan Winship <danw@...hat.com>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>
Subject: Re: [PATCH net v2] rtnetlink: always put ILFA_LINK for links with a
 link-netnsid

On 5/13/19 7:47 AM, Sabrina Dubroca wrote:
> Currently, nla_put_iflink() doesn't put the IFLA_LINK attribute when
> iflink == ifindex.
> 
> In some cases, a device can be created in a different netns with the
> same ifindex as its parent. That device will not dump its IFLA_LINK
> attribute, which can confuse some userspace software that expects it.
> For example, if the last ifindex created in init_net and foo are both
> 8, these commands will trigger the issue:
> 
>     ip link add parent type dummy                   # ifindex 9
>     ip link add link parent netns foo type macvlan  # ifindex 9 in ns foo
> 
> So, in case a device puts the IFLA_LINK_NETNSID attribute in a dump,
> always put the IFLA_LINK attribute as well.
> 
> Thanks to Dan Winship for analyzing the original OpenShift bug down to
> the missing netlink attribute.
> 
> Analyzed-by: Dan Winship <danw@...hat.com>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Sabrina Dubroca <sd@...asysnail.net>
> Acked-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>
> ---
> v2: change Fixes tag, it's been here forever as Nicolas said, and add his Ack
> 
>  net/core/rtnetlink.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index 2bd12afb9297..adcc045952c2 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -1496,14 +1496,15 @@ static int put_master_ifindex(struct sk_buff *skb, struct net_device *dev)
>  	return ret;
>  }
>  
> -static int nla_put_iflink(struct sk_buff *skb, const struct net_device *dev)
> +static int nla_put_iflink(struct sk_buff *skb, const struct net_device *dev,
> +			  bool force)
>  {
>  	int ifindex = dev_get_iflink(dev);
>  
> -	if (dev->ifindex == ifindex)
> -		return 0;
> +	if (force || dev->ifindex != ifindex)
> +		return nla_put_u32(skb, IFLA_LINK, ifindex);
>  
> -	return nla_put_u32(skb, IFLA_LINK, ifindex);
> +	return 0;
>  }
>  
>  static noinline_for_stack int nla_put_ifalias(struct sk_buff *skb,

why not always adding the attribute?

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