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Date:   Sat, 12 Feb 2022 12:46:25 -0700
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Subject: Re: [PATCH] ipv6: mcast: use rcu-safe version of ipv6_get_lladdr()
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To:     Ignat Korchagin <ignat@cloudflare.com>,
        "David S . Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     kernel-team@cloudflare.com, dpini@cloudflare.com
References: <20220211173042.112852-1-ignat@cloudflare.com>
From:   David Ahern <dsahern@gmail.com>
In-Reply-To: <20220211173042.112852-1-ignat@cloudflare.com>
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On 2/11/22 9:30 AM, Ignat Korchagin wrote:
> diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
> index f927c199a93c..3f23da8c0b10 100644
> --- a/net/ipv6/addrconf.c
> +++ b/net/ipv6/addrconf.c
> @@ -1839,8 +1839,8 @@ int ipv6_dev_get_saddr(struct net *net, const struct net_device *dst_dev,
>  }
>  EXPORT_SYMBOL(ipv6_dev_get_saddr);
>  
> -int __ipv6_get_lladdr(struct inet6_dev *idev, struct in6_addr *addr,
> -		      u32 banned_flags)
> +static int __ipv6_get_lladdr(struct inet6_dev *idev, struct in6_addr *addr,
> +			      u32 banned_flags)
>  {
>  	struct inet6_ifaddr *ifp;
>  	int err = -EADDRNOTAVAIL;
> diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
> index bed8155508c8..a8861db52c18 100644
> --- a/net/ipv6/mcast.c
> +++ b/net/ipv6/mcast.c
> @@ -1759,7 +1759,7 @@ static struct sk_buff *mld_newpack(struct inet6_dev *idev, unsigned int mtu)
>  	skb_reserve(skb, hlen);
>  	skb_tailroom_reserve(skb, mtu, tlen);
>  
> -	if (__ipv6_get_lladdr(idev, &addr_buf, IFA_F_TENTATIVE)) {
> +	if (ipv6_get_lladdr(dev, &addr_buf, IFA_F_TENTATIVE)) {
>  		/* <draft-ietf-magma-mld-source-05.txt>:
>  		 * use unspecified address as the source address
>  		 * when a valid link-local address is not available.

Why not add read_lock_bh(&idev->lock); ... read_unlock_bh(&idev->lock);
around the call to __ipv6_get_lladdr? you already have the idev.