netdev - Re: [PATCH net v2] udp6: set dst cache for a connected sk before udp_v6_send

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Message-ID: <20180326170037.36xikwnham3zaiel@kafai-mbp.dhcp.thefacebook.com>
Date:   Mon, 26 Mar 2018 10:02:47 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Alexey Kodanev <alexey.kodanev@...cle.com>
CC:     <netdev@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH net v2] udp6: set dst cache for a connected sk before
 udp_v6_send_skb

On Mon, Mar 26, 2018 at 05:48:47PM +0300, Alexey Kodanev wrote:
> After commit 33c162a980fe ("ipv6: datagram: Update dst cache of a
> connected datagram sk during pmtu update"), when the error occurs on
> sending datagram in udpv6_sendmsg() due to ICMPV6_PKT_TOOBIG type,
> error handler can trigger the following path and call ip6_dst_store():
> 
>     udpv6_err()
>         ip6_sk_update_pmtu()
>             ip6_datagram_dst_update()
>                 ip6_dst_lookup_flow(), can create a RTF_CACHE clone
Instead of ip6_dst_lookup_flow(),
you meant the RTF_CACHE route created in ip6_update_pmtu()

>                 ...
>                 ip6_dst_store()
> 
> It can happen before a connected UDP socket invokes ip6_dst_store()
> in the end of udpv6_sendmsg(), on destination release, as a result,
> the last one changes dst to the old one, preventing getting updated
> dst cache on the next udpv6_sendmsg() call.
> 
> This patch moves ip6_dst_store() in udpv6_sendmsg(), so that it is
> invoked after ip6_sk_dst_lookup_flow() and before udp_v6_send_skb().
After this patch, the above udpv6_err() path could not happen after
ip6_sk_dst_lookup_flow() and before the ip6_dst_store() in udpv6_sendmsg()?

> 
> Also, increase refcnt for dst, when passing it to ip6_dst_store()
> because after that the dst cache can be released by other calls
> to ip6_dst_store() with the same socket.
> 
> Fixes: 33c162a980fe ("ipv6: datagram: Update dst cache of a connected datagram sk during pmtu update")
> Signed-off-by: Alexey Kodanev <alexey.kodanev@...cle.com>
> ---
> 
> v2: * remove 'release_dst:' label
> 
>     * move ip6_dst_store() below MSG_CONFIRM check as
>       suggested by Eric and add dst_clone()
> 
>     * add 'Fixes' commit.
> 
> 
>  net/ipv6/udp.c | 29 +++++++++++------------------
>  1 file changed, 11 insertions(+), 18 deletions(-)
> 
> diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
> index 52e3ea0..4508e5a 100644
> --- a/net/ipv6/udp.c
> +++ b/net/ipv6/udp.c
> @@ -1303,6 +1303,16 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>  		goto do_confirm;
>  back_from_confirm:
>  
> +	if (connected)
> +		ip6_dst_store(sk, dst_clone(dst),
> +			      ipv6_addr_equal(&fl6.daddr, &sk->sk_v6_daddr) ?
> +			      &sk->sk_v6_daddr : NULL,
> +#ifdef CONFIG_IPV6_SUBTREES
> +			      ipv6_addr_equal(&fl6.saddr, &np->saddr) ?
> +			      &np->saddr :
> +#endif
> +			      NULL);
> +
>  	/* Lockless fast path for the non-corking case */
>  	if (!corkreq) {
>  		struct sk_buff *skb;
> @@ -1314,7 +1324,7 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>  		err = PTR_ERR(skb);
>  		if (!IS_ERR_OR_NULL(skb))
>  			err = udp_v6_send_skb(skb, &fl6);
> -		goto release_dst;
> +		goto out;
>  	}
>  
>  	lock_sock(sk);
> @@ -1348,23 +1358,6 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
>  		err = np->recverr ? net_xmit_errno(err) : 0;
>  	release_sock(sk);
>  
> -release_dst:
> -	if (dst) {
> -		if (connected) {
> -			ip6_dst_store(sk, dst,
> -				      ipv6_addr_equal(&fl6.daddr, &sk->sk_v6_daddr) ?
> -				      &sk->sk_v6_daddr : NULL,
> -#ifdef CONFIG_IPV6_SUBTREES
> -				      ipv6_addr_equal(&fl6.saddr, &np->saddr) ?
> -				      &np->saddr :
> -#endif
> -				      NULL);
> -		} else {
> -			dst_release(dst);
> -		}
> -		dst = NULL;
> -	}
> -
>  out:
>  	dst_release(dst);
>  	fl6_sock_release(flowlabel);
> -- 
> 1.8.3.1
>