netdev - Re: [PATCH net-next] tcp: Check daddr_cache before use in tracepoint

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Message-ID: <1508211908.31614.90.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Mon, 16 Oct 2017 20:45:08 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     netdev@...r.kernel.org, xiyou.wangcong@...il.com
Subject: Re: [PATCH net-next] tcp: Check daddr_cache before use in tracepoint

On Mon, 2017-10-16 at 14:29 -0700, David Ahern wrote:
> Running perf in one window to capture tcp_retransmit_skb tracepoint:
>     $ perf record -e tcp:tcp_retransmit_skb -a
> 
> And causing a retransmission on an active TCP session (e.g., dropping
> packets in the receiver, changing MTU on the interface to 500 and back
> to 1500) triggers a panic:
...

> Check that daddr_cache is non-NULL before de-referencing.
> 
> Fixes: e086101b150a ("tcp: add a tracepoint for tcp retransmission")
> Signed-off-by: David Ahern <dsahern@...il.com>
> ---
>  include/trace/events/tcp.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/trace/events/tcp.h b/include/trace/events/tcp.h
> index 3d1cbd072b7e..13e8ee8af2c8 100644
> --- a/include/trace/events/tcp.h
> +++ b/include/trace/events/tcp.h
> @@ -48,7 +48,8 @@ TRACE_EVENT(tcp_retransmit_skb,
>  			pin6 = (struct in6_addr *)__entry->saddr_v6;
>  			*pin6 = np->saddr;
>  			pin6 = (struct in6_addr *)__entry->daddr_v6;
> -			*pin6 = *(np->daddr_cache);
> +			if (np->daddr_cache)
> +				*pin6 = *(np->daddr_cache);
>  		} else {
>  			pin6 = (struct in6_addr *)__entry->saddr_v6;
>  			ipv6_addr_set_v4mapped(inet->inet_saddr, pin6);

This is weird.

IPV6 TCP uses sk->sk_v6_daddr and sk->->sk_v6_rcv_saddr

So I would rather remove the need to fetch np = inet6_sk(sk) in the
first place, and look at sk->sk_family instead.

No need to get to np at all.