netdev - Re: [PATCH net 2/3] sctp: hold transport before we access t->asoc in sctp proc

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Date:	Thu, 21 Jan 2016 11:27:36 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Xin Long <lucien.xin@...il.com>
Cc:	network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
	Vlad Yasevich <vyasevich@...il.com>, daniel@...earbox.net,
	davem@...emloft.net
Subject: Re: [PATCH net 2/3] sctp: hold transport before we access t->asoc
 in sctp proc

On Fri, 2016-01-22 at 01:49 +0800, Xin Long wrote:
> Previously, before rhashtable, /proc assoc listing was done by
> read-locking the entire hash entry and dumping all assocs at once, so we
> were sure that the assoc wasn't freed because it wouldn't be possible to
> remove it from the hash meanwhile.
> 
> Now we use rhashtable to list transports, and dump entries one by one.
> That is, now we have to check if the assoc is still a good one, as the
> transport we got may be being freed.
> 
> Signed-off-by: Xin Long <lucien.xin@...il.com>
> ---
>  net/sctp/proc.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/net/sctp/proc.c b/net/sctp/proc.c
> index 684c5b3..c74a810 100644
> --- a/net/sctp/proc.c
> +++ b/net/sctp/proc.c
> @@ -380,6 +380,8 @@ static int sctp_assocs_seq_show(struct seq_file *seq, void *v)
>  	}
>  
>  	transport = (struct sctp_transport *)v;

What protects you from this structure already being freed ?

> +	if (!sctp_transport_hold(transport))
> +		return 0;

If this is rcu, then you do not need to increment the refcount, and
decrement it later.