Subject: Re: [PATCH net-next v2] net: sctp: keep owned chunk in destructor_arg instead of skb->cb

From: Vlad Yasevich <[email-masked]>
Date: Wed, 19 Nov 2014 22:02:00 -0500

On 11/19/2014 07:54 PM, Daniel Borkmann wrote:
> It's just silly to hold the skb destructor argument around inside
> skb->cb[] as we currently do in SCTP.
> 
> Nowadays, we're sort of cheating on data accounting in the sense
> that due to commit 4c3a5bdae293 ("sctp: Don't charge for data in
> sndbuf again when transmitting packet"), we orphan the skb already
> in the SCTP output path, i.e. giving back charged data memory, and
> use a different destructor only to make sure the sk doesn't vanish
> on skb destruction time. Thus, cb[] is still valid here as we
> operate within the SCTP layer. (It's generally actually a big
> candidate for future rework, imho.)
> 
> However, storing the destructor in the cb[] can easily cause issues
> should an non sctp_packet_set_owner_w()'ed skb ever escape the SCTP
> layer, since cb[] may get overwritten by lower layers and thus can
> corrupt the chunk pointer. There are no such issues at present,
> but lets keep the chunk in destructor_arg, as this is the actual
> purpose for it.
> 
> Signed-off-by: Daniel Borkmann <[email-masked]>

Good clean-up.

Acked-by: Vlad Yasevich <[email-masked]>

-vlad

> ---
>  v1->v2:
>   - Only reworded commit message to make it more clear
> 
>  net/sctp/socket.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 2120292..85e0b65 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -162,7 +162,7 @@ static inline void sctp_set_owner_w(struct sctp_chunk *chunk)
>  
>  	chunk->skb->destructor = sctp_wfree;
>  	/* Save the chunk pointer in skb for sctp_wfree to use later.  */
> -	*((struct sctp_chunk **)(chunk->skb->cb)) = chunk;
> +	skb_shinfo(chunk->skb)->destructor_arg = chunk;
>  
>  	asoc->sndbuf_used += SCTP_DATA_SNDSIZE(chunk) +
>  				sizeof(struct sk_buff) +
> @@ -6870,14 +6870,10 @@ static void sctp_wake_up_waiters(struct sock *sk,
>   */
>  static void sctp_wfree(struct sk_buff *skb)
>  {
> -	struct sctp_association *asoc;
> -	struct sctp_chunk *chunk;
> -	struct sock *sk;
> +	struct sctp_chunk *chunk = skb_shinfo(skb)->destructor_arg;
> +	struct sctp_association *asoc = chunk->asoc;
> +	struct sock *sk = asoc->base.sk;
>  
> -	/* Get the saved chunk pointer.  */
> -	chunk = *((struct sctp_chunk **)(skb->cb));
> -	asoc = chunk->asoc;
> -	sk = asoc->base.sk;
>  	asoc->sndbuf_used -= SCTP_DATA_SNDSIZE(chunk) +
>  				sizeof(struct sk_buff) +
>  				sizeof(struct sctp_chunk);
>