On Thu, Nov 20, 2014 at 01:54:48AM +0100, 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 <dborkman@xxxxxxxxxx>
> ---
> 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);
> --
> 1.7.11.7
>
> --
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>
Acked-by: Neil Horman <nhorman@xxxxxxxxxxxxx>
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