Subject: Re: [PATCH net] net: sctp: use MAX_HEADER for headroom reserve in output path

From: Vlad Yasevich <[email-masked]>
Date: Wed, 03 Dec 2014 10:32:02 -0500

On 12/03/2014 06:13 AM, Daniel Borkmann wrote:
> To accomodate for enough headroom for tunnels, use MAX_HEADER instead
> of LL_MAX_HEADER. Robert reported that he has hit after roughly 40hrs
> of trinity an skb_under_panic() via SCTP output path (see reference).
> I couldn't reproduce it from here, but not using MAX_HEADER as elsewhere
> in other protocols might be one possible cause for this.
> 
> In any case, it looks like accounting on chunks themself seems to look
> good as the skb already passed the SCTP output path and did not hit
> any skb_over_panic(). Given tunneling was enabled in his .config, the
> headroom would have been expanded by MAX_HEADER in this case.
> 
> Reported-by: Robert Święcki <[email-masked]>
> Reference: https://lkml.org/lkml/2014/12/1/507
> Fixes: 594ccc14dfe4d ("[SCTP] Replace incorrect use of dev_alloc_skb with alloc_skb in sctp_packet_transmit().")
> Signed-off-by: Daniel Borkmann <[email-masked]>

Looks right.  If sctp path is over any kind of L3 tunnel, we'll see this.

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

-vlad

> ---
>  net/sctp/output.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/sctp/output.c b/net/sctp/output.c
> index 42dffd4..fc5e45b 100644
> --- a/net/sctp/output.c
> +++ b/net/sctp/output.c
> @@ -401,12 +401,12 @@ int sctp_packet_transmit(struct sctp_packet *packet)
>  	sk = chunk->skb->sk;
>  
>  	/* Allocate the new skb.  */
> -	nskb = alloc_skb(packet->size + LL_MAX_HEADER, GFP_ATOMIC);
> +	nskb = alloc_skb(packet->size + MAX_HEADER, GFP_ATOMIC);
>  	if (!nskb)
>  		goto nomem;
>  
>  	/* Make sure the outbound skb has enough header room reserved. */
> -	skb_reserve(nskb, packet->overhead + LL_MAX_HEADER);
> +	skb_reserve(nskb, packet->overhead + MAX_HEADER);
>  
>  	/* Set the owning socket so that we know where to get the
>  	 * destination IP address.
>