Subject: Re: [PATCH] ibmveth: Kernel crash LSO offload flag toggle

From: Daniel Axtens <[email-masked]>
Date: Wed, 15 Nov 2017 13:47:08 +1100

Hi Bryant,

This looks a bit better, but...

> The following patch ensures that the bounce_buffer is not null
> prior to using it within skb_copy_from_linear_data.

How would this occur?

Looking at ibmveth.c, I see bounce_buffer being freed in ibmveth_close()
and allocated in ibmveth_open() only. If allocation fails, the whole
opening of the device fails with -ENOMEM.

It seems your test case - changing TSO - causes ibmveth_set_tso() to
cause an adaptor restart - an ibmveth_close(dev) and then an
ibmveth_open(dev). Is this happening in parallel with an out of memory
condition - is the memory allocation failing?

Alternatively, could it be the case that you're closing the device while
packets are in flight, and then trying to use a bounce_buffer that's
been freed elsewhere? Do you need to decouple memory freeing from
ibmveth_close?

> The problem can be recreated toggling on/off Large send offload.
>
> The following script when run (along with some iperf traffic recreates the
> crash within 5-10 mins or so).
>
> while true
> do
> 	ethtool -k ibmveth0 | grep tcp-segmentation-offload
> 	ethtool -K ibmveth0 tso off
> 	ethtool -k ibmveth0 | grep tcp-segmentation-offload
> 	ethtool -K ibmveth0 tso on
> done
>
> Note: This issue happens the very first time largsesend offload is
> turned off too (but the above script recreates the issue all the times)
>
> [76563.914173] Unable to handle kernel paging request for data at address 0x00000000
> [76563.914197] Faulting instruction address: 0xc000000000063940
> [76563.914205] Oops: Kernel access of bad area, sig: 11 [#1]
> [76563.914210] SMP NR_CPUS=2048 NUMA pSeries
> [76563.914217] Modules linked in: rpadlpar_io rpaphp dccp_diag dccp tcp_diag udp_diag inet_diag unix_diag af_packet_diag netlink_diag nls_utf8 isofs binfmt_misc pseries_rng rtc_generic autofs4 ibmvfc scsi_transport_fc ibmvscsi ibmveth
> [76563.914251] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.4.0-34-generic #53-Ubuntu
                                                            ^--- yikes!

There are relevant changes to this area since 4.4:
2c42bf4b4317 ("ibmveth: check return of skb_linearize in ibmveth_start_xmit")
66aa0678efc2 ("ibmveth: Support to enable LSO/CSO for Trunk VEA.")

Does this crash occurs on a more recent kernel?

> diff --git a/drivers/net/ethernet/ibm/ibmveth.c b/drivers/net/ethernet/ibm/ibmveth.c
> index f210398200ece..1d29b1649118d 100644
> --- a/drivers/net/ethernet/ibm/ibmveth.c
> +++ b/drivers/net/ethernet/ibm/ibmveth.c
> @@ -1092,8 +1092,14 @@ static netdev_tx_t ibmveth_start_xmit(struct sk_buff *skb,
>  	 */
>  	if (force_bounce || (!skb_is_nonlinear(skb) &&
>  				(skb->len < tx_copybreak))) {
> -		skb_copy_from_linear_data(skb, adapter->bounce_buffer,
> -					  skb->len);
> +		if (adapter->bounce_buffer) {
> +			skb_copy_from_linear_data(skb, adapter->bounce_buffer,
> +						  skb->len);
> +		} else {
> +			adapter->tx_send_failed++;
> +			netdev->stats.tx_dropped++;
> +			goto out;
Finally, as I alluded to at the top of this message, isn't the
disappearance of the bounce-buffer a pretty serious issue? As I
understand it, it means your data structures are now inconsistent. Do
you need to - at the least - be more chatty here?

Regards,
Daniel
> +		}
>  
>  		descs[0].fields.flags_len = desc_flags | skb->len;
>  		descs[0].fields.address = adapter->bounce_buffer_dma;
> -- 
> 2.13.6 (Apple Git-96)