Subject: Re: [PATCH bpf-next] bpf: add skb->queue_mapping write access from tc clsact

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 19 Feb 2019 15:52:59 +0100

On Tue, 19 Feb 2019 12:46:57 +0100
Daniel Borkmann <[email-masked]> wrote:

> On 02/19/2019 11:24 AM, Jesper Dangaard Brouer wrote:
> > The skb->queue_mapping already have read access, via __sk_buff->queue_mapping.
> > 
> > This patch allow BPF tc qdisc clsact write access to the queue_mapping via
> > tc_cls_act_is_valid_access.
> > 
> > It is already possible to change this via TC filter action skbedit
> > tc-skbedit(8).  Due to the lack of TC examples, lets show one:
> > 
> >  # tc qdisc  add  dev ixgbe1 handle ffff: ingress
> >  # tc filter add  dev ixgbe1 parent ffff: matchall action skbedit queue_mapping 5
> >  # tc filter list dev ixgbe1 parent ffff:  
> 
> Using handles was in the old days, if we add examples, then lets do
> something more user friendly ;)
> 
>   # tc qdisc  add     dev ixgbe1 clsact
>   # tc filter replace dev ixgbe1 ingress matchall action skbedit queue_mapping 5
>   # tc filter list    dev ixgbe1 ingress
> 
> > The most common mistake is that XPS (Transmit Packet Steering) takes
> > precedence over setting skb->queue_mapping. XPS is configured per DEVICE
> > via /sys/class/net/DEVICE/queues/tx-*/xps_cpus via a CPU hex mask. To
> > disable set mask=00.
> > 
> > The purpose of changing skb->queue_mapping is to influence the selection of
> > the net_device "txq" (struct netdev_queue), which influence selection of
> > the qdisc "root_lock" (via txq->qdisc->q.lock) and txq->_xmit_lock. When
> > using the MQ qdisc the txq->qdisc points to different qdiscs and associated
> > locks, and HARD_TX_LOCK (txq->_xmit_lock), allowing for CPU scalability.
> > 
> > Due to lack of TC examples, lets show howto attach clsact BPF programs:
> > 
> >  # tc qdisc  add     dev ixgbe2 clsact
> >  # tc filter replace dev ixgbe2 egress bpf da obj XXX_kern.o sec tc_qmap2cpu
> >  # tc filter list    dev ixgbe2 egress
> > 
> > Signed-off-by: Jesper Dangaard Brouer <[email-masked]>
> > ---
> >  net/core/filter.c |   14 +++++++++++---
> >  1 file changed, 11 insertions(+), 3 deletions(-)
> > 
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 353735575204..d05ae8d05397 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -6238,6 +6238,7 @@ static bool tc_cls_act_is_valid_access(int off, int size,
> >  		case bpf_ctx_range(struct __sk_buff, tc_classid):
> >  		case bpf_ctx_range_till(struct __sk_buff, cb[0], cb[4]):
> >  		case bpf_ctx_range(struct __sk_buff, tstamp):
> > +		case bpf_ctx_range(struct __sk_buff, queue_mapping):
> >  			break;
> >  		default:
> >  			return false;
> > @@ -6642,9 +6643,16 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
> >  		break;
> >  
> >  	case offsetof(struct __sk_buff, queue_mapping):
> > -		*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
> > -				      bpf_target_off(struct sk_buff, queue_mapping, 2,
> > -						     target_size));
> > +		if (type == BPF_WRITE)
> > +			*insn++ = BPF_STX_MEM(BPF_H, si->dst_reg, si->src_reg,
> > +					      bpf_target_off(struct sk_buff,
> > +							     queue_mapping,
> > +							     2, target_size));
> > +		else
> > +			*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
> > +					      bpf_target_off(struct sk_buff,
> > +							     queue_mapping,
> > +							     2, target_size));  
> 
> One thing we should avoid would be to allow user to write NO_QUEUE_MAPPING
> into skb->queue_mapping so we don't hit the warn in sk_tx_queue_set(), I'd
> add this into the ctx rewrite here.

Makes sense. I would really appreciate if you could help me out writing
the needed BPF instructions, as I'm not an expert here.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer