netdev - Re: [PATCHv7 bpf-next 2/4] xdp: extend xdp_redirect

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Message-ID: <20210415022127.GQ2900@Leo-laptop-t470s>
Date:   Thu, 15 Apr 2021 10:21:27 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jiri Benc <jbenc@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        David Ahern <dsahern@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Björn Töpel <bjorn.topel@...il.com>
Subject: Re: [PATCHv7 bpf-next 2/4] xdp: extend xdp_redirect_map with
 broadcast support

On Wed, Apr 14, 2021 at 05:23:50PM -0700, Martin KaFai Lau wrote:
> On Wed, Apr 14, 2021 at 08:26:08PM +0800, Hangbin Liu wrote:
> [ ... ]
> 
> > +static __always_inline int __bpf_xdp_redirect_map(struct bpf_map *map, u32 ifindex,
> > +						  u64 flags, u64 flag_mask,
> >  						  void *lookup_elem(struct bpf_map *map, u32 key))
> >  {
> >  	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
> >  
> >  	/* Lower bits of the flags are used as return code on lookup failure */
> > -	if (unlikely(flags > XDP_TX))
> > +	if (unlikely(flags & ~(BPF_F_ACTION_MASK | flag_mask)))
> >  		return XDP_ABORTED;
> >  
> >  	ri->tgt_value = lookup_elem(map, ifindex);
> > -	if (unlikely(!ri->tgt_value)) {
> > +	if (unlikely(!ri->tgt_value) && !(flags & BPF_F_BROADCAST)) {
> >  		/* If the lookup fails we want to clear out the state in the
> >  		 * redirect_info struct completely, so that if an eBPF program
> >  		 * performs multiple lookups, the last one always takes
> > @@ -1482,13 +1484,21 @@ static __always_inline int __bpf_xdp_redirect_map(struct bpf_map *map, u32 ifind
> >  		 */
> >  		ri->map_id = INT_MAX; /* Valid map id idr range: [1,INT_MAX[ */
> >  		ri->map_type = BPF_MAP_TYPE_UNSPEC;
> > -		return flags;
> > +		return flags & BPF_F_ACTION_MASK;
> >  	}
> >  
> >  	ri->tgt_index = ifindex;
> >  	ri->map_id = map->id;
> >  	ri->map_type = map->map_type;
> >  
> > +	if (flags & BPF_F_BROADCAST) {
> > +		WRITE_ONCE(ri->map, map);
> Why only WRITE_ONCE on ri->map?  Is it needed?

I think this is make sure the map pointer assigned to ri->map safely.
which starts from commit f6069b9aa993 ("bpf: fix redirect to map under tail
calls")

> 
> > +		ri->flags = flags;
> > +	} else {
> > +		WRITE_ONCE(ri->map, NULL);
> > +		ri->flags = 0;
> > +	}
> > +
> >  	return XDP_REDIRECT;
> >  }
> >  
> [ ... ]
> 
> > +int dev_map_enqueue_multi(struct xdp_buff *xdp, struct net_device *dev_rx,
> > +			  struct bpf_map *map, bool exclude_ingress)
> > +{
> > +	struct bpf_dtab *dtab = container_of(map, struct bpf_dtab, map);
> > +	int exclude_ifindex = exclude_ingress ? dev_rx->ifindex : 0;
> > +	struct bpf_dtab_netdev *dst, *last_dst = NULL;
> > +	struct hlist_head *head;
> > +	struct hlist_node *next;
> > +	struct xdp_frame *xdpf;
> > +	unsigned int i;
> > +	int err;
> > +
> > +	xdpf = xdp_convert_buff_to_frame(xdp);
> > +	if (unlikely(!xdpf))
> > +		return -EOVERFLOW;
> > +
> > +	if (map->map_type == BPF_MAP_TYPE_DEVMAP) {
> > +		for (i = 0; i < map->max_entries; i++) {
> > +			dst = READ_ONCE(dtab->netdev_map[i]);
> > +			if (!is_valid_dst(dst, xdp, exclude_ifindex))
> > +				continue;
> > +
> > +			/* we only need n-1 clones; last_dst enqueued below */
> > +			if (!last_dst) {
> > +				last_dst = dst;
> > +				continue;
> > +			}
> > +
> > +			err = dev_map_enqueue_clone(last_dst, dev_rx, xdpf);
> > +			if (err)
> > +				return err;
> > +
> > +			last_dst = dst;
> > +		}
> > +	} else { /* BPF_MAP_TYPE_DEVMAP_HASH */
> > +		for (i = 0; i < dtab->n_buckets; i++) {
> > +			head = dev_map_index_hash(dtab, i);
> > +			hlist_for_each_entry_safe(dst, next, head, index_hlist) {
> hmm.... should it be hlist_for_each_entry_rcu() instead?

Ah, makes sense to me. I will fix it.

Thanks
Hangbin