Re: [PATCH bpf-next v5 2/3] bpf_xdp_redirect_map: Perform map lookup in eBPF helper

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Daniel Borkmann <daniel@iogearbox.net> writes:

> On 06/23/2019 04:17 AM, Toke Høiland-Jørgensen wrote:
>> From: Toke Høiland-Jørgensen <toke@redhat.com>
>> 
>> The bpf_redirect_map() helper used by XDP programs doesn't return any
>> indication of whether it can successfully redirect to the map index it was
>> given. Instead, BPF programs have to track this themselves, leading to
>> programs using duplicate maps to track which entries are populated in the
>> devmap.
>> 
>> This patch fixes this by moving the map lookup into the bpf_redirect_map()
>> helper, which makes it possible to return failure to the eBPF program. The
>> lower bits of the flags argument is used as the return code, which means
>> that existing users who pass a '0' flag argument will get XDP_ABORTED.
>> 
>> With this, a BPF program can check the return code from the helper call and
>> react by, for instance, substituting a different redirect. This works for
>> any type of map used for redirect.
>> 
>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> [...]
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 183bf4d8e301..a6779e1cc1b8 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -3605,17 +3605,13 @@ static int xdp_do_redirect_map(struct net_device *dev, struct xdp_buff *xdp,
>>  			       struct bpf_redirect_info *ri)
>>  {
>>  	u32 index = ri->ifindex;
>> -	void *fwd = NULL;
>> +	void *fwd = ri->item;
>>  	int err;
>>  
>>  	ri->ifindex = 0;
>> +	ri->item = NULL;
>>  	WRITE_ONCE(ri->map, NULL);
>>  
>> -	fwd = __xdp_map_lookup_elem(map, index);
>> -	if (unlikely(!fwd)) {
>> -		err = -EINVAL;
>> -		goto err;
>> -	}
>
> If you look at the _trace_xdp_redirect{,_err}(), we should also get rid of the
> extra NULL test in devmap_ifindex() which is not under tracepoint static key.

ACK, will add.

-Toke