Subject: Re: [PATCH bpf-next v3 2/3] bpf_xdp_redirect_map: Perform map lookup in eBPF helper

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 11 Jun 2019 20:17:02 +0200

Jesper Dangaard Brouer <[email-masked]> writes:

> On Tue, 11 Jun 2019 17:44:00 +0200
> Toke Høiland-Jørgensen <[email-masked]> wrote:
>
>> From: Toke Høiland-Jørgensen <[email-masked]>
>> 
>> 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 <[email-masked]>
>> ---
>>  include/linux/filter.h   |    1 +
>>  include/uapi/linux/bpf.h |    8 ++++++++
>>  net/core/filter.c        |   26 ++++++++++++--------------
>>  3 files changed, 21 insertions(+), 14 deletions(-)
>> 
>> diff --git a/include/linux/filter.h b/include/linux/filter.h
>> index 43b45d6db36d..f31ae8b9035a 100644
>> --- a/include/linux/filter.h
>> +++ b/include/linux/filter.h
>> @@ -580,6 +580,7 @@ struct bpf_skb_data_end {
>>  struct bpf_redirect_info {
>>  	u32 ifindex;
>>  	u32 flags;
>> +	void *item;
>>  	struct bpf_map *map;
>>  	struct bpf_map *map_to_flush;
>>  	u32 kern_flags;
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index 7c6aef253173..9931cf02de19 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -3098,6 +3098,14 @@ enum xdp_action {
>>  	XDP_REDIRECT,
>>  };
>>  
>> +/* Flags for bpf_xdp_redirect_map helper */
>> +
>> +/* The lower flag bits will be the return code of bpf_xdp_redirect_map() helper
>> + * if the map lookup fails.
>> + */
>> +#define XDP_REDIRECT_INVALID_MASK (XDP_ABORTED | XDP_DROP | XDP_PASS | XDP_TX)
>> +#define XDP_REDIRECT_ALL_FLAGS XDP_REDIRECT_INVALID_MASK
>> +
>
> Slightly confused about the naming of the define, see later.
>
>>  /* user accessible metadata for XDP packet hook
>>   * new fields must be added to the end of this structure
>>   */
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 7a996887c500..dd43be497480 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -3608,17 +3608,13 @@ 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 (ri->map_to_flush && unlikely(ri->map_to_flush != map))
>>  		xdp_do_flush_map();
>>  
>> @@ -3655,18 +3651,13 @@ static int xdp_do_generic_redirect_map(struct net_device *dev,
>>  {
>>  	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>>  	u32 index = ri->ifindex;
>> -	void *fwd = NULL;
>> +	void *fwd = ri->item;
>>  	int err = 0;
>>  
>>  	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 (map->map_type == BPF_MAP_TYPE_DEVMAP) {
>>  		struct bpf_dtab_netdev *dst = fwd;
>>  
>> @@ -3735,6 +3726,7 @@ BPF_CALL_2(bpf_xdp_redirect, u32, ifindex, u64, flags)
>>  
>>  	ri->ifindex = ifindex;
>>  	ri->flags = flags;
>> +	ri->item = NULL;
>>  	WRITE_ONCE(ri->map, NULL);
>>  
>>  	return XDP_REDIRECT;
>> @@ -3753,9 +3745,15 @@ BPF_CALL_3(bpf_xdp_redirect_map, struct bpf_map *, map, u32, ifindex,
>>  {
>>  	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>>  
>> -	if (unlikely(flags))
>> +	if (unlikely(flags & ~XDP_REDIRECT_ALL_FLAGS))
>>  		return XDP_ABORTED;
>>  
>> +	ri->item = __xdp_map_lookup_elem(map, ifindex);
>> +	if (unlikely(!ri->item)) {
>> +		WRITE_ONCE(ri->map, NULL);
>> +		return (flags & XDP_REDIRECT_INVALID_MASK);
>
> Maybe I'm reading it wrong, but shouldn't the mask be called the "valid" mask?

It's the mask that is applied when the index looked up is invalid (i.e.,
the entry doesn't exist)? But yeah, can see how the name can be
confusing; maybe it should just be "RETURN_MASK" or something like that?

-Toke