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: Wed, 12 Jun 2019 23:33:17 +0200

Maciej Fijalkowski <[email-masked]> writes:

> On Tue, 11 Jun 2019 20:17:02 +0200
> Toke Høiland-Jørgensen <[email-masked]> wrote:
>
>> 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;
>> >>  
>
> Here you don't allow the flags to get different value than
> XDP_REDIRECT_ALL_FLAGS.
>
>> >> +	ri->item = __xdp_map_lookup_elem(map, ifindex);
>> >> +	if (unlikely(!ri->item)) {
>> >> +		WRITE_ONCE(ri->map, NULL);
>> >> +		return (flags & XDP_REDIRECT_INVALID_MASK);  
>
> So here you could just return flags? Don't we know that the flags
> value is legit here? Am I missing something?

No, you're right. I was just worried that we would forget to change this
when we add another flag later on, so I thought I'd mask it from the
get-go. Can't the compiler figure out that the second mask is unnecessary?

> TBH the v2 was more clear to me.

Clearer how? As in, you prefer just always returning XDP_PASS on error?

-Toke

>> > 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?
>
> Maybe something along ALLOWED_RETVAL_MASK?

Yeah, good idea! :)

-Toke