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

From: Maciej Fijalkowski <[email-masked]>
Date: Wed, 12 Jun 2019 22:01:05 +0200

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? TBH the v2 was more clear to me.

> >
> > 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?

> -Toke