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Message-ID: <20190612220105.00000d39@gmail.com>
Date: Wed, 12 Jun 2019 22:01:05 +0200
From: Maciej Fijalkowski <maciejromanfijalkowski@...il.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Jesper Dangaard Brouer <brouer@...hat.com>, netdev@...r.kernel.org,
Daniel Borkmann <daniel@...earbox.net>,
Alexei Starovoitov <ast@...nel.org>,
David Miller <davem@...emloft.net>,
Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: [PATCH bpf-next v3 2/3] bpf_xdp_redirect_map: Perform map
lookup in eBPF helper
On Tue, 11 Jun 2019 20:17:02 +0200
Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> Jesper Dangaard Brouer <brouer@...hat.com> writes:
>
> > On Tue, 11 Jun 2019 17:44:00 +0200
> > Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >
> >> From: Toke Høiland-Jørgensen <toke@...hat.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@...hat.com>
> >> ---
> >> 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
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