Subject: Re: [PATCH net-next 2/2] devmap: Allow map lookups from eBPF

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 04 Jun 2019 22:22:47 +0200

Toke Høiland-Jørgensen <[email-masked]> writes:

> We don't currently allow lookups into a devmap from eBPF, because the map
> lookup returns a pointer directly to the dev->ifindex, which shouldn't be
> modifiable from eBPF.
>
> However, being able to do lookups in devmaps is useful to know (e.g.)
> whether forwarding to a specific interface is enabled. Currently, programs
> work around this by keeping a shadow map of another type which indicates
> whether a map index is valid.
>
> To allow lookups, simply copy the ifindex into a scratch variable and
> return a pointer to this. If an eBPF program does modify it, this doesn't
> matter since it will be overridden on the next lookup anyway. While this
> does add a write to every lookup, the overhead of this is negligible
> because the cache line is hot when both the write and the subsequent read
> happens.
>
> Signed-off-by: Toke Høiland-Jørgensen <[email-masked]>
> ---
>  kernel/bpf/devmap.c   |    8 +++++++-
>  kernel/bpf/verifier.c |    7 ++-----
>  2 files changed, 9 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index 5ae7cce5ef16..830650300ea4 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -65,6 +65,7 @@ struct xdp_bulk_queue {
>  struct bpf_dtab_netdev {
>  	struct net_device *dev; /* must be first member, due to tracepoint */
>  	struct bpf_dtab *dtab;
> +	int ifindex_scratch;

Just realised I forgot to make this per-cpu; I'll send an updated
version once we settle on a solution that works for xskmap as well...

-Toke