Subject: Re: [PATCH bpf-next 04/16] net: Add BPF_XDP_EGRESS as a bpf_attach_type

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Tue, 21 Apr 2020 12:14:56 +0200

David Ahern <[email-masked]> writes:

> From: David Ahern <[email-masked]>
>
> Add new bpf_attach_type, BPF_XDP_EGRESS, for BPF programs attached
> at the XDP layer, but the egress path.
>
> Since egress path will not have ingress_ifindex and rx_queue_index
> set, update xdp_is_valid_access to block access to these entries in
> the xdp context when a program is attached to egress path.
>
> Update dev_change_xdp_fd to verify expected_attach_type for a program
> is BPF_XDP_EGRESS if egress argument is set.
>
> The next patch adds support for the egress ifindex.
>
> Signed-off-by: Prashant Bhole <[email-masked]>
> Signed-off-by: David Ahern <[email-masked]>
> ---
>  include/uapi/linux/bpf.h       |  1 +
>  net/core/dev.c                 | 11 +++++++++++
>  net/core/filter.c              |  8 ++++++++
>  tools/include/uapi/linux/bpf.h |  1 +
>  4 files changed, 21 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 2e29a671d67e..a9d384998e8b 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -215,6 +215,7 @@ enum bpf_attach_type {
>  	BPF_TRACE_FEXIT,
>  	BPF_MODIFY_RETURN,
>  	BPF_LSM_MAC,
> +	BPF_XDP_EGRESS,
>  	__MAX_BPF_ATTACH_TYPE
>  };
>  
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 97180458e7cb..e8a62bdb395b 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -8732,6 +8732,17 @@ int dev_change_xdp_fd(struct net_device *dev, struct netlink_ext_ack *extack,
>  		if (IS_ERR(prog))
>  			return PTR_ERR(prog);
>  
> +		if (egress && prog->expected_attach_type != BPF_XDP_EGRESS) {
> +			NL_SET_ERR_MSG(extack, "XDP program in Tx path must use BPF_XDP_EGRESS attach type");
> +			bpf_prog_put(prog);
> +			return -EINVAL;
> +		}
> +		if (!egress && prog->expected_attach_type == BPF_XDP_EGRESS) {
> +			NL_SET_ERR_MSG(extack, "XDP program in Rx path can not use BPF_XDP_EGRESS attach type");
> +			bpf_prog_put(prog);
> +			return -EINVAL;
> +		}
> +
>  		if (!offload && bpf_prog_is_dev_bound(prog->aux)) {
>  			NL_SET_ERR_MSG(extack, "using device-bound program without HW_MODE flag is not supported");
>  			bpf_prog_put(prog);
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 7d6ceaa54d21..bcb56448f336 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -6935,6 +6935,14 @@ static bool xdp_is_valid_access(int off, int size,
>  				const struct bpf_prog *prog,
>  				struct bpf_insn_access_aux *info)
>  {
> +	if (prog->expected_attach_type == BPF_XDP_EGRESS) {
> +		switch (off) {
> +		case offsetof(struct xdp_md, ingress_ifindex):
> +		case offsetof(struct xdp_md, rx_queue_index):
> +			return false;
> +		}
> +	}

As I pointed out on the RFC patch, I'm concerned whether this will work
right with freplace programs attaching to XDP programs. It may just be
that I'm missing something, but in that case please explain why it
works? :)

-Toke