netdev - Re: [PATCH bpf-next V1] bpf: devmap dynamic map-value area based on BTF

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Message-ID: <20200603162257.nxgultkidnb7yb6q@ast-mbp.dhcp.thefacebook.com>
Date:   Wed, 3 Jun 2020 09:22:57 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     David Ahern <dsahern@...il.com>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, Daniel Borkmann <borkmann@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>
Subject: Re: [PATCH bpf-next V1] bpf: devmap dynamic map-value area based on
 BTF

On Wed, Jun 03, 2020 at 05:44:43PM +0200, Jesper Dangaard Brouer wrote:
> The recent commit fbee97feed9b ("bpf: Add support to attach bpf program to a
> devmap entry"), introduced ability to attach (and run) a separate XDP
> bpf_prog for each devmap entry. A bpf_prog is added via a file-descriptor,
> thus not using the feature requires using value minus-1. The UAPI is
> extended via tail-extending struct bpf_devmap_val and using map->value_size
> to determine the feature set.
> 
> There is a specific problem with dev_map_can_have_prog() check, which is
> called from net/core/dev.c in generic_xdp_install() to refuse usage of
> devmap's from generic-XDP that support these bpf_prog's. The check is size
> based. This means that all newer features will be blocked from being use by
> generic-XDP.
> 
> This patch allows userspace to skip handling of 'bpf_prog' on map-inserts.
> The feature can be skipped, via not including the member 'bpf_prog' in the
> map-value struct, which is propagated/described via BTF.
> 
> Fixes: fbee97feed9b ("bpf: Add support to attach bpf program to a devmap entry")
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com

The patch makes no sense to me.
please expose 'struct struct bpf_devmap_val' in uapi/bpf.h
That's what it is whether you want to acknowledge that or not.