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

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Message-ID: <20200604173341.rvfrjmt433knl3uv@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 4 Jun 2020 10:33:41 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.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>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH bpf-next V1] bpf: devmap dynamic map-value area based on
 BTF

On Thu, Jun 04, 2020 at 10:40:06AM -0600, David Ahern wrote:
> On 6/4/20 9:48 AM, Jesper Dangaard Brouer wrote:
> > I will NOT send a patch that expose this in uapi/bpf.h.  As I explained
> > before, this caused the issues for my userspace application, that
> > automatically picked-up struct bpf_devmap_val, and started to fail
> > (with no code changes), because it needed minus-1 as input.  I fear
> > that this will cause more work for me later, when I have to helpout and
> > support end-users on e.g. xdp-newbies list, as it will not be obvious
> > to end-users why their programs map-insert start to fail.  I have given
> > up, so I will not NACK anyone sending such a patch.

Jesper,

you gave wrong direction to David during development of the patches and
now the devmap uapi is suffering the consequences.

> > 
> > Why is it we need to support file-descriptor zero as a valid
> > file-descriptor for a bpf-prog?
> 
> That was a nice property of using the id instead of fd. And the init to
> -1 is not unique to this; adopters of the bpf_set_link_xdp_fd_opts for
> example have to do the same.

I think it's better to adopt "fd==0 -> invalid" approach.
It won't be unique here. We're already using it in other places in bpf syscall.
I agree with Jesper that requiring -1 init of 2nd field is quite ugly
and inconvenient.