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

From: Jesper Dangaard Brouer <brouer@redhat.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: David Ahern <dsahern@gmail.com>,
	bpf@vger.kernel.org, netdev@vger.kernel.org,
	Daniel Borkmann <borkmann@iogearbox.net>,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	Lorenzo Bianconi <lorenzo@kernel.org>,
	David Miller <davem@davemloft.net>,
	brouer@redhat.com
Subject: Re: [PATCH bpf-next V1] bpf: devmap dynamic map-value area based on BTF
Date: Fri, 5 Jun 2020 10:23:23 +0200	[thread overview]
Message-ID: <20200605102323.15c2c06c@carbon> (raw)
In-Reply-To: <20200604173341.rvfrjmt433knl3uv@ast-mbp.dhcp.thefacebook.com>

On Thu, 4 Jun 2020 10:33:41 -0700
Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> 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.

Great. If we can remove this requirement of -1 init (and let zero mean
feature isn't used), then I'm all for exposing expose in uapi/bpf.h.

For future extensions there is still a problem/challenge in
dev_map_can_have_prog() that blocks generic-XDP for using future
extensions.  BUT next person extending devmap can deal with that, so
it's not something we need to deal with now.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer