netdev - Re: [PATCH bpf-next v2 2/5] bpf: Add support for setting chain call sequence for programs

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Date:   Fri, 4 Oct 2019 16:18:42 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Marek Majkowski <marek@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>,
        Alan Maguire <alan.maguire@...cle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 2/5] bpf: Add support for setting chain call
 sequence for programs

On Fri, 04 Oct 2019 19:22:42 +0200, Toke Høiland-Jørgensen wrote:
> From: Alan Maguire <alan.maguire@...cle.com>
> 
> This adds support for setting and deleting bpf chain call programs through
> a couple of new commands in the bpf() syscall. The CHAIN_ADD and CHAIN_DEL
> commands take two eBPF program fds and a return code, and install the
> 'next' program to be chain called after the 'prev' program if that program
> returns 'retcode'. A retcode of -1 means "wildcard", so that the program
> will be executed regardless of the previous program's return code.
> 
> 
> The syscall command names are based on Alexei's prog_chain example[0],
> which Alan helpfully rebased on current bpf-next. However, the logic and
> program storage is obviously adapted to the execution logic in the previous
> commit.
> 
> [0] https://git.kernel.org/pub/scm/linux/kernel/git/ast/bpf.git/commit/?h=prog_chain&id=f54f45d00f91e083f6aec2abe35b6f0be52ae85b&context=15
> 
> Signed-off-by: Alan Maguire <alan.maguire@...cle.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>

It'd be good to explain why not just allocate a full prog array (or 
in fact get one from the user), instead of having a hidden one which
requires new command to interact with?