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Date: Fri, 1 Dec 2017 14:51:06 -0300
From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Alexei Starovoitov <ast@fb.com>
Cc: Martin KaFai Lau <kafai@fb.com>, Wang Nan <wangnan0@huawei.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        "David S. Miller" <davem@davemloft.net>,
        David Ahern <dsahern@gmail.com>, Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>
Subject: Re: [PATCH/RFC] Re: 'perf test BPF' failing, libbpf regression wrt
 "basic API for BPF obj name"
Message-ID: <20171201175106.GH3298@kernel.org>
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 <20171129223135.6iqvj6ho4ojxmhu6@kafai-mbp>
 <20171130030110.GA18880@kernel.org>
 <20171130165358.GN3298@kernel.org>
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Em Thu, Nov 30, 2017 at 01:51:15PM -0800, Alexei Starovoitov escreveu:
> On 11/30/17 11:00 AM, Arnaldo Carvalho de Melo wrote:
> > > Instead of sinking all future bpf_attr's backward compatibility
> > > requirements to sys_bpf,  I would push it up to its own BPF_* command
> > > helper which has a better sense of its bpf_attr, i.e. push it up
> > > to bpf_create_map_node() and bpf_load_program_name() in this case.
> > Humm, we could try that approach, but the one in this patch seemed good
> > enough.
> > 
> > And after all if the first syscall() invokation, with the latest kernel
> > and latest tooling will work, right?
> 
> I agree with Martin and I also don't think it will work to push
> logic of all bpf commands into single sys_bpf syscall wrapper.

Sure, that was just a POC, I'll work on something that takes into
account what you guys pointed out.

> This logic will become more and more complex over time.
> Like this case really belongs in bpf_create_map() which is a wrapper
> on top of single BPF_CREATE_MAP command.
 
> Note it's the first time we're facing this 'new libbpf.a running on
> top of old kernel' issue and should be very careful adding such
> fallback code to the generic bpf library, since all the selftests/bpf/
> are using this lib and relying on excepted behavior.

Right, tools/perf/ uses it as well and relies on its continued
functioning.

> We don't want tests that want to test the latest kernel feature all of
> a sudden pass on old kernel that doesn't have it.

Sure, neither do I :-)
 
> To some degree perf and selftests/bpf needs are diverging here,
> so adding #ifdef to libbpf.a to match testcase expectations may be
> necessary.

But this is not just testcase expectations, the usecase is someone
wanting to use a newer tool, with perhaps some new features of interest
that don't depend on changes in the kernel, in an older kernel on a
system where updating it is not possible or desirable.

- Arnaldo