netdev - Re: [PATCH RFC net-next 08/14] bpf: add eBPF verifier

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Date:	Tue, 1 Jul 2014 13:04:41 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Chema Gonzalez <chema@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC net-next 08/14] bpf: add eBPF verifier

On Tue, Jul 1, 2014 at 1:05 AM, Daniel Borkmann <dborkman@...hat.com> wrote:
> On 06/28/2014 02:06 AM, Alexei Starovoitov wrote:
>>
>> Safety of eBPF programs is statically determined by the verifier, which
>> detects:
>> - loops
>> - out of range jumps
>> - unreachable instructions
>> - invalid instructions
>> - uninitialized register access
>> - uninitialized stack access
>> - misaligned stack access
>> - out of range stack access
>> - invalid calling convention
>
> ...
>
>> More details in Documentation/networking/filter.txt
>>
>> Signed-off-by: Alexei Starovoitov <ast@...mgrid.com>
>> ---
>
> ...
>>
>>   kernel/bpf/verifier.c               | 1431
>> +++++++++++++++++++++++++++++++++++
>
>
> Looking at classic BPF verifier which checks safety of BPF
> user space programs, it's roughly 200 loc. :-/

I'm not sure what's your point comparing apples to oranges.
For the record 1431 lines include ~200 lines worth of comments
and 200 lines of verbose prints. Without them rejected eBPF
program is black box. Users need a way to understand why
verifier rejected it.

>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> new file mode 100644
>
> ...
>
>> +#define _(OP) ({ int ret = OP; if (ret < 0) return ret; })
>
> ...
>>
>> +       _(get_map_info(env, map_id, &map));
>
> ...
>>
>> +       _(size = bpf_size_to_bytes(bpf_size));
>
>
> Nit: such macros should be removed, please.

It may surely look unconventional, but alternative is to replace
every usage of _ macro with:
err = …
if (err)
  return err;

and since this macro is used 38 times, it will add ~120 unnecessary
lines that will only make code much harder to follow.
I tried not using macro and results were not pleasing.
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