linux-kernel - Re: [PATCH RFC v2 net-next 10/16] bpf: add eBPF verifier

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Message-ID: <CAMEtUuz2DS8Ks0L15K0Jsj_nFjgvDh0TBwb+5pHzrdU+o6co_g@mail.gmail.com>
Date:	Tue, 12 Aug 2014 13:00:40 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 net-next 10/16] bpf: add eBPF verifier

On Tue, Aug 12, 2014 at 12:32 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, Jul 24, 2014 at 12:25 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
>> On Thu, Jul 24, 2014 at 11:25 AM, Andy Lutomirski <luto@...capital.net> wrote:
>>> On Thu, Jul 17, 2014 at 9:20 PM, Alexei Starovoitov <ast@...mgrid.com> 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
>>>
>>> Is there something that documents exactly what conditions an eBPF
>>> program must satisfy in order to be considered valid?
>>
>> I did a writeup in the past on things that verifiers checks and gave it
>> to internal folks to review. Guys have said that now they understand very
>> well how it works, but in reality it didn't help at all to write valid programs.
>> What worked is 'verification trace' = the instruction by instruction dump
>> of verifier state while it's analyzing the program.
>> I gave few simple examples of it in
>> 'Understanding eBPF verifier messages' section:
>> https://git.kernel.org/cgit/linux/kernel/git/ast/bpf.git/diff/Documentation/networking/filter.txt?id=b22459133b9f52d2176c8c0f8b5eb036478a40c9
>> Every example there is what "program must satisfy to be valid"...
>>
>> Therefore I'm addressing two things:
>> 1. how verifier works and what it checks for.
>>   that is described in 'eBPF verifier' section of the doc and
>>   in 200 lines of comments inside verifier.c
>
> That doc is pretty good.  I'll try to read it carefully soon.  Sorry
> for the huge delay here -- I've been on vacation.

I've been sitting on v4 for few weeks, since it's a merge window.
So please hold on a careful review. I'll post v4 later today.
Mainly I've split the verifier into several patches to make it
easier to read.
Thanks!
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