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

[Andy Lutomirski]

On Thu, Jul 24, 2014 at 12:25 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
> On Thu, Jul 24, 2014 at 11:25 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Thu, Jul 17, 2014 at 9:20 PM, Alexei Starovoitov <ast@xxxxxxxxxxxx> 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.

--Andy

> 2. how to write valid programs
>  that's more important one, since it's a key to happy users.
>  'verification trace' is the first step. I'm planning to add debug info and
>  user space tool that points out to line in C instead of assembler trace.
>  In other words to bring errors to user as early as possible during
>  compilation process.
>  This is not a concern when programs are written in assembler,
>  since the programs will be much shorter and thought through by
>  the author. However I don't think there will be too many users
>  willing to understand ebpf assembler.
>
> I suspect you're more concerned about #1 at this point whereas
> I'm concerned about #2.



-- 
Andy Lutomirski
AMA Capital Management, LLC
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