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

From: Andy Lutomirski
Date: Tue Aug 12 2014 - 15:32:59 EST

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:
> 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.


> 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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