Re: perf, kprobes: fuzzer generates huge number of WARNings

[Alexei Starovoitov]

On Tue, Jul 07, 2015 at 05:08:51PM -0400, Vince Weaver wrote:
> On Tue, 7 Jul 2015, Alexei Starovoitov wrote:
> 
> > On Tue, Jul 07, 2015 at 12:00:12AM -0400, Vince Weaver wrote:
> > > 
> > > Well the BPF hack is in the fuzzer, not the kernel.  And it's not really a 
> > > hack, it just turned out to be a huge pain to figure out how to 
> > > manually create a valid BPF program in conjunction with a valid kprobe 
> > > event.
> > 
> > You mean automatically generating valid bpf program? That's definitely hard.
> > If you mean just few hardcoded programs then take them from samples or
> > from test_bpf ?
> 
> there's already code in trinity that in theory autogenerates bpf programs, 
> but for now I was just trying to hook up a short known valid one.
> 
> it might not be possible to really test things though, as you need to be 
> root to create a kprobe and attach a BPF program, but my fuzzer when run 
> as root often does all kinds of other stuff that will crash a machine.
> Is it ever planned to allow using bpf/kprobes without requiring full 
> CAP_ADMIN privledges?

I suspect kprobes will forever be root only, whereas for bpf I'm thinking
to introduce CAP_BPF, but before that we need to finish constant blinding
and add address leak prevention. So not soon.

> > > I did have to sprinkle printks in the kprobe and bpf code to find out 
> > > where various EINVAL returns were coming from, so potentially this is just 
> > > a problem of printks happening where they shouldn't.  I'll remove those 
> > > changes and try to reproduce this tomorrow.
> > 
> > could you please elaborate on this further. Which EINVALs you talking about?
> 
> When you are trying to create a kprobe and bpf file there's about 10 
> different ways to get EINVAL as a return value and no way of knowing which 
> one you are hitting.  I added printks so I could know what issue was 
> causing the einval.  (from memory, the problems I hit were not zeroing out 
> the attr structure, having a wrong instruction count, and a few others).

I see. I guess anyone trying to use syscall directly will be facing such
issues, but libbpf that is being developed to be used by perf and others
should solve these problems.

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