Re: [PATCH v3 net-next] bpf/verifier: track liveness for pruning

[Alexei Starovoitov]

On 8/21/17 1:24 PM, Edward Cree wrote:
> On 18/08/17 15:16, Edward Cree wrote:
> > On 18/08/17 04:21, Alexei Starovoitov wrote:
> > > It seems you're trying to sort-of do per-fake-basic block liveness
> > > analysis, but our state_list_marks are not correct if we go with
> > > canonical basic block definition, since we mark the jump insn and
> > > not insn after the branch and not every basic block boundary is
> > > properly detected.
> > I think the reason this works is that jump insns can't do writes.
> > [snip]
> > the sl->state will never have any write marks and it'll all just work.
> > But I should really test that!
> I tested this, and found that, no, sl->state can have write marks, and the
>  algorithm will get the wrong answer in that case.  So I've got a patch to
>  make the first iteration ignore write marks, as part of a series which I
>  will post shortly.  When I do so, please re-do your tests with adding
>  state_list_marks in strange and exciting places; it should work wherever
>  you put them.  Like you say, it "magically doesn't depend on proper basic
>  block boundaries", and that's because really pruning is just a kind of
>  checkpointing that just happens to be most effective when done just after
>  a jump (pop_stack).
>
> Can I have a SOB for your "grr" test program, so I can include it in the
>  series?

yes. of course. just give the test some reasonable name :)