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

[Edward Cree]

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?

-Ed