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

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Message-ID: <e4075575-b12c-5c6f-bac3-0c7ba8e04802@solarflare.com>
Date:   Mon, 21 Aug 2017 21:24:24 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Alexei Starovoitov <ast@...com>, <davem@...emloft.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>
CC:     <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        iovisor-dev <iovisor-dev@...ts.iovisor.org>
Subject: Re: [PATCH v3 net-next] bpf/verifier: track liveness for pruning

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