netdev - Re: [PATCH bpf-next 4/4] bpf: add self-check logic to liveness analysis

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Date:   Wed, 12 Dec 2018 22:26:17 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        "David S . Miller" <davem@...emloft.net>, <daniel@...earbox.net>,
        <jakub.kicinski@...ronome.com>, <jiong.wang@...ronome.com>,
        <netdev@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH bpf-next 4/4] bpf: add self-check logic to liveness
 analysis

On 12/12/18 22:00, Alexei Starovoitov wrote:
> On Wed, Dec 12, 2018 at 08:58:33PM +0000, Edward Cree wrote:
>> A different way I previously thought of was to have a refcount in
>>  verifier states (at the time we had a single parent rather than per-
>>  register parents) counting live children, that falls to 0 when all
>>  continuations have been walked.  That was something I did in my
>>  bounded loops RFC.
>> With something like that, we could check refcount != 0 in mark_reg_read
>>  and check refcount == 0 in explored states in is_state_visited.  Seems
>>  to me like that gets you the same thing and also adds the guarantee
>>  that our explored_states really are fully explored.
> refcnt was my initial approach, but it needs to walk parentage chain.
> Also push/pop_stack needs full walk of all chains too.
> That is too expensive.
> What kind of refcnt you had in mind?
Shallow, rather than deep, refcnt means that you only have to walk to the
 parent when your refcnt falls to zero.  push_stack never has to walk at
 all.  The refcnt only counts immediate children, not all descendants.
IIRC that's how I implemented it in my bounded loops RFC; see patch #9
 "bpf/verifier: count still-live children of explored_states" of that
 series.
Maybe it would still be too expensive, but I wonder if we should obtain
 numbers for that rather than guessing that it would or wouldn't.  Note
 that if a process_bpf_exit would walk N states dropping refs, then there
 are N states that would need to be marked DONE by your approach; and you
 re-do clean_live_states() for each one every time is_state_visited()
 comes back to the same insn, rather than just walking them once on exit.

>> Rest of series looks good, have my Ack for patches 1-3.
>> (Though, maybe use a few more capital letters in your commit messages?)
> Meaning capitalize first letter of the sentences?
Yes, that was what I meant.  (Also I think patch #2 is missing a full
 stop at the end of the sentence, but now I'm just being picky ;-)

-Ed