Date:   Wed, 12 Dec 2018 16:00:56 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Edward Cree <ecree@...arflare.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 Wed, Dec 12, 2018 at 10:26:17PM +0000, Edward Cree wrote:
> 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.

luckily found it in my email archives. next time could you send a link to
make sure we're talking about the same patch?
back then there was no per-register chains and push_stack()
has to do only one live_children++.
With per-register it would need to walk all frames and all regs and
all stack slots.

Old kill_thread() instead of:
+       while (cur && !--cur->live_children)
+               cur = cur->parent;
becomes such inner loop for all frames, all regs and all slots, right?

I have to agree that it is easier to understand to do such kill_thread()
in process_bpf_exit, but the cost seems excessive for safety feature.

> 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.

I can change clean_live_states() to be called only when equivalent state
is not found.
Since that's the place where I want to do state merging.
Since is_state_visited() just walked state lists and all data is hot,
it's the best place to walk them again either for safety check
or for state merging.

As far as state merging I see a pattern when a bunch of states are
overlapping in the register ranges, but not fully contained.
Essentially range_within() is too conservative.
The idea is to merge [1,5] with [3,10] if this is the only difference
between states. Merge, but don't declare it safe yet and proceed further.

To do any merging the verifier needs to be sure that reg_state is DONE.
Essentially the check of callsites with current state and whole idea
of this patch is a precursor of state merging.
I was thinking to enforce this safety first and keep it enforced,
but use clean_live_states() entry point and checks as a gate for
actual merging in the future patches.

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