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

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Date:   Thu, 13 Dec 2018 20:02:09 +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 13/12/18 00:00, Alexei Starovoitov wrote:
> luckily found it in my email archives. next time could you send a link to
> make sure we're talking about the same patch?
Sorry, will do.

> 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.
Thinking about it, since this is about control flow rather than data flow,
 maybe it makes sense to add in a separate parent pointer on the verifier
 state, and use that for this, rather than shoehorning it into the
 liveness machinery.

> 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?
If it's done with the per-reg parent pointers, then yes that's right and
 that does start to look a bit painful, agreed.

> 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.
Yep, makes sense.

-Ed