netdev - Re: bpf bounded loops. Was: [flamebait] xdp

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Message-ID: <5726302e-3502-99c8-a4d9-a5278761cb5a@solarflare.com>
Date:   Mon, 5 Dec 2016 16:40:15 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Tom Herbert <tom@...bertland.com>, Thomas Graf <tgraf@...g.ch>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: bpf bounded loops. Was: [flamebait] xdp

On 02/12/16 19:25, Hannes Frederic Sowa wrote:
> On 02.12.2016 19:39, Alexei Starovoitov wrote:
>> Hannes,
>> Not too long ago you proposed a very interesting idea to add
>> support for bounded loops without adding any new bpf instructions and
>> changing llvm (which was way better than my 'rep' like instructions
>> I was experimenting with). I thought systemtap guys also wanted bounded
>> loops and you were cooperating on the design, so I gave up on my work and
>> was expecting an imminent patch from you. I guess it sounds like you know
>> believe that bounded loops are impossible or I misunderstand your statement ?
> Your argument was that it would need a new verifier as the current first
> pass checks that we indeed can lay out the basic blocks as a DAG which
> the second pass depends on. This would be violated.
I may be completely mistaken here, but can't the verifier unroll the loop 'for
verification' without it actually being unrolled in the program?
I.e., any "proof that the loop terminates" should translate into "rewrite of
the directed graph to make it a DAG, possibly duplicating a lot of insns", and
you feed the rewritten graph to the verifier, while using the original loopy
version as the actual program to store and later execute.
Then the verifier happily checks things like array indices being valid, without
having to know about the bounded loops.

-Ed