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Date:	Tue, 12 Jan 2016 16:19:20 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Rabin Vincent <rabin@....in>, davem@...emloft.net,
	netdev@...r.kernel.org, ast@...nel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv2] net: bpf: reject invalid shifts

On Wed, Jan 13, 2016 at 12:59:46AM +0100, Hannes Frederic Sowa wrote:
> On 13.01.2016 00:47, Alexei Starovoitov wrote:
> >On Tue, Jan 12, 2016 at 03:28:22PM -0800, Eric Dumazet wrote:
> >>On Tue, 2016-01-12 at 12:46 -0800, Alexei Starovoitov wrote:
> >>>On Tue, Jan 12, 2016 at 09:42:39PM +0100, Daniel Borkmann wrote:
> >>
> >>>>>yep and we all know who was able to code hundreds of cBPF insns by hand ;)
> >>>>>But I'm sure that code doesn't have such broken shifts. :)))
> >>>>
> >>>>libpcap certainly supports raw filters now thanks to Chema [1]. Alternative
> >>>>could be to just mask them here, but not in eBPF verifier, but that would be
> >>>>even more inconsistent (on the other hand, we also allow holes in BPF but not
> >>>>in eBPF, so wouldn't be the first time we make things different), hmm.
> >>>
> >>>I would rather see broken classic bpf program fixed instead of continue
> >>>running them with undefined behavior.
> >>
> >>This is your choice, because you are a developer.
> >>
> >>Some people might be stuck with old software they can not update,
> >>because they do not have the money to pay developers.
> >>
> >>And no, I did not code BPF programs like that, but maybe others did, and
> >>I feel the pain of customers that might be stuck.
> >>
> >>Linus Torvalds always made clear we must provide backward compatibility,
> >>and really this discussion should not even take place.
> >>
> >>As I said, we used to load such BPF program in the past.
> >>
> >>The fact that ARM64 crashes because of a faulty JIT implementation is
> >>not an excuse.
> >
> >I would agree if those loaded programs would do something sensible,
> >but they're broken. As shown arm and arm64 would execute them
> >differently without JIT, because HW treats such shifts differently.
> >I also checked that libpcap is sane and doesn't generate broken shifts.
> >imo we're not breaking backward compatiblity here.
> 
> But on one specific platform those programs did something deterministic,
> reproducible and observable, no? Probably most developers only cared about
> that, probably especially in the embedded segment.

No, they were not. Say we do mask K&31 instead. That may match
what x86 cpu do, but it will not match arm. You just cannot
define previously undefined behavior without breaking something.
And with error the users can actually fix their stuff.
If their software is so old and cannot be upgraded, then
they shouldn't be upgrading the kernel either, something else will break.
Starting from kernel version. Remember 2.x -> 3.x -> 4.x ?
Also the arm64 JIT crash was noticed only because of fancy fuzzing,
so let's be sensible in our risk estimations.

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