Subject: Re: [RFC PATCH net-next 3/5] bpf/verifier: feed
 pointer-to-unknown-scalar casts into scalar ALU path
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
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CC: <davem@davemloft.net>, Alexei Starovoitov <ast@fb.com>,
        Daniel Borkmann <daniel@iogearbox.net>, <netdev@vger.kernel.org>,
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        LKML <linux-kernel@vger.kernel.org>
From: Edward Cree <ecree@solarflare.com>
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Date: Thu, 8 Jun 2017 18:12:39 +0100
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On 08/06/17 17:50, Alexei Starovoitov wrote:
> On Thu, Jun 08, 2017 at 04:25:39PM +0100, Edward Cree wrote:
>> On 08/06/17 03:35, Alexei Starovoitov wrote:
>>> such large back and forth move doesn't help reviewing.
>>> may be just merge it into previous patch?
>>> Or keep that function in the right place in patch 2 already?
>> I think 'diff' got a bit confused, and maybe with different options I could
>>  have got it to produce something more readable.  But I think I will just
>>  merge this into patch 2; it's only separate because it started out as an
>>  experiment.
> after sleeping on it I'm not sure we should be allowing such pointer
> arithmetic. In normal C code people do fancy tricks with lower 3 bits
> of the pointer, but in bpf code I cannot see such use case.
> What kind of realistic code will be doing ptr & 0x40 ?
Well, I didn't support it because I saw a use case.  I supported it because
 it seemed easy to do and the code came out reasonably elegant-looking.
Since this is guarded by env->allow_ptr_leaks, I can't see any reason _not_
 to let people try fancy tricks with the low bits of pointers.
I agree ptr & 0x40 is a crazy thing with no imaginable use case, but...
"Unix was not designed to stop its users from doing stupid things, as that
 would also stop them from doing clever things." ;-)

-Ed