netdev - Re: [PATCH net-next 6/9] bpf: fix stack_depth usage by test

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Message-Id: <20170531.193040.2290856192477071399.davem@davemloft.net>
Date:   Wed, 31 May 2017 19:30:40 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     ast@...com
Cc:     daniel@...earbox.net, netdev@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH net-next 6/9] bpf: fix stack_depth usage by test_bpf.ko

From: Alexei Starovoitov <ast@...com>
Date: Wed, 31 May 2017 15:41:09 -0700

> On 5/31/17 11:45 AM, Alexei Starovoitov wrote:
>> On 5/31/17 11:43 AM, David Miller wrote:
>>> From: Alexei Starovoitov <ast@...com>
>>> Date: Wed, 31 May 2017 11:39:37 -0700
>>>
>>>> On 5/31/17 11:15 AM, David Miller wrote:
>>>>> From: Alexei Starovoitov <ast@...com>
>>>>> Date: Tue, 30 May 2017 13:31:32 -0700
>>>>>
>>>>>> test_bpf.ko doesn't call verifier before selecting interpreter or
>>>>>> JITing,
>>>>>> hence the tests need to manually specify the amount of stack they
>>>>>> consume.
>>>>>>
>>>>>> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>>>>>> Acked-by: Daniel Borkmann <daniel@...earbox.net>
>>>>>
>>>>> I do not like this and the previous patch, it seems so error prone.
>>>>
>>>> in what sense 'error prone' ?
>>>
>>> In the sense that a human computes these numbers, and nothing checks
>>> if it is correct or not until program perhaps crashes if the value is
>>> wrong.
>>
>> right. that's how all these tests are.
>> See bpf_fill_ld_abs_vlan_push_pop() for example.
>> If that codegen has a bug, it will crash the kernel.
>> That's why it's done from kernel module to do things
>> that user space cannot do.
> 
> btw, when added a bunch of these '.stack_depth = 40'
> I was thinking to randomize these values in [40, 512]
> range to stress test interpreter and JITs more,
> but then decided not to do that to avoid questions
> why these numbers don't match the instructions.
> Now I'm thinking we should actually do it for two reasons:
> - to stress test stuff
> - and to demonstrate more clearly that test_bpf.ko can
> really do things that user space cannot and that's the
> purpose of this .ko
> Thoughts?

Ok I've applied the base series for now.

Feel free to build that kind of stuff on top.

Thanks for explaining.