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

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Date:   Wed, 31 May 2017 11:45:58 -0700
From:   Alexei Starovoitov <ast@...com>
To:     David Miller <davem@...emloft.net>
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

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.