Re: [PATCH bpf-next v5 7/9] selftests/bpf: Test kptr arrays and kptrs in nested struct fields.

[Kui-Feng Lee <sinquersw@gmail.com>]

On 5/10/24 15:31, Eduard Zingerman wrote:
> On Fri, 2024-05-10 at 15:25 -0700, Kui-Feng Lee wrote:
> 
>>>>> Also, in the tests below you check that a pointer to some object could
>>>>> be put into an array at different indexes. Tbh, I find it not very
>>>>> interesting if we want to check that offsets are correct.
>>>>> Would it be possible to create an array of object kptrs,
>>>>> put specific references at specific indexes and somehow check which
>>>>> object ended up where? (not necessarily 'bpf_cpumask').
>>>>
>>>> Do you mean checking index in the way like the following code?
>>>>
>>>>     if (array[0] != ref0 || array[1] != ref1 || array[2] != ref2 ....)
>>>>       return err;
>>>
>>> Probably, but I'd need your help here.
>>> There goal is to verify that offsets of __kptr's in the 'info' array
>>> had been set correctly. Where is this information is used later on?
>>> E.g. I'd like to trigger some action that "touches" __kptr at index N
>>> and verify that all others had not been "touched".
>>> But this "touch" action has to use offset stored in the 'info'.
>>
>> They are used for verifying the offset of instructions.
>> Let's assume we have an array of size 10.
>> Then, we have 10 infos with 10 different offsets.
>> And, we have a program includes one instruction for each element, 10 in
>> total, to access the corresponding element.
>> Each instruction has an offset different from others, generated by the
>> compiler. That means the verifier will fail to find an info for some of
>> instructions if there is one or more info having wrong offset.
> 
> That's a bit depressing, as there would be no way to check if e.g. all
> 10 refer to the same offset. Is it possible to trigger printing of the
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
How can that happen? Do you mean the compiler does it wrong?


> 'info.offset' to verifier log? E.g. via some 'illegal' action.
Yes if necessary!