Re: [PATCH bpf-next 07/11] bpf: check_map_access() with the knowledge of arrays.

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

On 4/12/24 12:08, Eduard Zingerman wrote:
> On Fri, 2024-04-12 at 09:32 -0700, Kui-Feng Lee wrote:
>>
>> On 4/11/24 15:14, Eduard Zingerman wrote:
>>> On Tue, 2024-04-09 at 17:41 -0700, Kui-Feng Lee wrote:
>>> [...]
>>>
>>>> Any access to elements other than the first one would be rejected.
>>>
>>> I'm not sure this is true, could you please point me to a specific
>>> check in the code that enforces access to go to the first element?
>>> The check added in this patch only enforces correct alignment with
>>> array element start.
>>
>> I mean accessing to elements other than the first one would be rejected
>> if we don't have this patch.
> 
> Oh, I misunderstood the above statement then.
> The way I read it was: "after this patch access to elements other than
> the first one would be rejected". While this patch explicitly allows
> access to the subsequent array elements, hence confusion.
> Sorry for the noise.

I will rephrase it to make it more clear.

> 
>>
>> Before the change, it enforces correct alignment with the start of the
>> whole array.  Once the array feature is enabled, the "size" of struct
>> btf_field will be the size of entire array. In another word, accessing
>> to later elements, other than the first one, doesn't align with the
>> beginning of entire array, and will be rejected.
>