Re: [PATCH bpf-next v13 04/14] bpf: add struct_ops_tab to btf.

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

On 12/15/23 17:19, Martin KaFai Lau wrote:
> On 12/15/23 1:42 PM, Kui-Feng Lee wrote:
>>
>>
>> On 12/14/23 18:22, Martin KaFai Lau wrote:
>>> On 12/8/23 4:26 PM, thinker.li@gmail.com wrote:
>>>> +const struct bpf_struct_ops_desc *btf_get_struct_ops(struct btf 
>>>> *btf, u32 *ret_cnt)
>>>> +{
>>>> +    if (!btf)
>>>> +        return NULL;
>>>> +    if (!btf->struct_ops_tab)
>>>
>>>          *ret_cnt = 0;
>>>
>>> unless the later patch checks the return value NULL before using 
>>> *ret_cnt.
>>> Anyway, better to set *ret_cnt to 0 if the btf has no struct_ops.
>>>
>>> The same should go for the "!btf" case above but I suspect the above 
>>> !btf check is unnecessary also and the caller should have checked for 
>>> !btf itself instead of expecting a list of struct_ops from a NULL 
>>> btf. Lets continue the review on the later patches for now to confirm 
>>> where the above !btf case might happen.
>>
>> Checking callers, I didn't find anything that make btf here NULL so far.
> 
>> It is safe to remove !btf check. For the same reason as assigning
>> *ret_cnt for safe, this check should be fine here as well, right?
> 
> If for safety, why ref_cnt is not checked for NULL also? The userspace 
> passed-in btf should have been checked for NULL long time before 
> reaching here. There is no need to be over protective here. It would 
> really need a BUG_ON instead if btf was NULL here (don't add a BUG_ON 
> though).
> 
> afaict, no function in btf.c is checking the btf argument for NULL also.
> 
>>
>> I don't have strong opinion here. What I though is to keep the values
>> as it is without any side-effect if the function call fails and if
>> possible. And, the callers should not expect the callee to set some
>> specific values when a call fails.
> 
> For *ref_cnt stay uninit, there is a bug in patch 10 which exactly 
> assumes 0 is set in *ret_cnt when there is no struct_ops. It is a good 
> signal on how this function will be used.
> 
> I think it is arguable whether returning NULL here is failure. I would 
> argue getting a 0 struct_ops_desc array is not a failure. It is why the 
> !btf case confuses the return NULL case to mean a never would happen 
> error instead of meaning there is no struct_ops. Taking out the !btf 
> case, NULL means there is no struct_ops (instead of failure), so 0 cnt.
> 
> Anyhow, either assign 0 to *ret_cnt here, or fix patch 10 to init the 
> local cnt 0 and write a warning comment here in btf_get_struct_ops() 
> saying ret_cnt won't be set when there is no struct_ops in the btf.


I will fix at the patch 10 by setting local cnt 0.

> 
> When looking at it again, how about moving the bpf_struct_ops_find_*() 
> to btf.c. Then it will avoid the need of the new btf_get_struct_ops() 
> function. bpf_struct_ops_find_*() can directly use the btf->struct_ops_tab.
> 

I prefer to keep them in bpf_struct_ops.c if it is ok to you.
Fixing the initialization issue of bpf_struct_ops_find()
should be enough.

> 
>>
>>>
>>>> +        return NULL;
>>>> +
>>>> +    *ret_cnt = btf->struct_ops_tab->cnt;
>>>> +    return (const struct bpf_struct_ops_desc 
>>>> *)btf->struct_ops_tab->ops;
>>>> +}
>>>
>