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

[Martin KaFai Lau <martin.lau@linux.dev>]

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.

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.


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