Re: [PATCH bpf-next] bpf: Support default .validate() and .update() behavior for struct_ops links

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

On 8/11/23 4:12 PM, Kui-Feng Lee wrote:
> 
> 
> On 8/11/23 15:49, Martin KaFai Lau wrote:
>> On 8/11/23 1:19 PM, David Vernet wrote:
>>> On Fri, Aug 11, 2023 at 10:35:03AM -0700, Martin KaFai Lau wrote:
>>>> On 8/10/23 4:15 PM, Stanislav Fomichev wrote:
>>>>> On 08/10, David Vernet wrote:
>>>>>> On Thu, Aug 10, 2023 at 03:46:18PM -0700, Stanislav Fomichev wrote:
>>>>>>> On 08/10, David Vernet wrote:
>>>>>>>> Currently, if a struct_ops map is loaded with BPF_F_LINK, it must also
>>>>>>>> define the .validate() and .update() callbacks in its corresponding
>>>>>>>> struct bpf_struct_ops in the kernel. Enabling struct_ops link is useful
>>>>>>>> in its own right to ensure that the map is unloaded if an application
>>>>>>>> crashes. For example, with sched_ext, we want to automatically unload
>>>>>>>> the host-wide scheduler if the application crashes. We would likely
>>>>>>>> never support updating elements of a sched_ext struct_ops map, so we'd
>>>>>>>> have to implement these callbacks showing that they _can't_ support
>>>>>>>> element updates just to benefit from the basic lifetime management of
>>>>>>>> struct_ops links.
>>>>>>>>
>>>>>>>> Let's enable struct_ops maps to work with BPF_F_LINK even if they
>>>>>>>> haven't defined these callbacks, by assuming that a struct_ops map
>>>>>>>> element cannot be updated by default.
>>>>>>>
>>>>>>> Any reason this is not part of sched_ext series? As you mention,
>>>>>>> we don't seem to have such users in the three?
>>>>>>
>>>>>> Hi Stanislav,
>>>>>>
>>>>>> The sched_ext series [0] implements these callbacks. See
>>>>>> bpf_scx_update() and bpf_scx_validate().
>>>>>>
>>>>>> [0]: https://lore.kernel.org/all/20230711011412.100319-13-tj@kernel.org/
>>>>>>
>>>>>> We could add this into that series and remove those callbacks, but this
>>>>>> patch is fixing a UX / API issue with struct_ops links that's not really
>>>>>> relevant to sched_ext. I don't think there's any reason to couple
>>>>>> updating struct_ops map elements with allowing the kernel to manage the
>>>>>> lifetime of struct_ops maps -- just because we only have 1 (non-test)
>>>>
>>>> Agree the link-update does not necessarily couple with link-creation, so
>>>> removing 'link' update function enforcement is ok. The intention was to
>>>> avoid the struct_ops link inconsistent experience (one struct_ops link
>>>> support update and another struct_ops link does not) because consistency was
>>>> one of the reason for the true kernel backed link support that Kui-Feng did.
>>>> tcp-cc is the only one for now in struct_ops and it can support update, so
>>>> the enforcement is here. I can see Stan's point that removing it now looks
>>>> immature before a struct_ops landed in the kernel showing it does not make
>>>> sense or very hard to support 'link' update. However, the scx patch set has
>>>> shown this point, so I think it is good enough.
>>>
>>> Sorry for sending v2 of the patch a bit prematurely. Should have let you
>>> weigh in first.
>>>
>>>> For 'validate', it is not related a 'link' update. It is for the struct_ops
>>>> 'map' update. If the loaded struct_ops map is invalid, it will end up having
>>>> a useless struct_ops map and no link can be created from it. I can see some
>>>
>>> To be honest I'm actually not sure I understand why .validate() is only
>>> called for when BPF_F_LINK is specified. Is it because it could break
>>
>> Regardless '.validate' must be enforced or not, the ->validate() should be 
>> called for the non BPF_F_LINK case also during map update. This should be fixed.
> 
> For the case of the TCP congestion control, its validation function is
> called by the implementations of ->validate() and ->reg(). I mean it
> expects ->reg() to do validation as well.

Right, for tcp-cc, the reg is doing the validation because it is how the kernel 
tcp-cc module is done.

For newer subsystem supporting struct_ops, it should expect the validation is 
done in the .validate alone.