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

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

On 8/11/23 13:19, 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
> existing programs if they defined a struct_ops map that wasn't valid
> _without_ using BPF_F_LINK? Whether or not a map is valid should inform
> whether we can load it regardless of whether there's a link, no? It
> seems like .init_member() was already doing this as well. That's why I
> got confused and conflated the two.

With the previous solution (without link), you can not update the values
of a struct_ops map directly.
You have to delete the existing value before update it.
Updating a value would register a value, a function set,
to the implementation of a struct_ops type. Deleting a value
would unregister the value. So, the validation can be performed
in the registration function.

For BPF_LINK, it provides a solution to update a function
set atomically.  You doesn't have to unregister an existing
one before installing a new one. That is why validate functions
are invented.

init_member() handles/validates per-member value.  It can not detect
what is necessary but absent.  validate() has a full set of function
pointers (all members), so it is able to determine if something
necessary is missing.

> 
>> struct_ops subsystem check all the 'ops' function for NULL before calling
>> (like the FUSE RFC). I can also see some future struct_ops will prefer not
>> to check NULL at all and prefer to assume a subset of the ops is always
>> valid. Does having a 'validate' enforcement is blocking the scx patchset in
>> some way? If not, I would like to keep this for now. Once it is removed,
> 
> No, it's not blocking scx at all. scx, as with any other struct_ops
> implementation, could and does just implement these callbacks. As
> Kui-Feng said in [0], this is really just about enabling a sane default
> to improve usability. If a struct_ops implementation actually should
> have implemented some validation but neglected to, that would be a bug
> in exactly the same manner as if it had implemented .validate(), but
> neglected to check some corner case that makes the map invalid.
> 
> [0]: https://lore.kernel.org/lkml/887699ea-f837-6ed7-50bd-48720cea581c@gmail.com/
> 
>> there is no turning back.
> 
> Hmm, why there would be no turning back from this? This isn't a UAPI
> concern, is it? Whether or not a struct_ops implementation needs to
> implement .validate() or can just rely on the default behavior of "no
> .validate() callback implies the map is valid" is 100% an implementation
> detail that's hidden from the end user. This is meant to be a UX
> improvement for a developr defining a struct bpf_struct_ops instance in
> the main kernel, not someone defining an instance of that struct_ops
> (e.g. struct tcp_congestion_ops) in a BPF prog.
>