Messages in this thread | | | Date | Fri, 11 Aug 2023 14:25:45 -0700 | Subject | Re: [PATCH bpf-next] bpf: Support default .validate() and .update() behavior for struct_ops links | From | Kui-Feng Lee <> |
| |
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. >
| |