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

[David Vernet <void@manifault.com>]

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.

> 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.