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

From: Kui-Feng Lee
Date: Fri Aug 11 2023 - 19:12:59 EST



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@xxxxxxxxxx/

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.

... SKIP ...