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

[David Vernet]

On Mon, Aug 14, 2023 at 09:55:37AM -0700, Martin KaFai Lau wrote:
> On 8/11/23 4:36 PM, David Vernet wrote:
> > I see, thanks for explaining. This is why sched_ext doesn't really work
> > with the BPF_F_LINK version of map update. We can't guarantee that a map
> > can be updated if we can't succeed in ->reg(), because we can also race
> > with e.g. sysrq unloading the scheduler between ->validate() and
> > ->reg(). In a sense, it feels like ->reg() in "updateable" struct_ops
> > implementations should be void, whereas in other struct_ops
> > implementations like scx() it has to be int *. If validate() is meant to
> > prevent the scenario you outlined, can you help me understand why we
> > still check the return value of ->reg() in bpf_struct_ops_link_create()?
> > Or at the very least it seems like we should WARN_ON()?
> 
> ->regs() can fail if another struct_ops under the same
> name has already been loaded to the subsystem. If another
> subsystem needs another return value to support .update, I
> believe it can be done if that is blocking scx to support
> "updateable" link.

Ok, so ->validate() is a static check that should either always succeed
or always fail, and ->reg() may fail due to runtime circumstances. So a
map that passes ->validate() could e.g. retry to create the link in a
loop or something. Or create a series of validated struct_ops maps and
then have a management layer that destroys and creates links for the map
you want to actually use. Thanks for explaining.

> > > If it needs to validate struct_ops as a while,
> 
> There was a typo: as a /whole/.
> 
> > > 
> > > 1. it must be implemented in .validate instead of .reg. Otherwise, it may
> > > end up having an unusable map.
> > 
> > Some clarity on this point (why we check ->reg() on the ->validate()
> > path) would help me write this comment more clearly.
> 
> 
> hmm... where does it check ->reg() on the ->validate() now?
> 
> I was meaning the struct_ops supported subsystem should
> validate the struct_ops map in '.validate' instead of in
> the '.reg'.
> 
> or I misunderstood the question?

I just meant that I wasn't understanding why we had to check the return
value of ->reg() in bpf_struct_ops_link_create(). Now that I understand
the semantics, I can document them.