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

[Martin KaFai Lau <martin.lau@linux.dev>]

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.

>> 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?

> 
>> 2. if the validation is implemented in '.reg' only, the map update behavior
>> will be different between BPF_F_LINK map and the non BPF_F_LINK map.
> 
> Ack, this is good to document regardless.
> 
> I'll send a v3 on Monday with these comments added both to the code, and
> to the commit summary.
> 
> Thanks,
> David