Re: [PATCH v2 bpf-next 01/13] bpf: Support multiple arg regs w/ ref_obj_id for kfuncs

David Vernet
2022-12-29T17:00:43Z

On Thu, Dec 29, 2022 at 08:50:19AM -0800, Alexei Starovoitov wrote:
> On Wed, Dec 28, 2022 at 10:40 PM David Vernet <void@manifault.com> wrote:
> >
> > On Sat, Dec 17, 2022 at 12:24:54AM -0800, Dave Marchevsky wrote:
> > > Currently, kfuncs marked KF_RELEASE indicate that they release some
> > > previously-acquired arg. The verifier assumes that such a function will
> > > only have one arg reg w/ ref_obj_id set, and that that arg is the one to
> > > be released. Multiple kfunc arg regs have ref_obj_id set is considered
> > > an invalid state.
> > >
> > > For helpers, RELEASE is used to tag a particular arg in the function
> > > proto, not the function itself. The arg with OBJ_RELEASE type tag is the
> > > arg that the helper will release. There can only be one such tagged arg.
> > > When verifying arg regs, multiple helper arg regs w/ ref_obj_id set is
> > > also considered an invalid state.
> > >
> > > Later patches in this series will result in some linked_list helpers
> > > marked KF_RELEASE having a valid reason to take two ref_obj_id args.
> > > Specifically, bpf_list_push_{front,back} can push a node to a list head
> > > which is itself part of a list node. In such a scenario both arguments
> > > to these functions would have ref_obj_id > 0, thus would fail
> > > verification under current logic.
> > >
> > > This patch changes kfunc ref_obj_id searching logic to find the last arg
> > > reg w/ ref_obj_id and consider that the reg-to-release. This should be
> > > backwards-compatible with all current kfuncs as they only expect one
> > > such arg reg.
> >
> > Can't say I'm a huge fan of this proposal :-( While I think it's really
> > unfortunate that kfunc flags are not defined per-arg for this exact type
> > of reason, adding more flag-specific semantics like this is IMO a step
> > in the wrong direction. It's similar to the existing __sz and __k
> > argument-naming semantics that inform the verifier that the arguments
> > have special meaning. All of these little additions of special-case
> > handling for kfunc flags end up requiring people writing kfuncs (and
> > sometimes calling them) to read through the verifier to understand
> > what's going on (though I will say that it's nice that __sz and __k are
> > properly documented in [0]).
>
> Before getting to pros/cons of KF_* vs name suffix vs helper style
> per-arg description...
> It's important to highlight that here we're talking about
> link list and rb tree kfuncs that are not like other kfuncs.
> Majority of kfuncs can be added by subsystems like hid-bpf
> without touching the verifier.

I hear you and I agree. It wasn't my intention to drag us into a larger
discussion about kfuncs vs. helpers, but rather just to point out that I
think we have to try hard to avoid adding special-case logic that
requires looking into the verifier to understand the semantics. I think
we're on the same page about this, based on this and your other
response.

> Here we're paving the way for graph (aka new gen data structs)
> and so far not only kfuncs, but their arg types have to have
> special handling inside the verifier.
> There is not much yet to generalize and expose as generic KF_
> flag or as a name suffix.
> Therefore I think it's more appropriate to implement them
> with minimal verifier changes and minimal complexity.

Agreed

> There is no 3rd graph algorithm on the horizon after link list
> and rbtree. Instead there is a big todo list for
> 'multi owner graph node' and 'bpf_refcount_t'.

In this case my point in [0] of the only option for generalizing being
to have something like KF_GRAPH_INSERT / KF_GRAPH_REMOVE is just not the
way forward (which I also said was my opinion when I pointed it out as
an option). Let's just special-case these kfuncs. There's already a
precedence for doing that in the verifier anyways. Minimal complexity,
minimal API changes. It's a win-win.

[0]: https://lore.kernel.org/all/Y63GLqZil9l1NzY4@maniforge.lan/

> Those will require bigger changes in the verifier,
> so I'd like to avoid premature generalization :) as analogous
> to premature optimization :)

And of course given my points above and in other threads: agreed. I
think we have an ideal middle-ground for minimizing complexity in the
short term, and some nice follow-on todo-list items to work on in the
medium-long term which will continue to improve things without
(negatively) affecting users in any way. All SGTM