Re: [PATCH bpf-next v4 3/6] mm: introduce bpf_get_root_mem_cgroup() BPF kfunc

[Matt Bobrowski <mattbobrowski@google.com>]

On Tue, Jan 06, 2026 at 04:13:24PM +0100, Kumar Kartikeya Dwivedi wrote:
> On Mon, 5 Jan 2026 at 22:04, Matt Bobrowski <mattbobrowski@google.com> wrote:
> >
> > On Mon, Jan 05, 2026 at 08:05:54AM -0800, Alexei Starovoitov wrote:
> > > On Sun, Jan 4, 2026 at 11:49 PM Matt Bobrowski <mattbobrowski@google.com> wrote:
> > > >
> > > > >
> > > > > No need for a new KF flag. Any struct returned by kfunc should be
> > > > > trusted or trusted_or_null if KF_RET_NULL was specified.
> > > > > I don't remember off the top of my head, but this behavior
> > > > > is already implemented or we discussed making it this way.
> > > >
> > > > Hm, I do not see any evidence of this kind of semantic currently
> > > > implemented, so perhaps it was only discussed at some point. Would you
> > > > like me to put forward a patch that introduces this kind of implicit
> > > > trust semantic for BPF kfuncs returning pointer to struct types?
> > >
> > > Hmm. What about these:
> > > BTF_ID_FLAGS(func, scx_bpf_cpu_rq)
> > > BTF_ID_FLAGS(func, scx_bpf_locked_rq, KF_RET_NULL)
> > > BTF_ID_FLAGS(func, scx_bpf_cpu_curr, KF_RET_NULL | KF_RCU_PROTECTED)
> > >
> > > I thought they're returning a trusted pointer without acquiring it.
> > > iirc the last one returns trusted in RCU CS,
> > > but the first two return just a legacy ptr_to_btf_id ?
> > > This is something to fix asap then.
> >
> > No, AFAIU they do not. These simply return a regular pointer to BTF ID
> > (PTR_TO_BTF_ID), rather than a formally "trusted" pointer (which would
> > carry the PTR_TRUSTED flag or a ref_obj_id). scx_bpf_cpu_curr returns
> > a MEM_RCU pointer (via KF_RCU_PROTECTED), which is somewhat considered
> > to be trusted within a RCU read-side critical section *ONLY*.
> >
> > Kumar/Tejun,
> 
> Yeah, they don't return a trusted pointer. I think it would make sense
> to change the behavior here by default.

Thanks for chiming in and confirming this Kumar! I also agree that any
BPF kfunc returning a pointer should be treated as being implicitly
trusted by default. I can't think of any scenario whereby a BPF kfunc
would want to return a pointer that'd fundamentally be untrusted, but
there always could be some exceptions. Anyway, I will work on this and
send something through for review soon.

> A non-trusted pointer cannot be passed to kfuncs taking trusted
> arguments, so hopefully it will only make things more permissive and
> doesn't break anything.

We can only hope! ;)