Re: [PATCH bpf-next v3 3/6] bpf: Disallow !kprobe_write_ctx progs tail-calling kprobe_write_ctx progs

[Kumar Kartikeya Dwivedi]

On Tue, 3 Mar 2026 at 17:04, <bot+bpf-ci@xxxxxxxxxx> wrote:
>
> > diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
>
> [ ... ]
>
> > +     switch (match) {
> > +     case BPF_MAP_OWNER_MATCH_FOR_INIT:
> > +             if (owner->kprobe_write_ctx != aux->kprobe_write_ctx)
> > +                     return false;
> > +             break;
> > +
> > +     case BPF_MAP_OWNER_MATCH_FOR_UPDATE:
> > +             if (!owner->kprobe_write_ctx && aux->kprobe_write_ctx)
> > +                     return false;
> > +             break;
> > +     }
>
> In the v1 review, Alexei Starovoitov asked for strict conformance
> both ways rather than one-directional validation:
>
>     "Don't be fancy. Require strict conformance both ways
>     in *all* patches."
>
> https://lore.kernel.org/bpf/CAADnVQ+-V0-1i8_+CYpK7G0CnV-2n8e9Szv43yM3Az197eL_0A@xxxxxxxxxxxxxx/
>
> The BPF_MAP_OWNER_MATCH_FOR_UPDATE case still uses one-directional
> matching, rejecting only !owner->kprobe_write_ctx && aux->kprobe_write_ctx
> but allowing the reverse.  Was this an intentional design choice, and
> if so, should the changelog note the disagreement?
>

Let's follow the approach Alexei outlined, while the changes look ok
to me, let's remove the one way check and just do
owner->kprobe_write_ctx != aux->kprobe_write_ctx, same for the other
two checks. With this we can also get rid of this INIT vs UPDATE
distinction.

Other than that I think patches are good, please also test both
directions in the selftest in next respin.

>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/22630000544