Re: [PATCH bpf v3 1/4] bpf: Keep refcount_acquire nullable for borrowed RCU kptrs

[Amery Hung]

On Mon, Aug 3, 2026 at 6:14 AM <bot+bpf-ci@xxxxxxxxxx> wrote:
>
> > bpf: Keep refcount_acquire nullable for borrowed RCU kptrs
> >
> > The verifier can mistake an RCU-loaded map kptr for an owned reference and
> > assume bpf_refcount_acquire() cannot return NULL. The kfunc uses
> > refcount_inc_not_zero(), so it can return NULL after the last reference is
> > dropped. Passing that unchecked result to bpf_obj_drop() can crash the
> > kernel.
> >
> > Only treat the argument as owned when the verifier tracks an acquired
> > reference for it. Borrowed pointers remain nullable, while owned pointers
> > keep the existing behavior.
> >
> > Fixes: 1b12171533a9 ("bpf: Mark direct ld of stashed bpf_{rb,list}_node as non-owning ref")

^^^ The fix tag is correct.

Reviewed-by: Amery Hung <ameryhung@xxxxxxxxx>

>
> Does this Fixes tag point to the right commit?
>
> Looking at the code change, the fix adds && reg_is_referenced(env, reg)
> to an existing condition that checks !type_is_non_owning_ref(reg->type).
> The problem being fixed is that the verifier incorrectly assumes pointers
> that are not non-owning refs must be owning refs, which fails to account
> for borrowed RCU pointers.
>
> The commit 1b12171533a9 added logic to mark certain pointers as
> non-owning refs, but the bug appears to originate from commit
> 7793fc3babe9 ("bpf: Make bpf_refcount_acquire fallible for non-owning
> refs"), which introduced the logic:
>
>   if (!type_is_non_owning_ref(reg->type))
>       meta->arg_owning_ref = true;
>
> That commit incorrectly assumed pointers that are not non-owning refs
> must be owning refs, missing the case of borrowed RCU pointers which are
> neither. The current fix addresses this by verifying the verifier tracks
> an acquired reference before treating the pointer as owned.
>
> Should the Fixes tag be:
>
>   Fixes: 7793fc3babe9 ("bpf: Make bpf_refcount_acquire fallible for non-owning refs")
>
>
> ---
> 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/30810949269