Re: [PATCH bpf-next v8 08/22] bpf: Require an unwind to leave a frame holding what it entered with

[Yonghong Song <yonghong.song@linux.dev>]

On 10/3/26 2:25 PM, Alexei Starovoitov wrote:
> On Thu, Oct 01, 2026 at 06:30 AM Yonghong Song <yonghong.song@linux.dev> wrote:
>> frame that dropped it. The rule does refuse a program that could be proved
>> safe, a callee releasing its caller's reference and a caller's pad that
>> relies on that, but compiled code does not do that. A pad only knows about
>> its own frame.
> Compiled code does exactly that. It's a move.
>
>    fn consume(rec: Record) { may_panic(); }
>    fn foo() { let rec = reserve(); consume(rec); }
>
> consume() owns rec, so the pad of consume() drops it.
> foo() has nothing left to drop and rustc emits a plain call with no
> record over it.
> 'call consume' is refused with
> "an unwind through this call keeps the reference".
> With a record over that call the resume in consume() is refused with
> "a resume does not leave the frame's references as it found it".
> pad_drops_caller_ref_frame() in patch 19 is what consume() compiles to.
> RcuReadGuard passed by value is the same.
>
> [...]
>
>> +	u32 entry_active_locks;
>> +	u32 entry_preempt_locks;
>> +	u32 entry_rcu_locks;
>> +	u32 entry_irq_id;
>> +	u32 entry_id_gen;
>> +	u32 entry_acquired_refs;
> The verifier doesn't need them anymore.
> "Unreleased reference id=%d alloc_insn=%d" at exit already points
> at the insn that acquired it.
> Drop this patch.

You are absolutely right. We only need to check resource
right before main prog return. This patch indeed not needed any more.