linux-kernel - Re: [PATCH v5 2/3] bpf/verifier: allow using bpf_kptr_xchg even if the NON_OWN

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Message-Id: <94483c320d8eaf29277fe60ca7d10ed70b0bb6ca636994a0eb07c26d5f139b0e@mail.kernel.org>
Date: Tue,  3 Feb 2026 03:04:40 +0000 (UTC)
From: bot+bpf-ci@...nel.org
To: pilgrimtao@...il.com,ast@...nel.org,daniel@...earbox.net,john.fastabend@...il.com,andrii@...nel.org,martin.lau@...ux.dev,eddyz87@...il.com,song@...nel.org,yonghong.song@...ux.dev,kpsingh@...nel.org,sdf@...ichev.me,haoluo@...gle.com,jolsa@...nel.org,shuah@...nel.org,yangfeng@...inos.cn,alexei.starovoitov@...il.com
Cc: linux-kernel@...r.kernel.org,bpf@...r.kernel.org,linux-kselftest@...r.kernel.org,chengkaitao@...inos.cn,ast@...nel.org,andrii@...nel.org,daniel@...earbox.net,martin.lau@...nel.org,eddyz87@...il.com,yonghong.song@...ux.dev,clm@...a.com,ihor.solodrai@...ux.dev
Subject: Re: [PATCH v5 2/3] bpf/verifier: allow using bpf_kptr_xchg even if the NON_OWN_REF flag is set

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 2dd860ae12..bb45fa5612 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9300,7 +9300,8 @@ static const struct bpf_reg_types timer_types = { .types = { PTR_TO_MAP_VALUE }
>  static const struct bpf_reg_types kptr_xchg_dest_types = {
>  	.types = {
>  		PTR_TO_MAP_VALUE,
> -		PTR_TO_BTF_ID | MEM_ALLOC
> +		PTR_TO_BTF_ID | MEM_ALLOC,
> +		PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF
>  	}
>  };

Is the MEM_RCU variant also needed here?

When ref_set_non_owning() marks a register as NON_OWN_REF, it also adds
MEM_RCU if the node has a bpf_refcount field (refcount_off >= 0):

    reg->type |= NON_OWN_REF;
    if (rec->refcount_off >= 0)
        reg->type |= MEM_RCU;

This means rbtree nodes with both a __kptr field and a bpf_refcount field
would have type PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU, which
won't match any entry in kptr_xchg_dest_types.

Should this also add:

    PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF | MEM_RCU

to handle that case?

> @@ -9460,6 +9461,7 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
>  	}
>  	case PTR_TO_BTF_ID | MEM_ALLOC:
>  	case PTR_TO_BTF_ID | MEM_PERCPU | MEM_ALLOC:
> +	case PTR_TO_BTF_ID | MEM_ALLOC | NON_OWN_REF:
>  		if (meta->func_id != BPF_FUNC_spin_lock && meta->func_id != BPF_FUNC_spin_unlock &&
>  		    meta->func_id != BPF_FUNC_kptr_xchg) {

Similarly, does this switch need a case for the MEM_RCU variant?

---
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/21614799102

AI-authorship-score: low
AI-authorship-explanation: The commit addresses a specific practical problem with minimal targeted changes, showing domain expertise typical of human kernel developers rather than AI-generated comprehensive solutions.
issues-found: 1
issue-severity-score: low
issue-severity-explanation: Missing MEM_RCU variant may prevent bpf_kptr_xchg on refcounted rbtree nodes, but this affects only a subset of use cases and causes verifier rejection rather than runtime issues.