Re: [PATCH bpf-next v3 2/4] bpf: Introduce __bpf_kfunc_mark_deprecated annotation

[sashiko-bot@kernel.org]

Sashiko AI review found 1 potential issue(s):
- [Medium] Programs loaded without BTF will silently bypass deprecation warnings.
--

commit 90e501a1948d112dbb3b96a16249f1f76f84b290
Author: Kumar Kartikeya Dwivedi <memxor@gmail.com>

bpf: Introduce __bpf_kfunc_mark_deprecated annotation

This commit introduces the __bpf_kfunc_mark_deprecated macro to annotate
deprecated kfuncs with a replacement. It applies this annotation to existing
impl-suffixed kfuncs and modifies the verifier to emit warnings when these
deprecated kfuncs are used.

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c

[ ... ]

> @@ -12987,6 +12987,45 @@ static int check_return_code(struct bpf_verifier_env *env, int regno, const char
>  static int process_bpf_exit_full(struct bpf_verifier_env *env,
>  				 bool *do_print_state, bool exception_exit);
>  
> +static void warn_for_deprecated_kfuncs(struct bpf_verifier_env *env,
> +				       struct bpf_kfunc_call_arg_meta *meta,
> +				       int insn_idx, s16 offset)
> +{
> +	const struct bpf_line_info *linfo;
> +	const char *replacement;
> +	const struct btf_type *t;
> +	struct bpf_kfunc_desc *desc;
> +	const char *file;
> +	int line_num;
> +
> +	if (!env->prog->aux->btf)
> +		return;

Will programs loaded without BTF silently bypass the deprecation warning?

The kfunc deprecation tags are stored in the kernel or module BTF
(meta->btf), not the program's BTF.

If we return early here, we skip evaluating the tag entirely. There is a
fallback branch below that handles missing line info which could be used to
emit the warning even if the program doesn't have BTF.

> +
> +	t = btf_type_by_id(meta->btf, meta->func_id);
> +	replacement = btf_find_decl_tag_value(meta->btf, t, -1, BPF_KFUNC_DECL_TAG_DEPRECATED);

[ ... ]

> +	linfo = bpf_find_linfo(env->prog, insn_idx);
> +	if (linfo) {
> +		bpf_get_linfo_file_line(env->prog->aux->btf, linfo, &file, NULL, &line_num);
> +		warn(env, "%s:%d (insn #%d) uses deprecated kfunc %s(), which will be removed.\n",
> +		     file, line_num, insn_idx, meta->func_name);
> +	} else {
> +		warn(env, "(insn #%d) uses deprecated kfunc %s(), which will be removed.\n",
> +		     insn_idx, meta->func_name);
> +	}
> +
> +	warn(env, "Switch to kfunc %s() instead.\n", replacement);
> +	warn(env, "For older kernels, choose the correct kfunc using bpf_ksym_exists().\n");
> +
> +	desc->warned_deprecated = true;
> +}

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260418171701.610025-1-memxor@gmail.com?part=2