linux-kernel - Re: [PATCH] bpf: Mark kfuncs as _

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Message-ID: <aNOQkZjLNwQOlioo@gpd4>
Date: Wed, 24 Sep 2025 08:32:49 +0200
From: Andrea Righi <arighi@...dia.com>
To: Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>
Cc: Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	David Vernet <void@...ifault.com>, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf: Mark kfuncs as __noclone

On Fri, Aug 22, 2025 at 04:05:53PM +0200, Andrea Righi wrote:
> Some distributions (e.g., CachyOS) support building the kernel with -O3,
> but doing so may break kfuncs, resulting in their symbols not being
> properly exported.
> 
> In fact, with gcc -O3, some kfuncs may be optimized away despite being
> annotated as noinline. This happens because gcc can still clone the
> function during IPA optimizations, e.g., by duplicating or inlining it
> into callers, and then dropping the standalone symbol. This breaks BTF
> ID resolution since resolve_btfids relies on the presence of a global
> symbol for each kfunc.
> 
> Currently, this is not an issue for upstream, because we don't allow
> building the kernel with -O3, but it may be safer to address it anyway,
> to prevent potential issues in the future if compilers become more
> aggressive with optimizations.
> 
> Therefore, add __noclone to __bpf_kfunc to ensure kfuncs are never
> cloned and remain distinct, globally visible symbols, regardless of
> the optimization level.
> 
> Fixes: 57e7c169cd6af ("bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs")
> Signed-off-by: Andrea Righi <arighi@...dia.com>

Gentle ping.

Anyone has any concerns with this? Do you think we can apply it (so we
don't have to keep carrying it out of tree)? :)

Thanks,
-Andrea

> ---
>  include/linux/btf.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index 9eda6b113f9b4..f06976ffb63f9 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -86,7 +86,7 @@
>   * as to avoid issues such as the compiler inlining or eliding either a static
>   * kfunc, or a global kfunc in an LTO build.
>   */
> -#define __bpf_kfunc __used __retain noinline
> +#define __bpf_kfunc __used __retain __noclone noinline
>  
>  #define __bpf_kfunc_start_defs()					       \
>  	__diag_push();							       \
> -- 
> 2.50.1
>