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

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Message-ID: <4vfoqnabquaeeewaxqi6xuflha6c542or6vsiiuvfrgcmqbrli@yyeuhp45relh>
Date: Tue, 26 Aug 2025 21:10:46 -0500
From: David Vernet <void@...ifault.com>
To: Andrea Righi <arighi@...dia.com>
Cc: Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	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>, 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>

Acked-by: David Vernet <void@...ifault.com>

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