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

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Message-ID: <20250822140553.46273-1-arighi@nvidia.com>
Date: Fri, 22 Aug 2025 16:05:53 +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: [PATCH] bpf: Mark kfuncs as __noclone

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>
---
 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