On 8/22/25 7:05 AM, 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.
I tried with gcc14 and can reproduced the issue described in the above.
I build the kernel like below with gcc14
make KCFLAGS='-O3' -j
and get the following build error
WARN: resolve_btfids: unresolved symbol bpf_strnchr
make[2]: *** [/home/yhs/work/bpf-next/scripts/Makefile.vmlinux:91: vmlinux] Error 255
make[2]: *** Deleting file 'vmlinux'
Checking the symbol table:
22276: ffffffff81b15260 249 FUNC LOCAL DEFAULT 1 bpf_strnchr.cons[...]
235128: ffffffff81b1f540 296 FUNC GLOBAL DEFAULT 1 bpf_strnchr
and the disasm code:
bpf_strnchr:
...
bpf_strchr:
...
bpf_strnchr.constprop.0
...
So in symbol table, we have both bpf_strnchr.constprop.0 and bpf_strnchr.
For such case, pahole will skip func bpf_strnchr hence the above resolve_btfids
failure.
The solution in this patch can indeed resolve this issue.
Fixes: 57e7c169cd6af ("bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs")
Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
---
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(); \
The llvm does not support __noclone so __noclone is noop for llvm.
I tried with
make KCFLAGS='-O3' LLVM=1 -j
and the kernel is build successfully and also the kernel can boot properly.
So ack the patch:
Acked-by: Yonghong Song <yonghong.song@xxxxxxxxx>
