linux-kernel - Re: [PATCH bpf] selftests/bpf: fix implicit-function-declaration errors

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Message-ID: <d108d59be611a63c73303347d07fe0ba5f2b74b7.camel@gmail.com>
Date: Fri, 03 Oct 2025 16:37:32 -0700
From: Eduard Zingerman <eddyz87@...il.com>
To: "Matthieu Baerts (NGI0)" <matttbe@...nel.org>, ihor.solodrai@...ux.dev, 
	alan.maguire@...cle.com
Cc: bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, Andrii Nakryiko	
 <andrii@...nel.org>, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann	
 <daniel@...earbox.net>, Martin KaFai Lau <martin.lau@...ux.dev>, 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>,  Shuah Khan <shuah@...nel.org>, Nathan Chancellor
 <nathan@...nel.org>, Nick Desaulniers	 <nick.desaulniers+lkml@...il.com>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH bpf] selftests/bpf: fix implicit-function-declaration
 errors

On Fri, 2025-10-03 at 17:24 +0200, Matthieu Baerts (NGI0) wrote:
> When trying to build the latest BPF selftests, with a debug kernel
> config, Pahole 1.30 and CLang 20.1.8 (and GCC 15.2), I got these errors:
> 
>   progs/dynptr_success.c:579:9: error: call to undeclared function 'bpf_dynptr_slice'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
>     579 |         data = bpf_dynptr_slice(&ptr, 0, NULL, 1);
>         |                ^
>   progs/dynptr_success.c:579:9: note: did you mean 'bpf_dynptr_size'?
>   .virtme/build-debug-btf//tools/include/vmlinux.h:120280:14: note: 'bpf_dynptr_size' declared here
>    120280 | extern __u32 bpf_dynptr_size(const struct bpf_dynptr *p) __weak __ksym;
>           |              ^
>   progs/dynptr_success.c:579:7: error: incompatible integer to pointer conversion assigning to '__u64 *' (aka 'unsigned long long *') from 'int' [-Wint-conversion]
>     579 |         data = bpf_dynptr_slice(&ptr, 0, NULL, 1);
>         |              ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   progs/dynptr_success.c:596:9: error: call to undeclared function 'bpf_dynptr_slice'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
>     596 |         data = bpf_dynptr_slice(&ptr, 0, NULL, 10);
>         |                ^
>   progs/dynptr_success.c:596:7: error: incompatible integer to pointer conversion assigning to 'char *' from 'int' [-Wint-conversion]
>     596 |         data = bpf_dynptr_slice(&ptr, 0, NULL, 10);
>         |              ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> I don't have these errors without the debug kernel config from
> kernel/configs/debug.config. With the debug kernel, bpf_dynptr_slice()
> is not declared in vmlinux.h. It is declared there without debug.config.
> 
> The fix is similar to what is done in dynptr_fail.c which is also using
> bpf_dynptr_slice(): bpf_kfuncs.h is now included.
> 
> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>
> ---

I can reproduce similar issue when including
kernel/configs/debug.config with my regular dev config, but for
different functions: bpf_rcu_read_{un,}lock().

However, this is not a way to fix this.
Kfuncs are not supposed to just disappear from DWARF.

Running pahole in verbose mode I see the following output:

  $ pahole -V \
      --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs \
      --btf_features=attributes \
      --lang_exclude=rust \
      --btf_encode_detached=/dev/null vmlinux
  ...
  matched function 'bpf_rcu_read_lock' with 'bpf_rcu_read_lock.cold'
  ...

Alan, Ihor, does this sound familiar?