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

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Message-ID: <43eebdf1-5ea9-4991-88c3-f0780d7c42c6@linux.dev>
Date: Fri, 3 Oct 2025 20:08:40 -0700
From: Ihor Solodrai <ihor.solodrai@...ux.dev>
To: Eduard Zingerman <eddyz87@...il.com>,
 "Matthieu Baerts (NGI0)" <matttbe@...nel.org>, 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 10/3/25 4:37 PM, Eduard Zingerman wrote:
> 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?

This is most likely the issue addressed in this patch:
https://lore.kernel.org/dwarves/f7553b3f-5827-4f50-81a9-9bd0802734b9@linux.dev/

There wasn't a new pahole release with it yet.