linux-kernel - Re: [PATCH v1 1/1] bpf: Mark BPF printing functions with _

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Message-ID: <CAADnVQ+SXe-CsPHnYkB4SOKct6iMN=PkexaKRd-MJFhC3i8M0A@mail.gmail.com>
Date: Tue, 9 Dec 2025 18:12:46 +0900
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	linux-trace-kernel <linux-trace-kernel@...r.kernel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	John Fastabend <john.fastabend@...il.com>, 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>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Matt Bobrowski <mattbobrowski@...gle.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Alan Maguire <alan.maguire@...cle.com>, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v1 1/1] bpf: Mark BPF printing functions with __printf() attribute

On Tue, Dec 9, 2025 at 1:21 AM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> The printing functions in BPF code are using printf() type of format,
> and compiler is not happy about them as is:
>
> kernel/bpf/helpers.c:1069:9: error: function ‘____bpf_snprintf’ might be a candidate for ‘gnu_printf’ format attribute [-Werror=suggest-attribute=format]
>  1069 |         err = bstr_printf(str, str_size, fmt, data.bin_args);
>       |         ^~~
>
> kernel/trace/bpf_trace.c:377:9: error: function ‘____bpf_trace_printk’ might be a candidate for ‘gnu_printf’ format attribute [-Werror=suggest-attribute=format]
>   377 |         ret = bstr_printf(data.buf, MAX_BPRINTF_BUF, fmt, data.bin_args);
>       |         ^~~
>
> kernel/trace/bpf_trace.c:433:9: error: function ‘____bpf_trace_vprintk’ might be a candidate for ‘gnu_printf’ format attribute [-Werror=suggest-attribute=format]
>   433 |         ret = bstr_printf(data.buf, MAX_BPRINTF_BUF, fmt, data.bin_args);
>       |         ^~~
>
> kernel/trace/bpf_trace.c:475:9: error: function ‘____bpf_seq_printf’ might be a candidate for ‘gnu_printf’ format attribute [-Werror=suggest-attribute=format]
>   475 |         seq_bprintf(m, fmt, data.bin_args);
>       |         ^~~~~~~~~~~
>
> Fix the compilation errors by adding __printf() attribute. For that
> we need to pass it down to the BPF_CALL_x() and wrap into PRINTF_BPF_CALL_*()
> to make code neater.
>
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202512061425.x0qTt9ww-lkp@intel.com/
> Closes: https://lore.kernel.org/oe-kbuild-all/202512061640.9hKTnB8p-lkp@intel.com/
> Closes: https://lore.kernel.org/oe-kbuild-all/202512081321.2h9ThWTg-lkp@intel.com/
> Fixes: 10aceb629e19 ("bpf: Add bpf_trace_vprintk helper")
> Fixes: 7b15523a989b ("bpf: Add a bpf_snprintf helper")
> Fixes: 492e639f0c22 ("bpf: Add bpf_seq_printf and bpf_seq_write helpers")
> Fixes: f3694e001238 ("bpf: add BPF_CALL_x macros for declaring helpers")
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
>
> This is combined change and I think there is no need to split it, but if required
> I can do it in a four changes. Note, the culprits are older than 4 years and stable
> kernels anyway don't go that deep nowadays.

This is pointless churn to shut up a warning.
Teach syzbot to stop this spam instead.
At the end this patch doesn't make any visible difference,
since user declarations of these helpers are auto generated
from uapi/bpf.h file and __printf attribute is not there.

pw-bot: cr