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

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Message-ID: <CAADnVQLkSoOL8+kELdmX5nzNcXm-s4VbA5+Q-MPcNySsSiu+RQ@mail.gmail.com>
Date: Wed, 10 Dec 2025 16:09:19 +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 10:37 PM Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
>
> On Tue, Dec 09, 2025 at 06:12:46PM +0900, Alexei Starovoitov wrote:
> > 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.
>
> > This is pointless churn to shut up a warning.
>
> In some cases, like mine, it's an error.
>
> > Teach syzbot to stop this spam instead.
>
> It prevents to perform `make W=1` builds with the default CONFIG_WERROR,
> which is 'y'.
>
> > 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.
>
> I see, thanks for the review.
> Any recommendations on how to fix this properly?

Add -Wno-suggest-attribute=format
to corresponding files in Makefile.

I think it's cleaner than __diag_ignore() in the .c