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

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Message-ID: <aTlyCimsBUsU5QgZ@smile.fi.intel.com>
Date: Wed, 10 Dec 2025 15:13:46 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.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 Wed, Dec 10, 2025 at 04:09:19PM +0900, Alexei Starovoitov wrote:
> 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.

Thanks, I just sent a new patch.

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

-- 
With Best Regards,
Andy Shevchenko