linux-kernel - Re: [PATCH v1 1/1] bpf: Disable -Wsuggest-attribute=format

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Message-ID: <CAADnVQLZPYc0HWqQw7ma=G-t9UMXXo+aXomVkYAzoQt=0ZrQ=Q@mail.gmail.com>
Date: Mon, 15 Dec 2025 10:40:16 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Alexei Starovoitov <ast@...nel.org>, 
	Steven Rostedt <rostedt@...dmis.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>, 
	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>, 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>, Masami Hiramatsu <mhiramat@...nel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v1 1/1] bpf: Disable -Wsuggest-attribute=format

On Mon, Dec 15, 2025 at 10:23 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Mon, Dec 15, 2025 at 10:13 AM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
> >
> > On Mon, Dec 15, 2025 at 9:36 AM Andrii Nakryiko
> > <andrii.nakryiko@...il.com> wrote:
> > >
> > > On Wed, Dec 10, 2025 at 5:12 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/bpf/stream.c:241:9: error: function ‘bpf_stream_vprintk_impl’ might be a candidate for ‘gnu_printf’ format attribute [-Werror=suggest-attribute=format]
> > > >   241 |         ret = bstr_printf(data.buf, MAX_BPRINTF_BUF, fmt__str, 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);
> > > >       |         ^~~~~~~~~~~
> > > >
> > >
> > > I just want to point out that the compiler suggestion is wrong here
> > > and these functions do not follow printf semantics. Yes, they have
> > > printf format string argument, but arguments themselves are passed
> > > using a special convention that the compiler won't know how to verify
> > > properly. So now, these are not candidates for gnu_printf, and it
> > > would be nice to have some way to shut up GCC for individual function
> > > instead of blanket -Wno-suggest-attribute for the entire file.
> > >
> > > Similarly, I see you marked bstr_printf() with __printf() earlier.
> > > That also seems wrong, so you might want to fix that mistake as well,
> > > while at it.
> > >
> > > Maybe the pragma push/pop approach would be a bit better and more
> > > explicit in the code?
> >
> > I suggested using makefile and file level disable to avoid polluting
> > the code. Even when attr-print applies it doesn't help definitions.
> > The attribute is only useful in declaration and in our case it's not
> > going to be in vmlinux.h or in bpf_helpers.h
> > So having it right or wrong in .c is misleading.
>
> Makefile is fine, even if it's a big hammer, I don't mind or care.

I rewrote the patch, commit log and pushed:
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/commit/?id=ba34388912b5326aac591508c953a0be67a15d5a

> But I think instead of Makefile changes we should fix the root cause
> here. And that seems to be just wrong __printf annotations for
> seq_bprintf and bstr_printf. They are not printf-like, they should not
> be marked as such, and then the compiler won't be wrongly suggesting
> bpf_stream_vprintk_impl (and others that make use of either
> bstr_printf or seq_bprintf) to be marked with __printf.

yeah. commit 7bf819aa992f ("vsnprintf: Mark binary printing functions
with __printf() attribute")
should be reverted,
but that somebody else problem and the revert would need to silence
that incorrect warning in lib/vsprintf.c too.