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

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Message-ID: <CAEf4BzZQ_OJehh=5jJgVBUjJBNAkWh2o8Yd9UTa9nFrRO4oAFg@mail.gmail.com>
Date: Mon, 15 Dec 2025 09:36:07 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc: Alexei Starovoitov <ast@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, 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 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?

> Fix the compilation errors by disabling that warning since the code is
> generated and warning is not so useful in this case — it can't check
> the parameters for now.
>
> 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: 5ab154f1463a ("bpf: Introduce BPF standard streams")
> 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")
> Suggested-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
>  kernel/bpf/Makefile   | 11 +++++++++--
>  kernel/trace/Makefile |  6 ++++++
>  2 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index 232cbc97434d..cf7e8a972f98 100644
> --- a/kernel/bpf/Makefile
> +++ b/kernel/bpf/Makefile
> @@ -6,7 +6,14 @@ cflags-nogcse-$(CONFIG_X86)$(CONFIG_CC_IS_GCC) := -fno-gcse
>  endif
>  CFLAGS_core.o += -Wno-override-init $(cflags-nogcse-yy)
>
> -obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o log.o token.o liveness.o
> +obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o tnum.o log.o token.o liveness.o
> +
> +obj-$(CONFIG_BPF_SYSCALL) += helpers.o stream.o
> +# The ____bpf_snprintf() uses the format string that triggers a compiler warning.
> +CFLAGS_helpers.o += -Wno-suggest-attribute=format
> +# The bpf_stream_vprintk_impl() uses the format string that triggers a compiler warning.
> +CFLAGS_stream.o += -Wno-suggest-attribute=format
> +
>  obj-$(CONFIG_BPF_SYSCALL) += bpf_iter.o map_iter.o task_iter.o prog_iter.o link_iter.o
>  obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o bloom_filter.o
>  obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o bpf_insn_array.o
> @@ -14,7 +21,7 @@ obj-$(CONFIG_BPF_SYSCALL) += bpf_local_storage.o bpf_task_storage.o
>  obj-${CONFIG_BPF_LSM}    += bpf_inode_storage.o
>  obj-$(CONFIG_BPF_SYSCALL) += disasm.o mprog.o
>  obj-$(CONFIG_BPF_JIT) += trampoline.o
> -obj-$(CONFIG_BPF_SYSCALL) += btf.o memalloc.o rqspinlock.o stream.o
> +obj-$(CONFIG_BPF_SYSCALL) += btf.o memalloc.o rqspinlock.o
>  ifeq ($(CONFIG_MMU)$(CONFIG_64BIT),yy)
>  obj-$(CONFIG_BPF_SYSCALL) += arena.o range_tree.o
>  endif
> diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
> index fc5dcc888e13..1673b395c14c 100644
> --- a/kernel/trace/Makefile
> +++ b/kernel/trace/Makefile
> @@ -104,7 +104,13 @@ obj-$(CONFIG_TRACE_EVENT_INJECT) += trace_events_inject.o
>  obj-$(CONFIG_SYNTH_EVENTS) += trace_events_synth.o
>  obj-$(CONFIG_HIST_TRIGGERS) += trace_events_hist.o
>  obj-$(CONFIG_USER_EVENTS) += trace_events_user.o
> +
>  obj-$(CONFIG_BPF_EVENTS) += bpf_trace.o
> +# The BPF printing functions use the format string that triggers a compiler warning.
> +# Since the code is generated and warning is not so useful in this case (it can't
> +# check the parameters for now) disable the warning.
> +CFLAGS_bpf_trace.o += -Wno-suggest-attribute=format
> +
>  obj-$(CONFIG_KPROBE_EVENTS) += trace_kprobe.o
>  obj-$(CONFIG_TRACEPOINTS) += error_report-traces.o
>  obj-$(CONFIG_TRACEPOINTS) += power-traces.o
> --
> 2.50.1
>