Re: [PATCH v1 1/1] bpf: Mark BPF printing functions with __printf() attribute

On Tue, Dec 9, 2025 at 10:37 PM Andy Shevchenko
[off-list ref] wrote:

On Tue, Dec 09, 2025 at 06:12:46PM +0900, Alexei Starovoitov wrote:

quoted

On Tue, Dec 9, 2025 at 1:21 AM Andy Shevchenko
[off-list ref] wrote:

quoted

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.

quoted

This is pointless churn to shut up a warning.

In some cases, like mine, it's an error.

quoted

Teach syzbot to stop this spam instead.

It prevents to perform `make W=1` builds with the default CONFIG_WERROR,
which is 'y'.

quoted

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