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

[Andy Shevchenko]

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@xxxxxxxxxxxxxxx> 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?

-- 
With Best Regards,
Andy Shevchenko