From: Arnd Bergmann <arnd@arndb.de>

Some internal functions in bpf produce a warning when -Wsuggest-attribute=format
is passed to the compiler, e.g. in 'make W=1':

kernel/trace/bpf_trace.c: In function '____bpf_trace_printk':
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);
      |         ^~~

The attribute here is useless since there are no callers from C code,
but it helps to shut up the output anyway so we can eventually turn
the warning option on by default.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 kernel/bpf/helpers.c     | 1 +
 kernel/bpf/stream.c      | 1 +
 kernel/trace/bpf_trace.c | 3 +++
 3 files changed, 5 insertions(+)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index b54ec0e945aa..45d026fc4e8a 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1046,6 +1046,7 @@ int bpf_bprintf_prepare(const char *fmt, u32 fmt_size, const u64 *raw_args,
 	return err;
 }
 
+__printf(3, 0)
 BPF_CALL_5(bpf_snprintf, char *, str, u32, str_size, char *, fmt,
 	   const void *, args, u32, data_len)
 {
diff --git a/kernel/bpf/stream.c b/kernel/bpf/stream.c
index 24730df55e69..816fd7fba3d2 100644
--- a/kernel/bpf/stream.c
+++ b/kernel/bpf/stream.c
@@ -212,6 +212,7 @@ __bpf_kfunc_start_defs();
  * Avoid using enum bpf_stream_id so that kfunc users don't have to pull in the
  * enum in headers.
  */
+__printf(2, 0)
 __bpf_kfunc int bpf_stream_vprintk(int stream_id, const char *fmt__str, const void *args,
 				   u32 len__sz, struct bpf_prog_aux *aux)
 {
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index eadaef8592a3..2d3de71ab86a 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -359,6 +359,7 @@ static const struct bpf_func_proto bpf_probe_write_user_proto = {
 #define MAX_TRACE_PRINTK_VARARGS	3
 #define BPF_TRACE_PRINTK_SIZE		1024
 
+__printf(1, 0)
 BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
 	   u64, arg2, u64, arg3)
 {
@@ -412,6 +413,7 @@ const struct bpf_func_proto *bpf_get_trace_printk_proto(void)
 	return &bpf_trace_printk_proto;
 }
 
+__printf(1, 0)
 BPF_CALL_4(bpf_trace_vprintk, char *, fmt, u32, fmt_size, const void *, args,
 	   u32, data_len)
 {
@@ -455,6 +457,7 @@ const struct bpf_func_proto *bpf_get_trace_vprintk_proto(void)
 	return &bpf_trace_vprintk_proto;
 }
 
+__printf(2, 0)
 BPF_CALL_5(bpf_seq_printf, struct seq_file *, m, char *, fmt, u32, fmt_size,
 	   const void *, args, u32, data_len)
 {
-- 
2.39.5

On Tue, Feb 3, 2026 at 8:27 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> Some internal functions in bpf produce a warning when -Wsuggest-attribute=format
> is passed to the compiler, e.g. in 'make W=1':
>
> kernel/trace/bpf_trace.c: In function '____bpf_trace_printk':
> 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);
>       |         ^~~
>
> The attribute here is useless since there are no callers from C code,
> but it helps to shut up the output anyway so we can eventually turn
> the warning option on by default.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

This was discussed and it's incorrect.
Commit 7bf819aa992f ("vsnprintf: Mark binary printing functions with
__printf() attribute")
should be reverted instead.

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