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

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

[Andy Shevchenko]

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.

| Reported-by: kernel test robot <lkp@intel.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:
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 include/linux/filter.h   | 24 +++++++++++++++---------
 kernel/bpf/helpers.c     |  2 +-
 kernel/trace/bpf_trace.c |  6 +++---
 3 files changed, 19 insertions(+), 13 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index fd54fed8f95f..31034a74af22 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -607,9 +607,9 @@ static inline bool insn_is_cast_user(const struct bpf_insn *insn)
 	__BPF_MAP(n, __BPF_DECL_ARGS, __BPF_N, u64, __ur_1, u64, __ur_2,       \
 		  u64, __ur_3, u64, __ur_4, u64, __ur_5)
 
-#define BPF_CALL_x(x, attr, name, ...)					       \
+#define BPF_CALL_x(x, __attr, attr, name, ...)				       \
 	static __always_inline						       \
-	u64 ____##name(__BPF_MAP(x, __BPF_DECL_ARGS, __BPF_V, __VA_ARGS__));   \
+	__attr u64 ____##name(__BPF_MAP(x, __BPF_DECL_ARGS, __BPF_V, __VA_ARGS__));     \
 	typedef u64 (*btf_##name)(__BPF_MAP(x, __BPF_DECL_ARGS, __BPF_V, __VA_ARGS__)); \
 	attr u64 name(__BPF_REG(x, __BPF_DECL_REGS, __BPF_N, __VA_ARGS__));    \
 	attr u64 name(__BPF_REG(x, __BPF_DECL_REGS, __BPF_N, __VA_ARGS__))     \
@@ -620,14 +620,20 @@ static inline bool insn_is_cast_user(const struct bpf_insn *insn)
 	u64 ____##name(__BPF_MAP(x, __BPF_DECL_ARGS, __BPF_V, __VA_ARGS__))
 
 #define __NOATTR
-#define BPF_CALL_0(name, ...)	BPF_CALL_x(0, __NOATTR, name, __VA_ARGS__)
-#define BPF_CALL_1(name, ...)	BPF_CALL_x(1, __NOATTR, name, __VA_ARGS__)
-#define BPF_CALL_2(name, ...)	BPF_CALL_x(2, __NOATTR, name, __VA_ARGS__)
-#define BPF_CALL_3(name, ...)	BPF_CALL_x(3, __NOATTR, name, __VA_ARGS__)
-#define BPF_CALL_4(name, ...)	BPF_CALL_x(4, __NOATTR, name, __VA_ARGS__)
-#define BPF_CALL_5(name, ...)	BPF_CALL_x(5, __NOATTR, name, __VA_ARGS__)
+#define BPF_CALL_0(name, ...)	BPF_CALL_x(0, __NOATTR, __NOATTR, name, __VA_ARGS__)
+#define BPF_CALL_1(name, ...)	BPF_CALL_x(1, __NOATTR, __NOATTR, name, __VA_ARGS__)
+#define BPF_CALL_2(name, ...)	BPF_CALL_x(2, __NOATTR, __NOATTR, name, __VA_ARGS__)
+#define BPF_CALL_3(name, ...)	BPF_CALL_x(3, __NOATTR, __NOATTR, name, __VA_ARGS__)
+#define BPF_CALL_4(name, ...)	BPF_CALL_x(4, __NOATTR, __NOATTR, name, __VA_ARGS__)
+#define BPF_CALL_5(name, ...)	BPF_CALL_x(5, __NOATTR, __NOATTR, name, __VA_ARGS__)
 
-#define NOTRACE_BPF_CALL_1(name, ...)	BPF_CALL_x(1, notrace, name, __VA_ARGS__)
+#define PRINTF_BPF_CALL_x(p, name, x, ...)				       \
+	BPF_CALL_x(x, __printf(p, 0), __NOATTR, name, __VA_ARGS__)
+
+#define PRINTF_BPF_CALL_4(p, name, ...)		PRINTF_BPF_CALL_x(p, name, 4, __VA_ARGS__)
+#define PRINTF_BPF_CALL_5(p, name, ...)		PRINTF_BPF_CALL_x(p, name, 5, __VA_ARGS__)
+
+#define NOTRACE_BPF_CALL_1(name, ...)	BPF_CALL_x(1, __NOATTR, notrace, name, __VA_ARGS__)
 
 #define bpf_ctx_range(TYPE, MEMBER)						\
 	offsetof(TYPE, MEMBER) ... offsetofend(TYPE, MEMBER) - 1
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index db72b96f9c8c..cbc66865e3dc 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1046,7 +1046,7 @@ int bpf_bprintf_prepare(const char *fmt, u32 fmt_size, const u64 *raw_args,
 	return err;
 }
 
-BPF_CALL_5(bpf_snprintf, char *, str, u32, str_size, char *, fmt,
+PRINTF_BPF_CALL_5(3, bpf_snprintf, char *, str, u32, str_size, char *, fmt,
 	   const void *, args, u32, data_len)
 {
 	struct bpf_bprintf_data data = {
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index d57727abaade..5fd46b4bcf48 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -359,7 +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
 
-BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
+PRINTF_BPF_CALL_5(1, bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
 	   u64, arg2, u64, arg3)
 {
 	u64 args[MAX_TRACE_PRINTK_VARARGS] = { arg1, arg2, arg3 };
@@ -412,7 +412,7 @@ const struct bpf_func_proto *bpf_get_trace_printk_proto(void)
 	return &bpf_trace_printk_proto;
 }
 
-BPF_CALL_4(bpf_trace_vprintk, char *, fmt, u32, fmt_size, const void *, args,
+PRINTF_BPF_CALL_4(1, bpf_trace_vprintk, char *, fmt, u32, fmt_size, const void *, args,
 	   u32, data_len)
 {
 	struct bpf_bprintf_data data = {
@@ -455,7 +455,7 @@ const struct bpf_func_proto *bpf_get_trace_vprintk_proto(void)
 	return &bpf_trace_vprintk_proto;
 }
 
-BPF_CALL_5(bpf_seq_printf, struct seq_file *, m, char *, fmt, u32, fmt_size,
+PRINTF_BPF_CALL_5(2, bpf_seq_printf, struct seq_file *, m, char *, fmt, u32, fmt_size,
 	   const void *, args, u32, data_len)
 {
 	struct bpf_bprintf_data data = {
-- 
2.50.1

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

[Andy Shevchenko]

On Mon, Dec 08, 2025 at 04:47:33PM +0100, Andy Shevchenko wrote:
> The printing functions in BPF code are using printf() type of format,
> and compiler is not happy about them as is:

Sorry, this is noise that has to not be sent, it's WIP.
I will send it later when I provide a proper commit message.

-- 
With Best Regards,
Andy Shevchenko