libbpf: fix libbpf_print

With the recent print rework we now have the following problem:
pr_{warning,info,debug} expand to __pr which calls libbpf_print.
libbpf_print does va_start and calls __libbpf_pr with va_list argument.
In __base_pr we again do va_start. Because the next argument is a
va_list, we don't get correct pointer to the argument (and print noting
in my case, I don't know why it doesn't crash tbh).

Fix this by changing libbpf_print_fn_t signature to accept va_list and
remove unneeded calls to va_start in the existing users.

Alternatively, this can we solved by exporting __libbpf_pr and
changing __pr macro to (and killing libbpf_print):
{
	if (__libbpf_pr)
		__libbpf_pr(level, "libbpf: " fmt, ##__VA_ARGS__)
}

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf.c

@@ -54,22 +54,16 @@
 
 #define __printf(a, b)	__attribute__((format(printf, a, b)))
 
-__printf(2, 3)
-static int __base_pr(enum libbpf_print_level level, const char *format, ...)
+static int __base_pr(enum libbpf_print_level level, const char *format,
+		     va_list args)
 {
-	va_list args;
-	int err;
-
 	if (level == LIBBPF_DEBUG)
 		return 0;
 
-	va_start(args, format);
-	err = vfprintf(stderr, format, args);
-	va_end(args);
-	return err;
+	return vfprintf(stderr, format, args);
 }
 
-static __printf(2, 3) libbpf_print_fn_t __libbpf_pr = __base_pr;
+static libbpf_print_fn_t __libbpf_pr = __base_pr;
 
 void libbpf_set_print(libbpf_print_fn_t fn)
 {

tools/lib/bpf/libbpf.h

@@ -54,8 +54,7 @@ enum libbpf_print_level {
 };
 
 typedef int (*libbpf_print_fn_t)(enum libbpf_print_level level,
-				 const char *, ...)
-	__attribute__((format(printf, 2, 3)));
+				 const char *, va_list ap);
 
 LIBBPF_API void libbpf_set_print(libbpf_print_fn_t fn);
 

tools/perf/util/bpf-loader.c

@@ -25,15 +25,9 @@
 #include "c++/clang-c.h"
 
 static int libbpf_perf_print(enum libbpf_print_level level __attribute__((unused)),
-			      const char *fmt, ...)
+			      const char *fmt, va_list args)
 {
-	va_list args;
-	int ret;
-
-	va_start(args, fmt);
-	ret = veprintf(1, verbose, pr_fmt(fmt), args);
-	va_end(args);
-	return ret;
+	return veprintf(1, verbose, pr_fmt(fmt), args);
 }
 
 struct bpf_prog_priv {

tools/testing/selftests/bpf/test_btf.c

@@ -52,19 +52,10 @@ static int count_result(int err)
 	return err;
 }
 
-#define __printf(a, b)	__attribute__((format(printf, a, b)))
-
-__printf(2, 3)
 static int __base_pr(enum libbpf_print_level level __attribute__((unused)),
-		     const char *format, ...)
+		     const char *format, va_list args)
 {
-	va_list args;
-	int err;
-
-	va_start(args, format);
-	err = vfprintf(stderr, format, args);
-	va_end(args);
-	return err;
+	return vfprintf(stderr, format, args);
 }
 
 #define BTF_INFO_ENC(kind, kind_flag, vlen)			\

tools/testing/selftests/bpf/test_libbpf_open.c

@@ -36,19 +36,13 @@ static void usage(char *argv[])
 
 static bool debug = 0;
 static int libbpf_debug_print(enum libbpf_print_level level,
-			      const char *fmt, ...)
+			      const char *fmt, va_list args)
 {
-	va_list args;
-	int ret;
-
 	if (level == LIBBPF_DEBUG && !debug)
 		return 0;
 
-	va_start(args, fmt);
 	fprintf(stderr, "[%d] ", level);
-	ret = vfprintf(stderr, fmt, args);
-	va_end(args);
-	return ret;
+	return vfprintf(stderr, fmt, args);
 }
 
 #define EXIT_FAIL_LIBBPF EXIT_FAILURE

tools/testing/selftests/bpf/test_progs.c

@@ -1785,18 +1785,12 @@ static void test_task_fd_query_tp(void)
 }
 
 static int libbpf_debug_print(enum libbpf_print_level level,
-			      const char *format, ...)
+			      const char *format, va_list args)
 {
-	va_list args;
-	int ret;
-
 	if (level == LIBBPF_DEBUG)
 		return 0;
 
-	va_start(args, format);
-	ret = vfprintf(stderr, format, args);
-	va_end(args);
-	return ret;
+	return vfprintf(stderr, format, args);
 }
 
 static void test_reference_tracking()