selftests/net: Provide test_snprintf() helper

Instead of pre-allocating a fixed-sized buffer of TEST_MSG_BUFFER_SIZE
and printing into it, call vsnprintf() with str = NULL, which will
return the needed size of the buffer. This hack is documented in
man 3 vsnprintf.

Essentially, in C++ terms, it re-invents std::stringstream, which is
going to be used to print different tracing paths and formatted strings.
Use it straight away in __test_print() - which is thread-safe version of
printing in selftests.
Signed-off-by: Dmitry Safonov <0x7f454c46@gmail.com>
Link: https://patch.msgid.link/20240823-tcp-ao-selftests-upd-6-12-v4-2-05623636fe8c@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/tcp_ao/lib/aolib.h

@@ -37,17 +37,58 @@ extern void __test_xfail(const char *buf);
 extern void __test_error(const char *buf);
 extern void __test_skip(const char *buf);
 
-__attribute__((__format__(__printf__, 2, 3)))
-static inline void __test_print(void (*fn)(const char *), const char *fmt, ...)
+static inline char *test_snprintf(const char *fmt, va_list vargs)
 {
-#define TEST_MSG_BUFFER_SIZE 4096
-	char buf[TEST_MSG_BUFFER_SIZE];
-	va_list arg;
-
-	va_start(arg, fmt);
-	vsnprintf(buf, sizeof(buf), fmt, arg);
-	va_end(arg);
-	fn(buf);
+	char *ret = NULL;
+	size_t size = 0;
+	va_list tmp;
+	int n = 0;
+
+	va_copy(tmp, vargs);
+	n = vsnprintf(ret, size, fmt, tmp);
+	if (n < 0)
+		return NULL;
+
+	size = n + 1;
+	ret = malloc(size);
+	if (!ret)
+		return NULL;
+
+	n = vsnprintf(ret, size, fmt, vargs);
+	if (n < 0 || n > size - 1) {
+		free(ret);
+		return NULL;
+	}
+	return ret;
+}
+
+static __printf(1, 2) inline char *test_sprintf(const char *fmt, ...)
+{
+	va_list vargs;
+	char *ret;
+
+	va_start(vargs, fmt);
+	ret = test_snprintf(fmt, vargs);
+	va_end(vargs);
+
+	return ret;
+}
+
+static __printf(2, 3) inline void __test_print(void (*fn)(const char *),
+					       const char *fmt, ...)
+{
+	va_list vargs;
+	char *msg;
+
+	va_start(vargs, fmt);
+	msg = test_snprintf(fmt, vargs);
+	va_end(vargs);
+
+	if (!msg)
+		return;
+
+	fn(msg);
+	free(msg);
 }
 
 #define test_print(fmt, ...)						\