test_suite_debug: test the printf used by debug.c

In `test_suite_debug`, test `mbedtls_debug_snprintf()`, which uses
`mbedtls_vsnprintf()` like `mbedtls_debug_print_msg()`. Do this instead of
testing `mbedtls_snprintf()`, which might be subtly different (older
Windows runtimes had slightly different behavior for vsnprintf() vs
snprintf(); TF-PSA-Crypto might pick up a different function if the
platform configuration is different in TF-PSA-Crypto and Mbed TLS).

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/debug.c

@@ -21,6 +21,16 @@
 /* DEBUG_BUF_SIZE must be at least 2 */
 #define DEBUG_BUF_SIZE      512
 
+int mbedtls_debug_snprintf(char *dest, size_t maxlen,
+                           const char *format, ...)
+{
+    va_list argp;
+    va_start(argp, format);
+    int ret = mbedtls_vsnprintf(dest, maxlen, format, argp);
+    va_end(argp);
+    return ret;
+}
+
 static int debug_threshold = 0;
 
 void mbedtls_debug_set_threshold(int threshold)

library/debug_internal.h

@@ -12,6 +12,19 @@
 
 #include "mbedtls/debug.h"
 
+/* This should be equivalent to mbedtls_snprintf(). But it might not be due
+ * to platform shenanigans. For example, Mbed TLS and TF-PSA-Crypto could
+ * have inconsistent platform definitions. On Mingw, some code might
+ * be built with a different setting of __USE_MINGW_ANSI_STDIO, resulting
+ * in an old non-C99 printf being used somewhere.
+ *
+ * Our library assumes that mbedtls_snprintf() and other printf functions
+ * are consistent throughout. This function is not an official API and
+ * is not meant to be used inside the library. It is provided to help
+ * debugging printf inconsistencies issues. If you need it, good luck!
+ */
+int mbedtls_debug_snprintf(char *dest, size_t maxlen,
+                           const char *format, ...) MBEDTLS_PRINTF_ATTRIBUTE(3, 4);
 /**
  * \brief    Print a message to the debug output. This function is always used
  *          through the MBEDTLS_SSL_DEBUG_MSG() macro, which supplies the ssl

tests/suites/test_suite_debug.function

@@ -116,11 +116,11 @@ void printf_int_expr(int format_indicator, intmax_t sizeof_x, intmax_t x, char *
     /* Nominal case: buffer just large enough */
     TEST_CALLOC(output, n + 1);
     if ((size_t) sizeof_x <= sizeof(int)) { // Any smaller integers would be promoted to an int due to calling a vararg function
-        TEST_EQUAL(n, mbedtls_snprintf(output, n + 1, format, (int) x));
+        TEST_EQUAL(n, mbedtls_debug_snprintf(output, n + 1, format, (int) x));
     } else if (sizeof_x == sizeof(long)) {
-        TEST_EQUAL(n, mbedtls_snprintf(output, n + 1, format, (long) x));
+        TEST_EQUAL(n, mbedtls_debug_snprintf(output, n + 1, format, (long) x));
     } else if (sizeof_x == sizeof(long long)) {
-        TEST_EQUAL(n, mbedtls_snprintf(output, n + 1, format, (long long) x));
+        TEST_EQUAL(n, mbedtls_debug_snprintf(output, n + 1, format, (long long) x));
     } else {
         TEST_FAIL(
             "sizeof_x <= sizeof(int) || sizeof_x == sizeof(long) || sizeof_x == sizeof(long long)");