Integrate contributed implementation into Mbed TLS

A reduced implementation of the snprintf function has been provided.
This commit integrates it with Mbed TLS by putting it in the platform.c
translation unit and making it configurable via the config file.
Additionally the code has been formated to fit Mbed TLS style
guidelines.

include/mbedtls/check_config.h

@@ -392,6 +392,16 @@
 #error "MBEDTLS_PLATFORM_SNPRINTF_ALT defined, but not all prerequisites"
 #endif
 
+#if defined(MBEDTLS_PLATFORM_TF_SNPRINTF) && !defined(MBEDTLS_PLATFORM_C)
+#error "MBEDTLS_PLATFORM_TF_SNPRINTF defined, but not all prerequisites"
+#endif
+
+#if defined(MBEDTLS_PLATFORM_TF_SNPRINTF) &&\
+    ( defined(MBEDTLS_PLATFORM_SNPRINTF_ALT) ||\
+      defined(MBEDTLS_PLATFORM_STD_SNPRINTF) )
+#error "MBEDTLS_PLATFORM_TF_SNPRINTF and MBEDTLS_PLATFORM_SNPRINTF_ALT/MBEDTLS_PLATFORM_STD_SNPRINTF, cannot be defined simultaneously"
+#endif
+
 #if defined(MBEDTLS_PLATFORM_SNPRINTF_MACRO) && !defined(MBEDTLS_PLATFORM_C)
 #error "MBEDTLS_PLATFORM_SNPRINTF_MACRO defined, but not all prerequisites"
 #endif
@@ -399,7 +409,7 @@
 #if defined(MBEDTLS_PLATFORM_SNPRINTF_MACRO) &&\
     ( defined(MBEDTLS_PLATFORM_STD_SNPRINTF) ||\
         defined(MBEDTLS_PLATFORM_SNPRINTF_ALT) )
-#error "MBEDTLS_PLATFORM_SNPRINTF_MACRO and MBEDTLS_PLATFORM_STD_SNPRINTF/MBEDTLS_PLATFORM_SNPRINTF_ALT cannot be defined simultaneously"
+#error "MBEDTLS_PLATFORM_SNPRINTF_MACRO and MBEDTLS_PLATFORM_STD_SNPRINTF/MBEDTLS_PLATFORM_SNPRINTF_ALT/MBEDTLS_PLATFORM_TF_SNPRINTF cannot be defined simultaneously"
 #endif
 
 #if defined(MBEDTLS_PLATFORM_STD_MEM_HDR) &&\

include/mbedtls/config.h

@@ -194,6 +194,22 @@
 //#define MBEDTLS_PLATFORM_NV_SEED_ALT
 //#define MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT
 
+/**
+ * \def MBEDTLS_PLATFORM_TF_SNPRINTF
+ *
+ * Uncomment to enable the reduced snprintf to be used for trusted firmware.
+ *
+ * \warning Only The following type specifiers are supported:
+ * %d or %i - signed decimal format
+ * %u - unsigned decimal format
+ * The print panics on all other formats specifiers.
+ *
+ * \warning MBEDTLS_PLATFORM_SNPRINTF_ALT cannot be defined at the same time as
+ * MBEDTLS_PLATFORM_TF_SNPRINTF!
+ *
+ */
+//#define MBEDTLS_PLATFORM_TF_SNPRINTF
+
 /**
  * \def MBEDTLS_DEPRECATED_WARNING
  *

include/mbedtls/platform.h

@@ -50,6 +50,7 @@ extern "C" {
 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
+#include <stdarg.h>
 #if !defined(MBEDTLS_PLATFORM_STD_SNPRINTF)
 #if defined(_WIN32)
 #define MBEDTLS_PLATFORM_STD_SNPRINTF   mbedtls_platform_win32_snprintf /**< The default \c snprintf function to use.  */

library/platform.c

@@ -98,6 +98,111 @@ int mbedtls_platform_win32_snprintf( char *s, size_t n, const char *fmt, ... )
 }
 #endif
 
+#if defined(MBEDTLS_PLATFORM_TF_SNPRINTF)
+
+static void unsigned_dec_print( char **s, size_t n, size_t *chars_printed,
+                                unsigned int unum )
+{
+    /* Enough for a 32-bit unsigned decimal integer (4294967295). */
+    unsigned char num_buf[10];
+    int i = 0, rem;
+
+    do
+    {
+        rem = unum % 10;
+        num_buf[i++] = '0' + rem;
+    } while( unum /= 10 );
+
+    while( --i >= 0 )
+    {
+        if( *chars_printed < n )
+            *(*s)++ = num_buf[i];
+        (*chars_printed)++;
+    }
+}
+
+int tf_snprintf( char *s, size_t n, const char *fmt, ... )
+{
+    va_list args;
+    int num;
+    unsigned int unum;
+    size_t chars_printed = 0;
+
+    if( n == 1 )
+    {
+        /* Buffer is too small to actually write anything else. */
+        *s = '\0';
+        n = 0;
+    }
+    else if( n >= 2 )
+    {
+        /* Reserve space for the terminator character. */
+        n--;
+    }
+
+    va_start( args, fmt );
+    while( *fmt ) {
+
+        if( *fmt == '%' )
+        {
+            fmt++;
+            /* Check the format specifier. */
+            switch( *fmt )
+            {
+            case 'i':
+            case 'd':
+                num = va_arg( args, int );
+
+                if( num < 0 )
+                {
+                    if( chars_printed < n )
+                        *s++ = '-';
+                    chars_printed++;
+
+                    unum = (unsigned int)-num;
+                }
+                else
+                {
+                    unum = (unsigned int)num;
+                }
+
+                unsigned_dec_print( &s, n, &chars_printed, unum );
+                break;
+            case 'u':
+                unum = va_arg( args, unsigned int );
+                unsigned_dec_print( &s, n, &chars_printed, unum );
+                break;
+            default:
+                /* Return error on any other specifier. */
+                chars_printed = -1;
+                goto exit;
+            }
+            fmt++;
+            continue;
+        }
+
+        if( chars_printed < n )
+            *s++ = *fmt;
+        fmt++;
+        chars_printed++;
+    }
+
+exit:
+    va_end( args );
+
+    if( n > 0 )
+        *s = '\0';
+
+    return( chars_printed );
+}
+
+#if defined(MBEDTLS_PLATFORM_STD_SNPRINTF)
+#undef MBEDTLS_PLATFORM_STD_SNPRINTF
+#endif
+#define MBEDTLS_PLATFORM_STD_SNPRINTF tf_snprintf;
+
+#endif /* MBEDTLS_PLATFORM_TF_SNPRINTF */
+
 #if defined(MBEDTLS_PLATFORM_SNPRINTF_ALT)
 #if !defined(MBEDTLS_PLATFORM_STD_SNPRINTF)
 /*