Add macro for bounds checking

This commit adds a macro for buffer bounds checks in the SSL
module. It takes the buffer's current and end position as the
first argument(s), followed by the needed space; if the
available space is too small, it returns an SSL_BUFFER_TOO_SMALL
error.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>

include/mbedtls/ssl_internal.h

@@ -299,6 +299,41 @@ static inline uint32_t mbedtls_ssl_get_input_buflen( const mbedtls_ssl_context *
 #define MBEDTLS_TLS_EXT_SUPPORTED_POINT_FORMATS_PRESENT (1 << 0)
 #define MBEDTLS_TLS_EXT_ECJPAKE_KKPP_OK                 (1 << 1)
 
+/**
+ * \brief        This function checks if the remaining size in a buffer is
+ *               greater or equal than a needed space.
+ *
+ * \param cur    Pointer to the current position in the buffer.
+ * \param end    Pointer to one past the end of the buffer.
+ * \param need   Needed space in bytes.
+ *
+ * \return       Non-zero if the needed space is available in the buffer, 0
+ *               otherwise.
+ */
+static inline int mbedtls_ssl_chk_buf_ptr( const uint8_t *cur,
+                                           const uint8_t *end, size_t need )
+{
+    return( cur <= end && need <= (size_t)( end - cur ) );
+}
+
+/**
+ * \brief        This macro checks if the remaining size in a buffer is
+ *               greater or equal than a needed space. If it is not the case,
+ *               it returns an SSL_BUFFER_TOO_SMALL error.
+ *
+ * \param cur    Pointer to the current position in the buffer.
+ * \param end    Pointer to one past the end of the buffer.
+ * \param need   Needed space in bytes.
+ *
+ */
+#define MBEDTLS_SSL_CHK_BUF_PTR( cur, end, need )                        \
+    do {                                                                 \
+        if( mbedtls_ssl_chk_buf_ptr( ( cur ), ( end ), ( need ) ) == 0 ) \
+        {                                                                \
+            return( MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL );                  \
+        }                                                                \
+    } while( 0 )
+
 #ifdef __cplusplus
 extern "C" {
 #endif