Add MISRA justification for use of dynamic memory (#581)

Author: AniruddhaKanhere

MISRA.md

@@ -14,6 +14,18 @@ with ( Assuming rule 11.4 violation; with justification in point 2 ):
 ```
 grep 'MISRA Ref 11.4.2' . -rI
 ```
+#### Directive 4.12
+
+_Ref 4.12.1_
+
+- MISRA C:2012 Directive 4.12: Dynamic memory allocation shall not be used.
+        MISRA warns against the use of dynamic memory allocation as it might
+        lead to undefined behavior if not used properly. However, the
+        FreeRTOS-Plus-TCP library only uses the memory allocation primitives
+        defined by the FreeRTOS-Kernel, which are deterministic. Additionally,
+        proper care is taken in the code to not use free'd pointers and to check
+        the validity of malloc'd memory before it is dereferenced or used.
+
 #### Rule 2.2
 
 _Ref 2.2.1_

source/FreeRTOS_DNS_Callback.c

@@ -110,6 +110,10 @@
                           TickType_t uxIdentifier )
     {
         size_t lLength = strlen( pcHostName );
+
+        /* MISRA Ref 4.12.1 [Use of dynamic memory]. */
+        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#directive-412. */
+        /* coverity[misra_c_2012_directive_4_12_violation] */
         DNSCallback_t * pxCallback = ( ( DNSCallback_t * ) pvPortMalloc( sizeof( *pxCallback ) + lLength ) );
 
         /* Translate from ms to number of clock ticks. */

source/FreeRTOS_Sockets.c

@@ -404,7 +404,7 @@ Socket_t FreeRTOS_socket( BaseType_t xDomain,
     if( prvDetermineSocketSize( xDomain, xType, xProtocolCpy, &uxSocketSize ) == pdFAIL )
     {
         /* MISRA Ref 11.4.1 [Socket error and integer to pointer conversion] */
-/* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-114 */
+        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-114 */
         /* coverity[misra_c_2012_rule_11_4_violation] */
         xReturn = FREERTOS_INVALID_SOCKET;
     }
@@ -414,12 +414,16 @@ Socket_t FreeRTOS_socket( BaseType_t xDomain,
         * size depends on the type of socket: UDP sockets need less space. A
         * define 'pvPortMallocSocket' will used to allocate the necessary space.
         * By default it points to the FreeRTOS function 'pvPortMalloc()'. */
+
+        /* MISRA Ref 4.12.1 [Use of dynamic memory]. */
+        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#directive-412. */
+        /* coverity[misra_c_2012_directive_4_12_violation] */
         pxSocket = ( ( FreeRTOS_Socket_t * ) pvPortMallocSocket( uxSocketSize ) );
 
         if( pxSocket == NULL )
         {
             /* MISRA Ref 11.4.1 [Socket error and integer to pointer conversion] */
-/* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-114 */
+            /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-114 */
             /* coverity[misra_c_2012_rule_11_4_violation] */
             xReturn = FREERTOS_INVALID_SOCKET;
             iptraceFAILED_TO_CREATE_SOCKET();
@@ -433,7 +437,7 @@ Socket_t FreeRTOS_socket( BaseType_t xDomain,
                 vPortFreeSocket( pxSocket );
 
                 /* MISRA Ref 11.4.1 [Socket error and integer to pointer conversion] */
-/* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-114 */
+                /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#rule-114 */
                 /* coverity[misra_c_2012_rule_11_4_violation] */
                 xReturn = FREERTOS_INVALID_SOCKET;
                 iptraceFAILED_TO_CREATE_EVENT_GROUP();
@@ -528,6 +532,9 @@ Socket_t FreeRTOS_socket( BaseType_t xDomain,
     {
         SocketSelect_t * pxSocketSet;
 
+        /* MISRA Ref 4.12.1 [Use of dynamic memory]. */
+        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#directive-412. */
+        /* coverity[misra_c_2012_directive_4_12_violation] */
         pxSocketSet = ( ( SocketSelect_t * ) pvPortMalloc( sizeof( *pxSocketSet ) ) );
 
         if( pxSocketSet != NULL )
@@ -4235,6 +4242,9 @@ void vSocketWakeUpUser( FreeRTOS_Socket_t * pxSocket )
 
         uxSize = ( sizeof( *pxBuffer ) + uxLength ) - sizeof( pxBuffer->ucArray );
 
+        /* MISRA Ref 4.12.1 [Use of dynamic memory]. */
+        /* More details at: https://github.com/FreeRTOS/FreeRTOS-Plus-TCP/blob/main/MISRA.md#directive-412. */
+        /* coverity[misra_c_2012_directive_4_12_violation] */
         pxBuffer = ( ( StreamBuffer_t * ) pvPortMallocLarge( uxSize ) );
 
         if( pxBuffer == NULL )