NPD.GEN.CALL.MUST

Possible assigned null pointer may be dereferenced through a function call

An attempt to access data using a null pointer causes a runtime error. When a program dereferences a pointer that is expected to be valid but turns out to be null, a null pointer dereference occurs. Null-pointer dereference defects often occur due to ineffective error handling or race conditions, and typically cause abnormal program termination. Before a pointer is dereferenced in C/C++ code, it must be checked to confirm that it is not equal to null.

The NPD checkers look for instances in which a null or possibly null pointer is dereferenced.

The NPD.GEN.CALL.MUST checker flags situations in which a null pointer that's been assigned a constant null value locally is subsequently passed to a function that dereferences it without checking it for null.

Vulnerability and risk

Null-pointer dereferences usually result in the failure of the process. These issues typically occur due to ineffective exception handling.

Mitigation and prevention

To avoid this vulnerability:

  • Check for a null value in the results of all functions that return values
  • Make sure all external inputs are validated
  • Explicitly initialize variables
  • Make sure that unusual exceptions are handled correctly

Vulnerable code example

Copy
  void reassign(int *argument, int *p) {
    if (goodEnough(argument)) return;
    *argument = *p;
  }
  
  void npd_gen_call_must(int *argument) {
    int *p = NULL;
    reassign(argument, p);
  }

Klocwork reports a defect in this example because *p, which is assigned a null constant value, is passed to function reassign, in which it's dereferenced. This type of vulnerability can produce unexpected and unintended results.

Fixed code example

Copy
  void reassign(int *argument, int *p) {
    if (goodEnough(argument)) return;
    *argument = *p;
  }
  
  void npd_gen_call_must(int *argument) {
     int *p = NULL;
     if (p != 0) reassign(argument, p);
  }

In the fixed code, *p is checked for null in line 8 before the dereference.

Security training

Application security training materials provided by Secure Code Warrior.

Extension

This checker can be extended through the Klocwork knowledge base. See Tuning C/C++ analysis for more information.