Checkers:NPD.GEN.CALL.MIGHT

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Reference > C/C++ checkers > NPD.GEN.CALL.MIGHT

Possible assigned null pointer may be dereferenced through a conditional function call

Null is a special value in C/C++ that is used to indicate that a pointer doesn't point to any object. 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.MIGHT checker flags situations in which a pointer that's possibly been assigned a constant null value locally might subsequently be 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

Code examples

Vulnerable code example

1  void reassign(int *argument, int *p) {
2    if (goodEnough(argument)) return;
3    *argument = *p;
4  }
5  
6  void npd_gen_call_might(int *argument) {
7    int *p = NULL;
8    if (someCondition()) {
9      p = f();
10   }
11   reassign(argument, p);
12 }

Klocwork reports a defect in this example because *p, which may be equal to null depending on the result of function npd_gen_call_might, may be passed to function reassign, in which it's dereferenced. This type of vulnerability can produce unexpected and unintended results.

Fixed code example

1  void reassign(int *argument, int *p) {
2    if (goodEnough(argument)) return;
3    *argument = *p;
4  }
5  
6  void npd_gen_call_might(int *argument) {
7    int *p = NULL;
8    if (someCondition()) {
9      p = f();
10   }
11   if (p != 0) reassign(argument, p);
12 }

In the fixed version, *p is checked for null in line 11 before the dereference.

Related checkers

External guidance

Extension

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

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