CTR39-CPP. Do not use pointer arithmetic on polymorphic objects

The definition of pointer arithmetic from The C++ Standard, [expr.add], paragraph 7, states [ISO/IEC 14882-2014]:

For addition or subtraction, if the expressions P or Q have type “pointer to cv T”, where T is different from the cv-unqualified array element type, the behavior is undefined. [Note: In particular, a pointer to a base class cannot be used for pointer arithmetic when the array contains objects of a derived class type. —end note]

Pointer arithmetic does not account for polymorphic object sizes and attempting to perform pointer arithmetic on a polymorphic object value results in undefined behavior.

The C++ Stardard, [expr.sub], paragraph 1, defines array subscripting as being identical to pointer arithmetic. Specifically, it states:

The expression E1[E2] is identical (by definition) to *((E1)+(E2))...

Do not use pointer arithmetic, including array subscripting, on polymorphic objects.

Noncompliant Code Example

In this noncompliant code example, f() accepts an array of S objects as its first parameter. However, main() passes an array of T objects as the first argument to f(), which results in undefined behavior due to the pointer arithmetic used within the for loop.

#include <iostream>

int GlobI;
double GlobD;

struct S {
  int I;
  
  S() : I(GlobI++) {}
};

struct T : S {
  double D;
  
  T() : S(), D(GlobD++) {}
};

void f(const S *SomeSes, std::size_t Count) { 
  for (const S *End = SomeSes + Count; SomeSes != End; ++SomeSes) {
    std::cout << SomeSes->I << std::endl;
  }
}

int main() {
  T Test[5];
  f(Test, 5);
}

This example would still be noncompliant if the for loop had instead been written to use array subscripting, like:

for (std::size_t i = 0; i < Count; ++i) {
  std::cout << SomeSes[i].I << std::endl;
}

Compliant Solution (Array)

Instead of having an array of objects, an array of pointers solves the problem of the objects being of different sizes, as in this compliant solution:

#include <iostream>

int GlobI;
double GlobD;

struct S {
  int I;
  
  S() : I(GlobI++) {}
};

struct T : S {
  double D;
  
  T() : S(), D(GlobD++) {}
};

void f(const S * const *SomeSes, std::size_t Count) { 
  for (const S * const *End = SomeSes + Count; SomeSes != End; ++SomeSes) {
    std::cout << (*SomeSes)->I << std::endl;
  }
}

int main() {
  S *Test[] = {new T, new T, new T, new T, new T};
  f(Test, 5);
  for (auto V : Test) {
    delete V;
  }
}

The elements in the arrays are no longer polymorphic objects (instead, they are pointers to polymorphic objects), and so there is no undefined behavior with the pointer arithmetic.

Compliant Solution (std::vector)

Another approach is to use an STL container instead of an array and have f() accept iterators as parameters, as in this compliant solution. However, since STL containers require homogeneous elements, pointers are still required within the container.

#include <iostream>
#include <vector>

int GlobI;
double GlobD;

struct S {
  int I;
  
  S() : I(GlobI++) {}
};

struct T : S {
  double D;
  
  T() : S(), D(GlobD++) {}
};

template <typename Iter>
void f(Iter I, Iter E) {
  for (; I != E; ++I) {
    std::cout << (*I)->I << std::endl;
  }
}

int main() {
  std::vector<S *> Test{new T, new T, new T, new T, new T};
  f(Test.cbegin(), Test.cend());
  for (auto V : Test) {
    delete V;
  }
}

Risk Assessment

Using arrays polymorphically can result in memory corruption, which could lead to an attacker being able to execute arbitrary code.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography