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

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++ Standard, [expr.sub], paragraph 1 [ISO/IEC 14882-2014], defines array subscripting as being identical to pointer arithmetic. Specifically, it states the following:

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

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

The following code examples assume the following static variables and class definitions.

int globI;
double globD;

struct S {
  int i;
  
  S() : i(globI++) {

Because pointer arithmetic does not take account of polymorphism, a major problem with arrays is that they do not interact well with polymorphism (see Stroustrup 06, Meyers 06), as the following example illustrates:

Non-Compliant Code Example

class Base {
public:
	virtual void func(void) {
		cout << "Base" << endl;
	}
};

classstruct DerivedT : public BaseS {
public:
	int i;
	Derived  double d;
  
  T() { i = 0; }

	void func(void) {
		cout << "Derived " << ++i << endl;
	}
};

void walk(class Base *bar, int count) {
	for (int i = 0; i < count; i++) {
		bar[i].func();
	: S(), d(globD++) {}
};

Noncompliant Code Example (Pointer Arithmetic)

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>
 
// ... definitions for S, T, globI, 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(void) {
	Base dis[3  T test[5];
	Derived dat[3];

	walk(dis, 3);
	walk(dat, 3); // crashes
}

In the last call to {{walk()}}, {{dat\[\]}} is treated as a {{Base\[\]}} and the subscripting no longer works correctly when {{sizeof(Derived) \!= sizeof(Base)}}.  This is because {{walk()}} incorrectly believes that the size of each element in {{bar\[\]}} is {{sizeof(Base)}}.  To locate the second element in the array (located at {{bar\[1\]}}), {{walk()}} adds the {{sizeof(Base)}} to the address {{bar}}.  Assuming the derived object is larger (which is often the case), the resulting pointer refers to a point within the first element and not to the start of the second element located at {{bar + sizeof(Derived)}}.

 f(test, 5);
}

Noncompliant Code Example (Array Subscripting)

In this noncompliant code example, the for loop uses array subscripting. Since array subscripts are computed using pointer arithmetic, this code also results in undefined behavior.

#include <iostream>
 
// ... definitions for S, T, globI, globD ...

void f(const S *someSes, std::size_t count) { 
  for (std::size_t i = 0; i < count; ++i) {
    std::cout << someSes[i].i << std::endl;
  }
}

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

Compliant Solution (Array)

...

Instead of having an array of objects, an array of pointers solves the problem of the objects being of different sizes. With the Base and Derived classes as above, we can define the walk and main methods as followsas in this compliant solution.

Code Block
bgColor #ccccff lang cpp
#include <iostream> // ... definitions for S, T, globI, globD ... void walkf(classconst BaseS *bar [], int const *someSes, std::size_t count) { for (int i = 0; i < count; i++) { (bar[i])->func(); const S * const *end = someSes + count; someSes != end; ++someSes) { std::cout << (*someSes)->i << std::endl; } } int main(void) { Base* dis[3 S *test[] = {new BaseT, new BaseT, new Base}; Base* dat[3] = {new DerivedT, new DerivedT, new DerivedT}; walk(dis f(test, 35); walk(dat, 3); for (auto v : test) { delete v; } }

The elements in the arrays are now all the same size (because no longer polymorphic objects (instead, they are pointers to Base or Derived polymorphic objects), and so there is no problem no undefined behavior with the array indexingpointer arithmetic.

Compliant Solution

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A better approach would be to use vectors and iterators, instead of arrays, as follows. (Note, however, that we have to have vectors of pointers because containers must be homogeneous.)

void walk(vector<Base*>bar) {
	for_each (bar.begin(), bar.end(), mem_fun(&Base::func));
}

int main(void) {
	vector<Base*> dis(3);
        for (int i=0; i<3; i++) dis[i] = new Base;

	vector<Base*> dat(3);
        for (int i=0; i<3; i++) dat[i] = new Derived;

	walk(dis);
	walk(dat);
}

(std::vector)

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

#include <iostream>
#include <vector>

// ... definitions for S, T, globI, 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

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

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
CTR56-CPP	High	Likely	High	P9	L2

Automated Detection

References

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• Meyers 96 Item 3: Never treat arrays polymorphically.

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Related Vulnerabilities

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

Related Guidelines

Bibliography

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