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Do not violate the one-definition rule; violations result in undefined behavior.

Noncompliant Code Example

In this noncompliant code example, two different translation units define a class of the same name with differing definitions. Although the two definitions are functionally equivalent (they both define a class named S with a single, public, nonstatic data member int a), they are not defined using the same sequence of tokens. This code example violates the ODR and results in undefined behavior.

// a.cpp
struct S {
  int a;
};
 
// b.cpp
class S {
public:
  int a;
};

Compliant Solution

The correct mitigation depends on programmer intent. If the programmer intends for the same class definition to be visible in both translation units because of common usage, the solution is to use a header file to introduce the object into both translation units, as shown in this compliant solution:

// S.h
struct S {
  int a;
};
 
// a.cpp
#include "S.h"
 
// b.cpp
#include "S.h"

Compliant Solution

If the ODR violation was a result of accidental name collision, the best mitigation solution is to ensure that both class definitions are unique, as in this compliant solution:

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Alternatively, the classes could be given distinct names in each translation unit to avoid violating the ODR.

Noncompliant Code Example (Microsoft Visual Studio)

In this noncompliant code example, a class definition is introduced into two translation units using #include. However, one of the translation units uses an implementation-defined #pragma that is supported by Microsoft Visual Studio to specify structure field alignment requirements. Consequently, the two class definitions may have differing layouts in each translation unit, which is a violation of the ODR.

// s.h
struct S {
  char c;
  int a;
};
 
void init_s(S &s);
 
// s.cpp
#include "s.h"
 
void init_s(S &s); {
  s.c = 'a';
  s.a = 12;
}
 
// a.cpp
#pragma pack(push, 1)
#include "s.h"
#pragma pack(pop)
 
void f() {
  S s;
  init_s(s);
}

Implementation Details

It is possible for the preceding noncompliant code example to result in a.cpp allocating space for an object with a different size than expected by init_s() in s.cpp. When translating s.cpp, the layout of the structure may include padding bytes between the c and a data members. When translating a.cpp, the layout of the structure may remove those padding bytes as a result of the #pragma pack directive, and so the object passed to init_s() may be smaller than expected. Consequently, when init_s() initializes the data members of s, it may result in a buffer overrun.

For more information on the behavior of #pragma pack, see the vendor documentation for your implementation, such as Microsoft Visual Studio or GCC.

Compliant Solution

In this compliant solution, the implementation-defined structure member alignment directive is removed, ensuring that all definitions of S comply with the ODR:

...

struct S {
  TCHAR Buffer[1024];
};

Noncompliant Code Example

In this noncompliant code example, the constant object n has internal linkage but is odr-used within f(), which has external linkage. Because f() is declared as an inline function, the definition of f() must be identical in all translation units. However, each translation unit has a unique instance of n, resulting in a violation of the ODR.

const int n = 42;
 
int g(const int &lhs, const int &rhs);
 
inline int f(int k) {
  return g(k, n);
}

Compliant Solution

A compliant solution must change one of three factors: it must not odr-use n within f(), it must declare n such that it has external linkage, or it must not use an inline definition of f().

If circumstances allow modification of the signature of g() to accept parameters by value instead of by reference, then n will not be odr-used within f() because n would then qualify as a constant expression. This solution is compliant, but it is not ideal. It may not be possible (or desirable) to modify the signature of g(), such as if g() represented std::max() from <algorithm>. Also, because of the differing linkage used by n and f(), accidental violations of the ODR are still likely if the definition of f() is modified to odr-use n.

const int n = 42;
 
int g(int lhs, int rhs);
 
inline int f(int k) {
  return g(k, n);
}

Compliant Solution

In this compliant solution, the constant object n is replaced with an enumerator of the same name. Named enumerations defined at namespace scope have the same linkage as the namespace they are contained in. The global namespace has external linkage, and so the definition of the named enumeration, and its contained enumerators, also have external linkage. Although less aesthetically pleasing, this compliant solution does not suffer from the same maintenance burdens of the previous code because n and f() have the same linkage:

enum Constants {
  N = 42
};

int g(const int &lhs, const int &rhs);
 
inline int f(int k) {
  return g(k, N);
}

Risk Assessment

Violating the ODR causes undefined behavior, which can result in exploits as well as denial-of-service attacks. As shown in "Support for Whole-Program Analysis and the Verification of the One-Definition Rule in C++" [Quinlan 06], failing to enforce the ODR enables a virtual function pointer attack known as the VPTR exploit. In this exploit, an object's virtual function table is corrupted so that calling a virtual function on the object results in malicious code being executed. See the paper by Quinlan and colleagues for more details. However, note that to introduce the malicious class, the attacker must have access to the system building the code.

Automated Detection

Related Vulnerabilities

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

Bibliography

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