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It is possible to reference An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Referencing a volatile object by using a non-volatile value but the resulting behavior lvalue is undefined . According to C99 Section behavior. The C Standard, 6.7.3 Type qualifiers Paragraph 5:4 paragraph 7 [ISO/IEC 9899:2024], states

If an attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified type, the behavior is undefined.

This also applies to objects that behave as if they were defined with qualified types such as an object at a memory-mapped input/output address.

Non-Compliant Code Example

This non-compliant code example allows a constant value to be modified.

volatile char **cpp;
char *cp;
volatile char c = 'A';

cpp = &cp; // constraint violation
*cpp = &c; // valid
*cp = 'B'; // valid

The first assignment is unsafe because it would allow the following valid code to attempt to reference the
value of the volatile object c.

Implementation Specific Details

If cpp, cp, and c are declared as automatic (stack) variables, this example compiles without warning on Microsoft Visual C++ .NET (2003) and on MS Visual Studio 2005. In both cases, the resulting program changes the value of c. Version 3.2.2 of the gcc compiler generates a warning but compiles. The resulting program changes the value of c.

If cpp, cp, and c are declared with static storage duration this program terminates abnormally in both cases.

Compliant Solution

The compliant solution depends on the intention of the programmer. If the intention is that the value of c is modifiable, than it should not be declared as a constant. If the intention is that the value of c is not meant to change, then do not write non-compliant code that attempts to modify it.

Priority: P2 Level: L3

Integer truncation errors can lead to buffer overflows and the execution of arbitrary code by an attacker.

References

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See undefined behavior 65.

Noncompliant Code Example

In this noncompliant code example, a volatile object is accessed through a non-volatile-qualified reference, resulting in undefined behavior:

#include <stdio.h>
 
void func(void) {
  static volatile int **ipp;
  static int *ip;
  static volatile int i = 0;

  printf("i = %d.\n", i);

  ipp = &ip; /* May produce a warning diagnostic */
  ipp = (int**) &ip; /* Constraint violation; may produce a warning diagnostic */
  *ipp = &i; /* Valid */
  if (*ip != 0) { /* Valid */
    /* ... */
  }
}

The assignment ipp = &ip is not safe because it allows the valid code that follows to reference the value of the volatile object i through the non-volatile-qualified reference ip. In this example, the compiler may optimize out the entire if block because *ip != 0 must be false if the object to which ip points is not volatile.

Implementation Details

This example compiles without warning on Microsoft Visual Studio 2013 when compiled in C mode (/TC) but causes errors when compiled in C++ mode (/TP).

GCC 4.8.1 generates a warning but compiles successfully.

Compliant Solution

In this compliant solution, ip is declared volatile:

#include <stdio.h>

void func(void) {
  static volatile int **ipp;
  static volatile int *ip;
  static volatile int i = 0;

  printf("i = %d.\n", i);

  ipp = &ip;
  *ipp = &i;
  if (*ip != 0) {
    /* ... */
  }

}

Risk Assessment

Accessing an object with a volatile-qualified type through a reference with a non-volatile-qualified type is undefined behavior.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

Bibliography

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