Page History

Modifying a variable through a pointer of an incompatible type (other than unsigned char) can lead to unpredictable results. Subclause 6.2.7 of the C standard mentions Standard states that two types may be distinct yet compatible , and addresses precisely  when two distinct types are compatible.

This problem is often caused by a violation of aliasing rules. The C Standard, subclause 6.5, paragraph 7 [ ISO/IEC 9899:20112024 ], specifies those circumstances in which an object may or may not be aliased.

An object shall have its stored value accessed only by an lvalue expression that has one of the following types:

• a type compatible with the effective type of the object,
• a qualified version of a type compatible with the effective type of the object, 
• a type that is the signed or unsigned type corresponding to compatible with the underlying type of the effective type of the object, a type that is
• the signed or unsigned type corresponding to compatible with a qualified version of the underlying type of the effective type of the object, 
• an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), oror 
• a character type. 

Accessing an object by means of any other lvalue expression (other than unsigned char) results in is undefined behavior 3437.

Noncompliant Code Example

In this noncompliant example, an object of type float is incremented through an int *. The programmer can use the unit in the last place can be used to get the next representable value for a floating-point type.  However, accessing an object through a pointer of an incompatible types type is undefined behavior.

#include <stdio.h>
 
void f(void) {
  if (sizeof(int) == sizeof(float)) {
    float f = 0.0f;
    int *ip = (int *)&f;
    (*ip)++;
    printf("float is %f\n", f);
  }
}

Compliant Solution

In this compliant solution, the standard C function nextafterf() is used to round towards toward the highest representable floating-point value:

...

Recent versions of GCC turn on the option -fstrict-aliasing (, which allows alias-based optimizations) , by default with -O2. Some architectures then print "1111 1111" as a result. Without optimization, the executable generates the expected output "2222 2222."

To disable optimizations based on alias analysis for faulty legacy code, the option -fno-strict-aliasing can be used as a workaround. The option -Wstrict-aliasing (, which is included in -Wall) , warns about some, but not all, violations of aliasing rules when -fstrict-aliasing is active.

When GCC 3.4.6 compiles this code with optimization, the assignment through the aliased pointer is effectively eliminated.

Compliant Solution

This compliant solution uses a union type that includes a type compatible with the effective type of the object:

#include <stdio.h>
 
void func(void) {
  union {
    short a[2];
    int i;
  } u;

  u.a[0]=0x1111;
  u.a[1]=0x1111;
  u.i = 0x22222222;

  printf("%x %x\n", u.a[0], u.a[1]);

  /* ... */
}

The C standard states:

If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called “type punning”). This might be a trap representation

The call to printf() typically outputs "2222 2222". However, there is no guarantee that this will be true; the object representations of a and i are unspecified and need not be compatible in this way, despite this operation being commonly accepted as an implementation extension. (See unspecified behavior 11.)This code example now reliably outputs "2222 2222."

Noncompliant Code Example

...

#include <stdlib.h>
 
struct gadget {
  int i;
  double d;
  char *p;
};
 
struct widget {
  char *q;
  int j;
  double e;
};
 
void func(void) {
  struct gadget *gp;
  struct widget *wp;
 
  gp = (struct gadget *)malloc(sizeof (struct gadget));
  if (!gp) {
    /* Handle error */
  }
  /* ... Initialize gadget ... */
  wp = (struct widget *)realloc(gp, sizeof(struct widget));
  if (!wp) {
    free(gp);
    /* Handle error */
  }
  if (wp->j == 12) {
    /* ... */
  }
  /* ... */
  free(wp);
}

Compliant Solution

This compliant solution reuses the memory from the gadget object but reinitializes the memory to a consistent state before reading from it:

#include <stdlib.h>
#include <string.h>
 
struct gadget {
  int i;
  double d;
  char *p;
};
 
struct widget {
  char *q;
  int j;
  double e;
};
 
void func(void) {
  struct gadget *gp;
  struct widget *wp;
 
  gp = (struct gadget *)malloc(sizeof (struct gadget));
  if (!gp) {
    /* Handle error */
  }
  /* ... */
  wp = (struct widget *)realloc(gp, sizeof(struct widget));
  if (!wp) {
    free(gp);
    /* Handle error */
  }
  memset(wp, 0, sizeof(struct widget));
  /* ... Initialize widget ... */

  if (wp->j == 12) {
    /* ... */
  }
  /* ... */
  free(wp);
}

Noncompliant Code Example

According to subclause the C Standard, 6.7.6.2 of the C Standard 7.3 [ISO/IEC 9899:20112024], using two or more incompatible arrays in an expression results in is undefined behavior. (See also undefined behavior 76 of Appendix J.)

For two array types to be compatible, both should have compatible underlying element types, and both size specifiers should have the same constant value. If either of these properties is violated, the resulting behavior is undefined.

...

Optimizing for performance can lead to aliasing errors that can be quite difficult to detect. Furthermore, as in the preceding example, unexpected results can lead to buffer overflow attacks and/or , bypassing security checks and/, or unexpected execution.

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)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-119 and EXP39-C

Independent( ARR30-C, ARR38-C, ARR32-C, INT30-C, INT31-C, EXP39-C, EXP33-C, FIO37-C) STR31-C = Subset( Union( ARR30-C, ARR38-C)) STR32-C = Subset( ARR38-C)

Intersection( EXP39-C, CWE-119) =

• Reading memory assigned to one type, but being accessed through a pointer to a larger type.

EXP39-C – CWE-119 =

• Writing to memory assigned to one type, but accessed through a pointer to a larger type

• Reading memory assigned to one type, but being accessed through a pointer to a smaller (or equal-sized) type

CWE-119 – EXP39-C =

• Reading beyond a buffer using a means other than accessing a variable through an incompatible pointer.

CWE-123 and EXP39-C

Intersection( CWE-123, EXP39-C) = Ø

EXP39-C allows overflowing a (small) buffer, but not arbitrary memory writes. (Possibly an arbitrary-memory write exploit could be devised using a “perfect storm” of incompatible types, but this would be uncommon in practice.)

CWE-125 and EXP39-C

Independent( ARR30-C, ARR38-C, EXP39-C, INT30-C) STR31-C = Subset( Union( ARR30-C, ARR38-C)) STR32-C = Subset( ARR38-C)

Intersection( EXP39-C, CWE-125) =

• Reading memory assigned to one type, but being accessed through a pointer to a larger type.

ESP39-C – CWE-125 =

• Reading memory assigned to one type, but being accessed through a pointer to a smaller (or equal-sized) type

CWE-125 – EXP39-C =

• Reading beyond a buffer using a means other than accessing a variable through an incompatible pointer.

CWE-188 and EXP39-C

Intersection( CWE-188, EXP39-C) = Ø

CWE-188 appears to be about making assumptions about the layout of memory between distinct variables (that are not part of a larger struct or array). Such assumptions typically involve pointer arithmetic (which violates ARR30-C). EXP39-C involves only one object in memory being (incorrectly) interpreted as if it were another object. EG a float being treated as an int (usually via pointers and typecasting)

CWE-704 and EXP39-C

CWE-704 = Union( EXP39-C, list) where list =

• Incorrect (?) typecast that is not incompatible

Bibliography

...

...

Image Modified Image Modified Image Modified