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This problem is often caused by a violation of aliasing rules. The C Standard, 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) is undefined behavior 37.

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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.

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#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() printf() behavior in this compliant solution is unspecified, but it is commonly accepted as an implementation extension (see unspecified behavior 11).This function typically outputs "2222 2222". " However, there is no guarantee that this will be true, even on implementations that defined the unspecified behavior; values of type short need not have the same representation as values of type int.; 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.)

Noncompliant Code Example

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#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 the C Standard, 6.7.67.2 3 [ISO/IEC 9899:20112024], using two or more incompatible arrays in an expression is undefined behavior. (see See also undefined behavior 76.).

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.

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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, bypassing security checks, 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

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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

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