EXP36-C. Do not convert pointers into more strictly aligned pointer types

Different alignments are possible for different types of objects. If the type checking system is overridden by an explicit cast, or the pointer is converted to void * and then to a different type, the alignment of an object may be changed. As a result, if a pointer to one object type is converted to a pointer to a different object type, the second object type must not require stricter alignment than the first.

Non-Compliant Code Example

C99 allows a pointer to be cast into and out of void *. As a result, it is possible to silently convert from one pointer type to another without the compiler diagnosing the problem by storing or casting a pointer to void * and then storing or casting it to the final type. In this non-compliant code example, the type checking system is circumvented due to the caveats of void pointers.

char *loop_ptr;
int *int_ptr;

int *loop_function(void *v_pointer){
  return v_pointer;
}
int_ptr = loop_function(loop_ptr);

This example compiles without warning. However, v_pointer may be aligned on a one byte boundary. Once it is cast to an int *, some architectures will require that the object is aligned on a four byte boundary. If int_ptr is later dereferenced, the program may terminate abnormally.

One solution is to ensure that loop_ptr points to an object returned by malloc(), because this object is guaranteed to be aligned properly for any need. However, this is a subtlety that is easily missed when the program is modified in the future. It is cleaner to let the type system document the alignment needs.

Compliant Solution

Because the input parameter directly influences the return value, and loopFunction() returns an int *, the formal parameter v_pointer is redeclared to only accept int *.

int *loop_ptr;
int *int_ptr;

int *loopFunction(int *v_pointer) {
  return v_pointer;
}
int_ptr = loopFunction(loop_ptr);

Risk Assessment

Accessing a pointer or an object that is no longer on the correct access boundary can cause a program to crash, give wrong information, or may cause slow pointer accesses (if the architecture allows misaligned accesses).

Related Vulnerabilities

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

References

[[Bryant 03]]
[[ISO/IEC 9899-1999]] Section 6.2.5, "Types"

EXP35-C. Do not access or modify the result of a function call after a subsequent sequence point      03. Expressions (EXP)       EXP37-C. Call functions with the arguments intended by the API