MSC39-C. Do not call va_arg() on a va_list that has indeterminate value

Variadic functions access their variable arguments by using va_start() to initialize an object of type va_list, iteratively invoking the va_arg() macro, and finally calling va_end(). The va_list may be passed as an argument to another function, but calling va_arg() within that function causes the va_list to have an indeterminate value in the calling function. As a result, attempting to read variable arguments without reinitializing the va_list can have unexpected behavior. According to the C Standard, subclause 7.16, paragraph 3 [ISO/IEC 9899:2011],

If access to the varying arguments is desired, the called function shall declare an object (generally referred to as ap in this subclause) having type va_list. The object ap may be passed as an argument to another function; if that function invokes the va_arg macro with parameter ap, the value of ap in the calling function is indeterminate and shall be passed to the va_end macro prior to any further reference to ap.²⁵³
253) It is permitted to create a pointer to a va_list and pass that pointer to another function, in which case the original function may take further use of the original list after the other function returns.

Noncompliant Code Example

The following noncompliant code tries to check that none of its variable arguments are zero by passing a va_list to helper function contains_zero(). After the call to contains_zero(), the value of ap is indeterminate, which could cause undesired behavior.

#include <stdarg.h>
#include <stdio.h>
 
int contains_zero(int count, va_list ap) {
  for(int i = 1; i < count; ++i) {
    if(va_arg(ap, double) == 0.0) {
      return 1;
    }
  }
  return 0;
}

int print_reciprocals(int count, ...) {
  va_list ap;  
  va_start(ap, count);

  if(contains_zero(count, ap)) {
    va_end(ap);
    return 1;
  }

  for(int i = 0; i < count; ++i) {
    printf("%f ", 1.0 / va_arg(ap, double));
  }

  va_end(ap);
  return 0;
}

Compliant Solution

The compliant solution modifies print_reciprocals to create a copy of the va_list by using the va_copy macro. The copy is passed to contains_zero, and the variable arguments are accessed through the original.

#include <stdarg.h>
#include <stdio.h>
 
int contains_zero(int count, va_list ap) {
  for(int i = 1; i < count; ++i) {
    if(va_arg(ap, double) == 0.0) {
      return 1;
    }
  }
  return 0;
}
 
int print_reciprocals(int count, ...) {
  int status;
  va_list ap1;
  va_list ap2;  

  va_start(ap1, count);
  va_copy(ap2, ap1);

  if (contains_zero(count, ap2)) {
    status = 1;
  } else {
    for (int i = 0; i < count; i++) {
      printf("%f ", 1.0 / va_arg(ap1, double));
    }
    status = 0;
  }

  va_end(ap2);
  va_end(ap1);

  return status;
}

Risk Assessment

Reading variable arguments using a va_list that has an indeterminate value could lead to unexpected results.

Related Vulnerabilities

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

Bibliography