DCL11-A. Understand the type issues associated with variadic functions

The parameters of a variadic function are interpreted by the va_arg() macro. va_arg() is used to pull the next argument from an initialized argument list, the size of which is determined by the parameterized type. If type is inconsistent with what was the original parameter to the variadic function, the behavior is undefined and misinterpreted data or an alignment error (see EXP36-C. Do not cast between pointers between objects or types with differing alignments) could result.

Therefore, correct typing must always be maintained when using variadic functions.

Non-Compliant Code Example 1

For example, some C99 functions, such as printf(), are implemented as variadic functions, if care is not taken to ensure that the conversion specifiers to these do not match up with the type of the intended parameter, the result may be abnormal program termination or misinterpreted data.

The following non-compliant code swaps its null terminated byte string and integer parameters with respect to how they were specified in the format string. This means that the integer is silently casted into a pointer to a null terminated byte string and then dereferenced, possibly causing the program to abnormally terminate (error_message pointer is likewise silently converted into an integer).

char const *error_msg = "Error occurred";
/* ... */
printf("%s:%d", 15, error_msg);

Compliant Solution 1

This compliant solution is formatted so that the specifiers are consistent with their parameters.

char const *error_msg = "Error occurred";
/* ... */
printf("%d:%s", 15, error_msg);

As shown, care should be taken that the arguments passed to a format string function match up with the supplied format string.

Non-Compliant Code Example 2

In the following non-compliant code, a type long long integer is parsed by the printf() function with just a %d specifier, resulting in data truncation or misrepresentation when the value is pulled from the argument list.

long long a = 1;
char msg[128] = "Default message";
/* ... */
printf("%d %s", a, msg);

Because a long long was not interpreted, if the architecture is set up in a way that long long uses more bits for storage, the subsequent format specifier %s is unexpectedly offset, causing unknown data to be used instead of the pointer to the message.

Compliant Solution 2

This compliant solution adds in the length modifier ll to the %d format specifier so that the variadic function parser for printf() pulls the right amount of space off of the variable argument list for the long long argument.

long long a = 1;
char msg[128] = "Default message";
/* ... */
printf("%lld %s", a, msg);

Risk Assessment

Inconsistent typing in variadic functions can result in abnormal program termination or unintended information disclosure.

Related Vulnerabilities

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

References

[[ISO/IEC 9899-1999:TC2]] Section 7.15, "Variable arguments"