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

The variable parameters of a variadic function, this is, those that correspond with the position of the ellipsis, are interpreted by the va_arg() macro. The va_arg() macro is used to extract the next argument from an initialized argument list within the body of a variadic function implementation. The size of each parameter is determined by the specified type. If the type is inconsistent with the corresponding argument, the behavior is undefined and may result in misinterpreted data or an alignment error [[EXP36-C. Do not convert between pointers to objects with differing alignments]].

The variable arguments to a variadic function are not checked for type by the compiler.  Therefore, the programmer is responsible for ensuring that they are compatible with the corresponding parameter after the default argument promotions:

• integer arguments of types ranked lower than int are promoted to int, if int can hold all the values of that type, otherwise they are promoted to unsigned int (the "integer promotions");
• arguments of type float are promoted to double.

Non-Compliant Code Example (type interpretation error)

The C99 printf() function is implemented as a variadic function. This non-compliant code example swaps its null-terminated byte string and integer parameters with respect to how they were specified in the format string. Consequently, the integer is interpreted as a pointer to a null-terminated byte string and dereferenced. This will likely cause the program to abnormally terminate. Note that the error_message pointer is likewise interpreted as an integer.

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

Compliant Solution (type interpretation error)

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

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

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

Non-Compliant Code Example (type alignment error)

In this non-compliant code example, a type long long integer is incorrectly parsed by the printf() function with a %d specifier. This code may result in data truncation or misrepresentation when the value is extracted 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 long long uses more bytes 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 (type alignment error)

This compliant solution adds the length modifier ll to the %d format specifier so that the variadic function parser for printf() extracts the correct number of bytes from 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]] Section 6.5.2.2, "Function calls"; 7.15, "Variable arguments"

DCL10-A. Maintain the contract between the writer and caller of variadic functions      02. Declarations and Initialization (DCL)       DCL12-A. Create and use abstract data types