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• 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.

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

GCC Compiler Version 3.4.4 will warn about inconsistent types for the printf() family of functions when compiled with -Wall (see MSC00-A. Compile cleanly at high warning levels).

Compliant Solution (Type Interpretation Error)

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

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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.

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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 const msg[] = "Default message";
/* ... */
printf("%lld %s", a, msg);

Automated Detection

GCC Compiler Version 3.4.4 warns about inconsistently typed arguments to formatted output functions when compiled with -Wall (see MSC00-A. Compile cleanly at high warning levels).

The tool Compass/ROSE does not currently detect violations of this rule. While the rule in general can not cannot be automated, due to the difficulty in enforcing contracts between a variadic function and its invokers, it would be fairly easy to enforce type correctness on arguments to the printf() family of functions.

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

\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.5.2.2, "Function calls," and Section 7.15, "Variable arguments"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "IHN Type system" and "OTR Subprogram Signature Mismatch"
\[[MISRA 04|AA. C References#MISRA 04]\] Rule 16.1

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