EXP37-C. Call functions with the arguments intended by the API

An application programming interface (API) specifies how a function is intended to be called. Calling a function with incorrect arguments can result in unexpected or unintended program behavior. Functions that are appropriately declared (see DCL07-C. Include the appropriate type information in function declarators) will typically fail compilation if they are supplied with the wrong number or types of arguments. However, there are cases where supplying the incorrect arguments to a function will at best generate compiler warnings. These warnings should be resolved (see MSC00-C. Compile cleanly at high warning levels) but do not prevent program compilation.

Noncompliant Code Example (Function Pointers)

In this example, the function pointer fp is used to refer to the function strchr(). However, fp is declared without a function prototype. As a result, there is no type checking performed on the call to fp(12,2);. A typedef is included for compatibility with DCL05-C. Use typedefs to improve code readability.

#include <stdio.h>
#include <string.h>

typedef char *(*fp) ();

int main(void) {
  char *c;
  fp = strchr;
  c = fp(12, 2);
  printf("%s\n", c);
  return 0;
}

Compliant Solution (Function Pointers)

Declaring fp with a function prototype corrects this example.

#include <string.h>

typedef char *(*fp) (const char *, int);

int main(void) {
  char *c;
  fp = strchr;
  c = fp("Hello",'H');
  printf("%s\n", c);
  return 0;
}

Noncompliant Code Example (Variadic Functions)

The POSIX function open() [[Open Group 04]] is a variadic function with the following prototype:

int open(const char *path, int oflag, ... );

The open() function accepts a third argument to determine a newly created file's access mode. If open() is used to create a new file and the third argument is omitted, the file may be created with unintended access permissions (see FIO06-C. Create files with appropriate access permissions).

In this noncompliant code example from a vulnerability in the useradd() function of the shadow-utils package CVE-2006-1174 , the third argument to open() has been accidentally omitted.

fd = open(ms, O_CREAT|O_EXCL|O_WRONLY|O_TRUNC);

Note that technically it is also incorrect to pass a third argument to open() when not creating a new file (that is, with the O_CREAT flag not set). A POSIX implementation could, if it wished, return an EINVAL error in this case. However, in practise it is unlikely to cause a problem.

Compliant Solution (Variadic Functions)

To correct this example, a third argument is specified in the call to open().

/* ... */
fd = open(ms, O_CREAT|O_EXCL|O_WRONLY|O_TRUNC, file_access_permissions);
if (fd == -1){
  /* Handle Error */
}
/* ... */

Risk Assessment

Calling a function with incorrect arguments can result in unexpected or unintended program behavior.

Automated Detection

GCC Compiler can detect violation of this rule when the -Wstrict-prototypes flag is used. However, it cannot detect violations involving variadic functions, such as the open() example above.

Compass/ROSE can detect some violations of this rule. In particular, it ensures that all calls to open() supply exactly two arguments if the second argument does not involve O_CREAT, and exactly three arguments if the second argument does involve O_CREAT.

Related Vulnerabilities

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

References

[[CVE]] CVE-2006-1174
[[ISO/IEC 9899:1999]] Forward and Section 6.9.1, "Function definitions"
[[ISO/IEC PDTR 24772]] "OTR Subprogram Signature Mismatch"
[[MISRA 04]] Rule 16.6
[[Spinellis 06]] Section 2.6.1, "Incorrect Routine or Arguments"

EXP36-C. Do not convert pointers into more strictly aligned pointer types      03. Expressions (EXP)       EXP38-C. Do not call offsetof() on bit-field members or invalid types