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Avoid in-band error indicators while designing interfaces. This practice is commonly used by C library functions but is not recommended. One example from the C standard Standard of a troublesome in-band error indicator is EOF (see FIO34-C. Use int to capture the return value of character IO functions and FIO35-C. Use feof() and ferror() to detect end-of-file and file errors when sizeof(int) == sizeof(char)).

Non-Compliant Code Example

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Compliant Solution

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Exception

. Distinguish between characters read from a file and EOF or WEOF). Another problematic use of in-band error indicators from the C Standard involving the size_t and time_t types is described by 

• INT31-C. Ensure that integer conversions do not result in lost or misinterpreted data
• FLP07-C. Cast the return value of a function that returns a floating-point type
• INT18-C. Evaluate integer expressions in a larger size before comparing or assigning to that size

Noncompliant Code Example (sprintf())

This noncompliant code example is from the Linux Kernel Mailing List archive site, although similar examples are common:

int i;
ssize_t count = 0;

for (i = 0; i < 9; ++i) {
  count += sprintf(
    buf + count, "%02x ", ((u8 *)&slreg_num)[i]
  );
}
count += sprintf(buf + count, "\n");

The sprintf() function returns the number of characters written in the array, not counting the terminating null character. This number is frequently added to an existing counter to keep track of the location of the index into the array. However, the call to sprintf() can (and will) return −1 on error conditions, such as an encoding error. If this error happens on the first call (which is likely), the count variable, already at 0, is decremented. If this index is subsequently used, it will result in an out-of-bounds read or write.

Compliant Solution (sprintf_m())

This compliant solution shows the redesigned API for sprintf() from the CERT managed string library [Burch 2006]:

errno_t sprintf_m(
  string_m buf, 
  const string_m fmt, 
  int *count, 
  ...
);

The sprintf_m() API separates the return status of the function from information about the number of characters written. In this case, *count is set to the number of characters written in buf, and the return value indicates the return status. Returning the status as the return value of the function increases the likelihood that a programmer will check the return status of the function.

The preceding code example can be amended as follows:

int i;
rsize_t count = 0;
errno_t err;

for (i = 0; i < 9; ++i) {
  err = sprintf_m(
    buf + count, "%02x ", &count, ((u8 *)&slreg_num)[i]
  );
  if (err != 0) {
    /* Handle print error */
  }
}
err = sprintf_m(
  buf + count, "%02x ", &count, ((u8 *)&slreg_num)[i]
);
if (err != 0) {
  /* Handle print error */
}

Noncompliant Code Example (POSIX ssize_t)

The ssize_t data type is designed as a "signed representation of size_t." Consequently, it is often used as a return type for functions that can return an unsigned value upon success and a negative value upon error. For instance, the POSIX read() function has the following signature:

ssize_t read(int fildes, void *buf, size_t nbyte);

read() returns −1 if an error occurs; if no errors occur, it returns the number of bytes actually read.

As with all in-band error indicators, this type is not recommended because developers are tempted to ignore the possibility that a ssize_t value is negative.

Compliant Solution (POSIX size_t)

An alternative hypothetical signature for the read() function is

errno_t read(int fildes, void *buf, size_t nbyte, size_t* rbytes);

where rbytes is a pointer to a size_t. If no error occurs, and rbytes is not NULL, its value is set to the total number of bytes read, and read() returns 0. If an error occurs, read() returns a nonzero value indicating the error.

Exceptions

ERR02-EX1: Null pointers are another NULL is an example of an in-band error indicator, which is not so bad because the language supports it.

Risk Analysis

. Use of null pointers is allowed because it is supported by the language. According to the C Standard, subclause 6.3.2.3 [ISO/IEC 9899:2011]:

If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

ERR02-EX2: You may design a function that returns in-band error indicators if it is possible to securely guarantee that the program will not try to continue processing should an error occur in the function. For example, after calling the glibc function feenableexcept(FE_OVERFLOW), you can call the exp function without needing to check for overflow on the result yourself, because if it occurs, then a SIGFPE will cause the program to terminate in a similar manner as abort().

Risk Assessment

The risk in using in-band error indicators is difficult to quantify and is consequently given as low. However, if the use of in-band error indicators results in programmers' failing to check status codes or incorrectly checking them, the consequences can be more severeFailure to adopt and implement a consistent and comprehensive error-handling policy is detrimental to system survivability, and can result in a broad range of vulnerabilities depending on the operational characteristics of the system.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography

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