MSC11-C. Incorporate diagnostic tests using assertions

Incorporate diagnostic tests into your program using, for example, the assert() macro.

The assert macro expands to a void expression:

#include <assert.h>
void assert(scalar expression);

When it is executed, if expression (which must have a scalar type) is false, the assert macro outputs information about the failed assertion (including the text of the argument, the name of the source file, the source line number, and the name of the enclosing function) on the standard error stream in an implementation-defined format and calls the abort() function.

In the following example, the test for integer wrap was omitted for the unsigned multiplication based on the assumption that MAX_TABLE_SIZE * sizeof(char *) cannot exceed SIZE_MAX. While we know this is true, it cannot do any harm to codify this assumption.

assert(size <= SIZE_MAX/sizeof(char *));
table_size = size * sizeof(char *);

Assertions are primarily intended for use during debugging, and are generally turned off before code is deployed by defining the NDEBUG macro (typically as a flag passed to the compiler). Consequently, assertions should be used to protect against incorrect programmer assumptions and not for runtime error checking.

Assertions should never be used to verify the absence of runtime (as opposed to logic) errors such as:

• invalid user input (including command line arguments and environment variables)
• file errors (for example, errors opening, reading or writing files)
• network errors (including network protocol errors)
• out of memory conditions (for example, malloc() or similar failures)
• system resource exhaustion (for example, out of file descriptors, processes, threads)
• system call errors (for example, errors executing files, locking or unlocking mutexes)
• invalid permissions (for example, file, memory, user)

Code that protects against a buffer overflow, for example, cannot be implemented as an assertion because this code must be presented in the deployed executable.

In particular, assertions are generally unsuitable for server programs or embedded systems in deployment. A failed assertion can lead to a denial-of-service attack if triggered by a malicious user such as if size were in some way derived from client input. In such situations, a soft failure mode such as writing to a log file and rejecting the request is more appropriate.

if (size > SIZE_MAX/sizeof(char *)) {
  fprintf(
    log_file, 
    __FILE__ ": size %zu exceeds SIZE_MAX/sizeof(char *)\n", 
    size
  );
  size = SIZE_MAX/sizeof(char *);
}
table_size = size * sizeof(char *);

Noncompliant Code Example (malloc())

The noncompliant code example below uses the assert() macro to verify that memory allocation succeeded. Because memory availability depends on the overall state of the system and may become exhausted at any point during a process lifetime, a robust program must be prepared to gracefully handle and recover from its exhaustion. Therefore, using the assert() macro to verify that a memory allocation succeeded would be inappropriate as doing so might lead to an abrupt termination of the process and open up the possibility of a denial-of-service attack. See also MEM11-C. Do not assume infinite heap space and MEM32-C. Detect and handle memory allocation errors.

char *dupstring(const char *str) {
  size_t len;
  char *dup;

  len = strlen(str);
  dup = (char *)malloc(len + 1);
  assert(NULL != dup);

  memcpy(dup, str, len + 1);
  return dup;
}

Compliant Solution (malloc())

The compliant solution below demonstrates how to detect and handle possible memory exhaustion.

char *dupstring(const char *str) {
  size_t len;
  char *dup;

  len = strlen(str);
  dup = (char*)malloc(len + 1);
  /* detect and handle memory allocation error */
  if (NULL == dup) {
      return NULL; 
  }

  memcpy(dup, str, len + 1);
  return dup;
}

Risk Assessment

Assertions are a valuable diagnostic tool for finding and eliminating software defects that may result in vulnerabilities. The absence of assertions, however, does not mean that code is incorrect.

Related Vulnerabilities

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

Other Languages

This rule appears in the C++ Secure Coding Standard as MSC11-CPP. Incorporate diagnostic tests using assertions.

References

[[ISO/IEC 9899:1999]] Section 7.2.1, "Program diagnostics"

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