CERT C: Rule POS54-C

Detect and handle POSIX library errors

Description

Rule Definition

Detect and handle POSIX library errors.¹

Polyspace Implementation

The rule checker checks for Returned value of a sensitive POSIX function not checked.

Examples

Returned value of a sensitive POSIX function not checked

Issue

Returned value of a sensitive POSIX function not checked occurs when you call sensitive POSIX functions, but you:

• Ignore the return value.

• Use an output or a return value without testing the validity of the return value.

For this defect, two type of functions are considered: sensitive and critical sensitive.

A sensitive function is a standard function that can encounter:

• Exhausted system resources (for example, when allocating resources)

• Changed privileges or permissions

• Tainted sources when reading, writing, or converting data from external sources

• Unsupported features despite an existing API

A critical sensitive function is a sensitive function that performs one of these critical or vulnerable tasks:

• Set privileges (for example, setuid)

• Create a jail (for example, chroot)

• Create a process (for example, fork)

• Create a thread (for example, pthread_create)

• Lock or unlock mutex (for example, pthread_mutex_lock)

• Lock or unlock memory segments (for example, mlock)

Risk

If you do not check the return value of functions that perform sensitive or critical sensitive tasks, your program can behave unexpectedly. Errors from these functions can propagate throughout the program causing incorrect output, security vulnerabilities, and possibly system failures.

Fix

Before continuing with the program, test the return value of critical sensitive functions.

For sensitive functions, you can explicitly ignore a return value by casting the function to void. Polyspace^® does not raise this defect for sensitive functions cast to void. This resolution is not accepted for critical sensitive functions because they perform more vulnerable tasks.

Example - Sensitive Function Return Ignored

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

void initialize() {
    pthread_attr_t attr;

    pthread_attr_init(&attr);//Noncompliant 
}
int read_file(int argc, char *argv[])
{
  FILE *in;
  if (argc != 2) {
    /* Handle error */
  }

  in = fmemopen (argv[1], strlen (argv[1]), "r");  
  return 0;  //Noncompliant

}

This example shows calls to the sensitive POSIX functions pthread_attr_init and fmemopen. Their return values are ignored, causing defect.

Correction — Cast Function to (void)

One possible correction is to cast the function to void. This fix informs Polyspace and any reviewers that you are explicitly ignoring the return value of the sensitive function.

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

void initialize() {
    pthread_attr_t attr;

    (void)pthread_attr_init(&attr);//Compliant 
}
int read_file(int argc, char *argv[])
{
  FILE *in;
  if (argc != 2) {
    /* Handle error */
  }

  (void)fmemopen (argv[1], strlen (argv[1]), "r"); //Compliant
  
  return 0; 
}

Correction — Test Return Value

One possible correction is to test the return value of pthread_attr_init and fmemopen to check for errors.

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

void initialize() {
    pthread_attr_t attr;

    int result = pthread_attr_init(&attr);//Compliant 
	if(result != 0){
		//Handle fatal error
	} 
}
int read_file(int argc, char *argv[])
{
  FILE *in;
  if (argc != 2) {
    /* Handle error */
  }

  in = fmemopen (argv[1], strlen (argv[1]), "r"); //Compliant
  if (in==NULL){
	  // Handle error
  }
  return 0; 
}

Example - Critical Function Return Ignored

#include <pthread.h>
extern void *start_routine(void *);

void returnnotchecked() {
    pthread_t thread_id;
    pthread_attr_t attr;
    void *res;

    (void)pthread_attr_init(&attr);
    (void)pthread_create(&thread_id, &attr, &start_routine, ((void *)0)); //Noncompliant
    pthread_join(thread_id,  &res);  //Noncompliant
}

In this example, two critical functions are called: pthread_create and pthread_join. The return value of the pthread_create is ignored by casting to void, but because pthread_create is a critical function (not just a sensitive function), Polyspace does not ignore this Return value of a sensitive function not checked defect. The other critical function, pthread_join, returns value that is ignored implicitly. pthread_join uses the return value of pthread_create, which was not checked.

Correction — Test the Return Value of Critical Functions

The correction for this defect is to check the return value of these critical functions to verify the function performed as expected.

#include <pthread.h>
#include <stdlib.h>
#define fatal_error() abort()

extern void *start_routine(void *);

void returnnotchecked() {
    pthread_t thread_id;
    pthread_attr_t attr;
    void *res;
    int result;

    (void)pthread_attr_init(&attr);
    result = pthread_create(&thread_id, &attr, &start_routine, NULL);
    if (result != 0) {
        /* Handle error */
        fatal_error();
    }

    result = pthread_join(thread_id,  &res);
    if (result != 0) {
        /* Handle error */
        fatal_error();
    }
}

Check Information

Group: Rule 50. POSIX (POS)

Version History

Introduced in R2019a

R2021b: Checker flags unvalidated pointers from sensitive functions

The checker is now raised on sensitive functions that return pointers if the return value is not checked. For instance:

FILE *in;
in = fmemopen (argv[1], strlen (argv[1]), "r")

Polyspace now raises a defect if in is not tested before it is used for accessing a file.