FIO44-C. Only use values for fsetpos() that are returned from fgetpos()

Subclause 7.21.9.3 of the C Standard [ISO/IEC 9899:2011] defines the following behavior for fsetpos():

The fsetpos function sets the mbstate_t object (if any) and file position indicator for the stream pointed to by stream according to the value of the object pointed to by pos, which shall be a value obtained from an earlier successful call to the fgetpos function on a stream associated with the same file.

Consequently, using any other values for pos results in undefined behavior.

Noncompliant Code Example

This noncompliant code example attempts to read three values from a file and then set the cursor position back to the beginning of the file and return to the caller:

#include <stdio.h>
#include <string.h>
 
enum { NO_FILE_POS_VALUES = 3 };

int opener(FILE *file) {
  int rc;
  fpos_t offset;

  memset(&offset, 0, sizeof(offset));

  if (file == NULL) { return -1; }

  /* Read in data from file */

  rc = fsetpos(file, &offset);
  if (rc != 0 ) { return rc; }

  return 0;
}

However, because only the return value of an fgetpos() call is a valid argument to fsetpos(), passing an fpos_t value that was created in any other way is undefined behavior.

Compliant Solution

In this compliant solution, the initial file position indicator is stored by first calling fgetpos(), which is used to restore the state to the beginning of the file in the later call to fsetpos():

#include <stdio.h>
#include <string.h>
 
enum { NO_FILE_POS_VALUES = 3 };

int opener(FILE *file) {
  int rc;
  fpos_t offset;

  if (file == NULL) { return -1; }

  rc = fgetpos(file, &offset);
  if (rc != 0 ) { return rc; }

  /* Read in data from file */

  rc = fsetpos(file, &offset);
  if (rc != 0 ) { return rc; }

  return 0;
}

Risk Assessment

The misuse of fsetpos() can move a file stream read to a unintended location in the file. If this location held input from the user, the user would then gain control of the variables being read from the file.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Bibliography