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

Section 7.19.9.3 of C99 defines the following behavior for fsetpos(): [[ISO/IEC 9899:1999]]

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 will result in undefined behavior and should be avoided.

Non-Compliant Code Example

The following non-compliant code 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.

enum { NO_FILE_POS_VALUES = 3 };

int opener(
  FILE* file, 
  unsigned int *width, 
  unsigned int *height, 
  unsigned int *data_offset
) {
  unsigned int file_w;
  unsigned int file_h;
  unsigned int file_o;
  int rc;
  fpos_t offset;

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

  if (file == NULL) { return EINVAL; }

  if (fscanf(file, "%u %u %u", &file_w, &file_h, &file_o)  
      != NO_FILE_POS_VALUES) { return EIO; }

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

  *width = file_w;
  *height = file_h;
  *data_offset = file_o;

  return 0;
}

However, because only the return value of a fgetpos() call is a valid argument to fsetpos(); passing an fpos_t value that was created in any other way may not work. It is possible that the position will be set to an arbitrary location in the file.

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().

enum { NO_FILE_POS_VALUES = 3 };

int opener(
  FILE* file, 
  unsigned int *width, 
  unsigned int *height, 
  unsigned int *data_offset
) {
  unsigned int file_w;
  unsigned int file_h;
  unsigned int file_o;
  int rc;
  fpos_t offset;

  if (file == NULL) { return EINVAL; }

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

  if (fscanf(file, "%u %u %u", &file_w, &file_h, &file_o)  
      != NO_FILE_POS_VALUES) { return EIO; }

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

  *width = file_w;
  *height = file_h;
  *data_offset = file_o;

  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

Fortify SCA Version 5.0 with CERT C Rule Pack can detect violations of this rule.

Related Vulnerabilities

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

References

[[ISO/IEC 9899:1999]] Section 7.19.9.3, "The fsetpos function"

FIO43-C. Do not create temporary files in shared directories      09. Input Output (FIO)       10. Environment (ENV)