home | O'Reilly's CD bookshelfs | FreeBSD | Linux | Cisco | Cisco Exam    

8.17. Testing a File for Trustworthiness


You want to read from a file, perhaps because it has configuration information. You only want to use the file if it can't be written to (or perhaps not even be read from) by anyone else than its owner.


Use the stat call to retrieve ownership and file permissions information. You can use the built-in version, which returns a list:

( $dev, $ino, $mode, $nlink, 
  $uid, $gid, $rdev, $size, 
  $atime, $mtime, $ctime, 
  $blksize, $blocks )       = stat($filename)
        or die "no $filename: $!";

$mode &= 07777;             # discard file type info

Or you can use the by-name interface in:

$info = stat($filename)     or die "no $filename: $!";
if ($info->uid == 0) {
    print "Superuser owns $filename\n";
if ($info->atime > $info->mtime) {
    print "$filename has been read since it was written.\n";


Usually you trust users to set file permissions as they wish. If they want others to read their files, or even to write to them, that's their business. Applications like editors, mailers, and shells are often more discerning, though, refusing to evaluate code in configuration files if anyone but the owner can write to them. This helps avoid Trojan horses attacks. Security-minded programs like ftp and rlogin may even reject config files that can be read by anyone but their owner.

If the file is writable by someone other than the owner or is owned by someone other than the current user or the superuser, it shouldn't be trusted. To figure out file ownership and permissions, the stat function is used. The following function returns true if the file is deemed safe and false otherwise. If the stat fails, undef is returned.

use File::stat;

sub is_safe {
    my $path = shift;
    my $info = stat($path);
    return unless $info;

    # owner neither superuser nor me 
    # the real uid is in stored in the $< variable
    if (($info->uid != 0) && ($info->uid != $<)) {
        return 0;

    # check whether group or other can write file.
    # use 066 to detect either reading or writing
    if ($info->mode & 022) {   # someone else can write this
        return 0 unless -d _;  # non-directories aren't safe
            # but directories with the sticky bit (01000) are
        return 0 unless $info->mode & 01000;        
    return 1;

A directory is considered safe even if others can write to it, provided that its mode 01000 (owner delete only) bit is set.

Careful programmers also ensure that no enclosing directory is writable. This is due to systems with the " chown giveaway" problem in which any user can give away a file they own and make it owned by someone else. The following function handles that by using the is_safe function to check every enclosing directory up to the root if it detects that you have the chown problem, for which it queries the POSIX::sysconf . If you don't have an unrestricted version of chown , the is_verysafe subroutine just calls is_safe . If you do have the problem, it walks up the filesystem tree until it reaches the root.

use Cwd;
sub is_verysafe {
    my $path = shift;
    return is_safe($path) if sysconf(_PC_CHOWN_RESTRICTED);
    $path = getcwd() . '/' . $path if $path !~ m{^/};
    do {
        return unless is_safe($path);
        $path =~ s#([^/]+|/)$##;               # dirname
        $path =~ s#/$## if length($path) > 1;  # last slash
    } while length $path;

    return 1;

To use this in a program, try something like this:

$file = "$ENV{HOME}/.myprogrc";
readconfig($file) if is_safe($file);

This has potential for a race condition, because it's presumed that the hypothetical readconfig function will open the file. Between the time when is_safe checks the file's stats and when readconfig opens it, something wicked could theoretically occur. To avoid this, pass is_safe the already open filehandle, which is set up to handle this:

$file = "$ENV{HOME}/.myprogrc";
if (open(FILE, "< $file")) { 
    readconfig(*FILE) if is_safe(*FILE);

You would still have to arrange for readconfig to accept a filehandle instead of a filename, though.