Brief digression: printlp

• printlp is the program that actually copies the data to /dev/lp

• Such a program needs special permissions

• It must run setuid or setgid

• So we want to make it as simple as possible

        #!/usr/bin/suidperl

        use Fcntl 'O_WRONLY';

        my $printer = shift;
        exit 1 unless defined $printer;
        exit 2 unless $printer =~ m{^/dev/lp[a-zA-Z.0-9-]*\z};
        exit 3 unless -c $printer;
        my $group =  getgrgid((stat _)[5]);
        exit 4 unless $group eq 'lp';
        untaint($printer);
        sysopen PRINTER, $printer, O_WRONLY or exit 5;

        my($br, $buf);
        while ($br = read STDIN, $buf, 8192) {
          print PRINTER $buf;
        }
        exit 6 unless defined $br;
        close PRINTER or exit 7;
        exit 0;

        sub untaint {
          $_[0] =~ /(.*)/;
          $_[0] = $1;
        }

• This is probably the only time in my life I have used the -c operator

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Copyright © 2012 M. J. Dominus