## 38.5. Trusted and Untrusted PL/Perl

Normally, PL/Perl is installed as a “trusted” programming language named `plperl`. In this setup, certain Perl operations are disabled to preserve security. In general, the operations that are restricted are those that interact with the environment. This includes file handle operations, `require`, and `use` (for external modules). There is no way to access internals of the database server process or to gain OS-level access with the permissions of the server process, as a C function can do. Thus, any unprivileged database user may be permitted to use this language.

Here is an example of a function that will not work because file system operations are not allowed for security reasons:

    CREATE FUNCTION badfunc() RETURNS integer AS $$
        my $tmpfile = "/tmp/badfile";
        open my $fh, '>', $tmpfile
            or elog(ERROR, qq{Could not open the file "$tmpfile": $!});
        print $fh "Testing writing to a file\n";
        close $fh or elog(ERROR, qq{Could not close the file "$tmpfile": $!});
        return 1;
    $$ LANGUAGE plperl;

The creation of this function will fail as its use of a forbidden operation will be be caught by the validator.

Sometimes it is desirable to write Perl functions that are not restricted. For example, one might want a Perl function that sends mail. To handle these cases, PL/Perl can also be installed as an “untrusted” language (usually called PL/PerlU). In this case the full Perl language is available. If the `createlang` program is used to install the language, the language name `plperlu` will select the untrusted PL/Perl variant.

The writer of a PL/PerlU function must take care that the function cannot be used to do anything unwanted, since it will be able to do anything that could be done by a user logged in as the database administrator. Note that the database system allows only database superusers to create functions in untrusted languages.

If the above function was created by a superuser using the language `plperlu`, execution would succeed.