Perl::Critic::Policy::InputOutput::RequireEncodingWithUTF8Layer3pm

Affiliation

This Policy is part of the core Perl::Critic distribution.

Overview

Use of the ":utf8" I/O layer (as opposed to :encoding(UTF8) or :encoding(UTF-8)) was suggested in the Perl documentation up to version 5.8.8. This may be OK for output, but on input ":utf8" does not validate the input, leading to unexpected results.

An exploit based on this behavior of ":utf8" is exhibited on PerlMonks at <http://www.perlmonks.org/?node_id=644786>. The exploit involves a string read from an external file and sanitized with "m/^(\w+)$/", where $1 nonetheless ends up containing shell meta-characters.

To summarize:

 open $fh, '<:utf8', 'foo.txt';             # BAD
 open $fh, '<:encoding(UTF8)', 'foo.txt';   # GOOD
 open $fh, '<:encoding(UTF-8)', 'foo.txt';  # BETTER

See the Encode documentation for the difference between "UTF8" and "UTF-8". The short version is that "UTF-8" implements the Unicode standard, and "UTF8" is liberalized.

For consistency's sake, this policy checks files opened for output as well as input. For complete coverage it also checks binmode() calls, where the direction of operation can not be determined.

Configuration

This Policy is not configurable except for the standard options.

Notes

Because "Perl::Critic" does a static analysis, this policy can not detect cases like

 my $encoding = ':utf8';
 binmode $fh, $encoding;

where the encoding is computed.