PERLFILTER(1) Perl Programmers Reference Guide PERLFILTER(1)NAMEperlfilter - Source Filters
DESCRIPTION
This article is about a little-known feature of Perl called
source filters. Source filters alter the program text of a
module before Perl sees it, much as a C preprocessor alters
the source text of a C program before the compiler sees it.
This article tells you more about what source filters are,
how they work, and how to write your own.
The original purpose of source filters was to let you
encrypt your program source to prevent casual piracy. This
isn't all they can do, as you'll soon learn. But first, the
basics.
CONCEPTS
Before the Perl interpreter can execute a Perl script, it
must first read it from a file into memory for parsing and
compilation. If that script itself includes other scripts
with a "use" or "require" statement, then each of those
scripts will have to be read from their respective files as
well.
Now think of each logical connection between the Perl parser
and an individual file as a source stream. A source stream
is created when the Perl parser opens a file, it continues
to exist as the source code is read into memory, and it is
destroyed when Perl is finished parsing the file. If the
parser encounters a "require" or "use" statement in a source
stream, a new and distinct stream is created just for that
file.
The diagram below represents a single source stream, with
the flow of source from a Perl script file on the left into
the Perl parser on the right. This is how Perl normally
operates.
file -------> parser
There are two important points to remember:
1. Although there can be any number of source streams in
existence at any given time, only one will be active.
2. Every source stream is associated with only one file.
A source filter is a special kind of Perl module that inter-
cepts and modifies a source stream before it reaches the
parser. A source filter changes our diagram like this:
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file ----> filter ----> parser
If that doesn't make much sense, consider the analogy of a
command pipeline. Say you have a shell script stored in the
compressed file trial.gz. The simple pipeline command below
runs the script without needing to create a temporary file
to hold the uncompressed file.
gunzip -c trial.gz | sh
In this case, the data flow from the pipeline can be
represented as follows:
trial.gz ----> gunzip ----> sh
With source filters, you can store the text of your script
compressed and use a source filter to uncompress it for
Perl's parser:
compressed gunzip
Perl program ---> source filter ---> parser
USING FILTERS
So how do you use a source filter in a Perl script? Above, I
said that a source filter is just a special kind of module.
Like all Perl modules, a source filter is invoked with a use
statement.
Say you want to pass your Perl source through the C prepro-
cessor before execution. You could use the existing "-P"
command line option to do this, but as it happens, the
source filters distribution comes with a C preprocessor
filter module called Filter::cpp. Let's use that instead.
Below is an example program, "cpp_test", which makes use of
this filter. Line numbers have been added to allow specific
lines to be referenced easily.
1: use Filter::cpp;
2: #define TRUE 1
3: $a = TRUE;
4: print "a = $a\n";
When you execute this script, Perl creates a source stream
for the file. Before the parser processes any of the lines
from the file, the source stream looks like this:
cpp_test ---------> parser
Line 1, "use Filter::cpp", includes and installs the "cpp"
filter module. All source filters work this way. The use
statement is compiled and executed at compile time, before
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any more of the file is read, and it attaches the cpp filter
to the source stream behind the scenes. Now the data flow
looks like this:
cpp_test ----> cpp filter ----> parser
As the parser reads the second and subsequent lines from the
source stream, it feeds those lines through the "cpp" source
filter before processing them. The "cpp" filter simply
passes each line through the real C preprocessor. The output
from the C preprocessor is then inserted back into the
source stream by the filter.
.-> cpp --.
| |
| |
| <-'
cpp_test ----> cpp filter ----> parser
The parser then sees the following code:
use Filter::cpp;
$a = 1;
print "a = $a\n";
Let's consider what happens when the filtered code includes
another module with use:
1: use Filter::cpp;
2: #define TRUE 1
3: use Fred;
4: $a = TRUE;
5: print "a = $a\n";
The "cpp" filter does not apply to the text of the Fred
module, only to the text of the file that used it
("cpp_test"). Although the use statement on line 3 will pass
through the cpp filter, the module that gets included
("Fred") will not. The source streams look like this after
line 3 has been parsed and before line 4 is parsed:
cpp_test ---> cpp filter ---> parser (INACTIVE)
Fred.pm ----> parser
As you can see, a new stream has been created for reading
the source from "Fred.pm". This stream will remain active
until all of "Fred.pm" has been parsed. The source stream
for "cpp_test" will still exist, but is inactive. Once the
parser has finished reading Fred.pm, the source stream asso-
ciated with it will be destroyed. The source stream for
"cpp_test" then becomes active again and the parser reads
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line 4 and subsequent lines from "cpp_test".
You can use more than one source filter on a single file.
Similarly, you can reuse the same filter in as many files as
you like.
For example, if you have a uuencoded and compressed source
file, it is possible to stack a uudecode filter and an
uncompression filter like this:
use Filter::uudecode; use Filter::uncompress;
M'XL(".H<US4''V9I;F%L')Q;>7/;1I;_>_I3=&E=%:F*I"T?22Q/
M6]9*<IQCO*XFT"0[PL%%'Y+IG?WN^ZYN-$'J.[.JE$,20/?K=_[>
...
Once the first line has been processed, the flow will look
like this:
file ---> uudecode ---> uncompress ---> parser
filter filter
Data flows through filters in the same order they appear in
the source file. The uudecode filter appeared before the
uncompress filter, so the source file will be uudecoded
before it's uncompressed.
WRITING A SOURCE FILTER
There are three ways to write your own source filter. You
can write it in C, use an external program as a filter, or
write the filter in Perl. I won't cover the first two in any
great detail, so I'll get them out of the way first. Writing
the filter in Perl is most convenient, so I'll devote the
most space to it.
WRITING A SOURCE FILTER IN C
The first of the three available techniques is to write the
filter completely in C. The external module you create
interfaces directly with the source filter hooks provided by
Perl.
The advantage of this technique is that you have complete
control over the implementation of your filter. The big
disadvantage is the increased complexity required to write
the filter - not only do you need to understand the source
filter hooks, but you also need a reasonable knowledge of
Perl guts. One of the few times it is worth going to this
trouble is when writing a source scrambler. The "decrypt"
filter (which unscrambles the source before Perl parses it)
included with the source filter distribution is an example
of a C source filter (see Decryption Filters, below).
Decryption Filters
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All decryption filters work on the principle of "secu-
rity through obscurity." Regardless of how well you
write a decryption filter and how strong your encryp-
tion algorithm, anyone determined enough can retrieve
the original source code. The reason is quite simple -
once the decryption filter has decrypted the source
back to its original form, fragments of it will be
stored in the computer's memory as Perl parses it. The
source might only be in memory for a short period of
time, but anyone possessing a debugger, skill, and lots
of patience can eventually reconstruct your program.
That said, there are a number of steps that can be
taken to make life difficult for the potential cracker.
The most important: Write your decryption filter in C
and statically link the decryption module into the Perl
binary. For further tips to make life difficult for the
potential cracker, see the file decrypt.pm in the
source filters module.
CREATING A SOURCE FILTER AS A SEPARATE EXECUTABLE
An alternative to writing the filter in C is to create a
separate executable in the language of your choice. The
separate executable reads from standard input, does whatever
processing is necessary, and writes the filtered data to
standard output. "Filter:cpp" is an example of a source
filter implemented as a separate executable - the executable
is the C preprocessor bundled with your C compiler.
The source filter distribution includes two modules that
simplify this task: "Filter::exec" and "Filter::sh". Both
allow you to run any external executable. Both use a copro-
cess to control the flow of data into and out of the exter-
nal executable. (For details on coprocesses, see Stephens,
W.R. "Advanced Programming in the UNIX Environment."
Addison-Wesley, ISBN 0-210-56317-7, pages 441-445.) The
difference between them is that "Filter::exec" spawns the
external command directly, while "Filter::sh" spawns a shell
to execute the external command. (Unix uses the Bourne
shell; NT uses the cmd shell.) Spawning a shell allows you
to make use of the shell metacharacters and redirection
facilities.
Here is an example script that uses "Filter::sh":
use Filter::sh 'tr XYZ PQR';
$a = 1;
print "XYZ a = $a\n";
The output you'll get when the script is executed:
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PQR a = 1
Writing a source filter as a separate executable works fine,
but a small performance penalty is incurred. For example, if
you execute the small example above, a separate subprocess
will be created to run the Unix "tr" command. Each use of
the filter requires its own subprocess. If creating sub-
processes is expensive on your system, you might want to
consider one of the other options for creating source
filters.
WRITING A SOURCE FILTER IN PERL
The easiest and most portable option available for creating
your own source filter is to write it completely in Perl. To
distinguish this from the previous two techniques, I'll call
it a Perl source filter.
To help understand how to write a Perl source filter we need
an example to study. Here is a complete source filter that
performs rot13 decoding. (Rot13 is a very simple encryption
scheme used in Usenet postings to hide the contents of
offensive posts. It moves every letter forward thirteen
places, so that A becomes N, B becomes O, and Z becomes M.)
package Rot13;
use Filter::Util::Call;
sub import {
my ($type) = @_;
my ($ref) = [];
filter_add(bless $ref);
}
sub filter {
my ($self) = @_;
my ($status);
tr/n-za-mN-ZA-M/a-zA-Z/
if ($status = filter_read()) > 0;
$status;
}
1;
All Perl source filters are implemented as Perl classes and
have the same basic structure as the example above.
First, we include the "Filter::Util::Call" module, which
exports a number of functions into your filter's namespace.
The filter shown above uses two of these functions,
"filter_add()" and "filter_read()".
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Next, we create the filter object and associate it with the
source stream by defining the "import" function. If you know
Perl well enough, you know that "import" is called automati-
cally every time a module is included with a use statement.
This makes "import" the ideal place to both create and
install a filter object.
In the example filter, the object ($ref) is blessed just
like any other Perl object. Our example uses an anonymous
array, but this isn't a requirement. Because this example
doesn't need to store any context information, we could have
used a scalar or hash reference just as well. The next sec-
tion demonstrates context data.
The association between the filter object and the source
stream is made with the "filter_add()" function. This takes
a filter object as a parameter ($ref in this case) and
installs it in the source stream.
Finally, there is the code that actually does the filtering.
For this type of Perl source filter, all the filtering is
done in a method called "filter()". (It is also possible to
write a Perl source filter using a closure. See the
"Filter::Util::Call" manual page for more details.) It's
called every time the Perl parser needs another line of
source to process. The "filter()" method, in turn, reads
lines from the source stream using the "filter_read()" func-
tion.
If a line was available from the source stream,
"filter_read()" returns a status value greater than zero and
appends the line to $_. A status value of zero indicates
end-of-file, less than zero means an error. The filter func-
tion itself is expected to return its status in the same
way, and put the filtered line it wants written to the
source stream in $_. The use of $_ accounts for the brevity
of most Perl source filters.
In order to make use of the rot13 filter we need some way of
encoding the source file in rot13 format. The script below,
"mkrot13", does just that.
die "usage mkrot13 filename\n" unless @ARGV;
my $in = $ARGV[0];
my $out = "$in.tmp";
open(IN, "<$in") or die "Cannot open file $in: $!\n";
open(OUT, ">$out") or die "Cannot open file $out: $!\n";
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print OUT "use Rot13;\n";
while (<IN>) {
tr/a-zA-Z/n-za-mN-ZA-M/;
print OUT;
}
close IN;
close OUT;
unlink $in;
rename $out, $in;
If we encrypt this with "mkrot13":
print " hello fred \n";
the result will be this:
use Rot13;
cevag "uryyb serq\a";
Running it produces this output:
hello fred
USING CONTEXT: THE DEBUG FILTER
The rot13 example was a trivial example. Here's another
demonstration that shows off a few more features.
Say you wanted to include a lot of debugging code in your
Perl script during development, but you didn't want it
available in the released product. Source filters offer a
solution. In order to keep the example simple, let's say you
wanted the debugging output to be controlled by an environ-
ment variable, "DEBUG". Debugging code is enabled if the
variable exists, otherwise it is disabled.
Two special marker lines will bracket debugging code, like
this:
## DEBUG_BEGIN
if ($year > 1999) {
warn "Debug: millennium bug in year $year\n";
}
## DEBUG_END
When the "DEBUG" environment variable exists, the filter
ensures that Perl parses only the code between the
"DEBUG_BEGIN" and "DEBUG_END" markers. That means that when
"DEBUG" does exist, the code above should be passed through
the filter unchanged. The marker lines can also be passed
through as-is, because the Perl parser will see them as com-
ment lines. When "DEBUG" isn't set, we need a way to disable
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the debug code. A simple way to achieve that is to convert
the lines between the two markers into comments:
## DEBUG_BEGIN
#if ($year > 1999) {
# warn "Debug: millennium bug in year $year\n";
#}
## DEBUG_END
Here is the complete Debug filter:
package Debug;
use strict;
use warnings;
use Filter::Util::Call;
use constant TRUE => 1;
use constant FALSE => 0;
sub import {
my ($type) = @_;
my (%context) = (
Enabled => defined $ENV{DEBUG},
InTraceBlock => FALSE,
Filename => (caller)[1],
LineNo => 0,
LastBegin => 0,
);
filter_add(bless \%context);
}
sub Die {
my ($self) = shift;
my ($message) = shift;
my ($line_no) = shift || $self->{LastBegin};
die "$message at $self->{Filename} line $line_no.\n"
}
sub filter {
my ($self) = @_;
my ($status);
$status = filter_read();
++ $self->{LineNo};
# deal with EOF/error first
if ($status <= 0) {
$self->Die("DEBUG_BEGIN has no DEBUG_END")
if $self->{InTraceBlock};
return $status;
}
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if ($self->{InTraceBlock}) {
if (/^\s*##\s*DEBUG_BEGIN/ ) {
$self->Die("Nested DEBUG_BEGIN", $self->{LineNo})
} elsif (/^\s*##\s*DEBUG_END/) {
$self->{InTraceBlock} = FALSE;
}
# comment out the debug lines when the filter is disabled
s/^/#/ if ! $self->{Enabled};
} elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
$self->{InTraceBlock} = TRUE;
$self->{LastBegin} = $self->{LineNo};
} elsif ( /^\s*##\s*DEBUG_END/ ) {
$self->Die("DEBUG_END has no DEBUG_BEGIN", $self->{LineNo});
}
return $status;
}
1;
The big difference between this filter and the previous
example is the use of context data in the filter object. The
filter object is based on a hash reference, and is used to
keep various pieces of context information between calls to
the filter function. All but two of the hash fields are used
for error reporting. The first of those two, Enabled, is
used by the filter to determine whether the debugging code
should be given to the Perl parser. The second, InTra-
ceBlock, is true when the filter has encountered a
"DEBUG_BEGIN" line, but has not yet encountered the follow-
ing "DEBUG_END" line.
If you ignore all the error checking that most of the code
does, the essence of the filter is as follows:
sub filter {
my ($self) = @_;
my ($status);
$status = filter_read();
# deal with EOF/error first
return $status if $status <= 0;
if ($self->{InTraceBlock}) {
if (/^\s*##\s*DEBUG_END/) {
$self->{InTraceBlock} = FALSE
}
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# comment out debug lines when the filter is disabled
s/^/#/ if ! $self->{Enabled};
} elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
$self->{InTraceBlock} = TRUE;
}
return $status;
}
Be warned: just as the C-preprocessor doesn't know C, the
Debug filter doesn't know Perl. It can be fooled quite
easily:
print <<EOM;
##DEBUG_BEGIN
EOM
Such things aside, you can see that a lot can be achieved
with a modest amount of code.
CONCLUSION
You now have better understanding of what a source filter
is, and you might even have a possible use for them. If you
feel like playing with source filters but need a bit of
inspiration, here are some extra features you could add to
the Debug filter.
First, an easy one. Rather than having debugging code that
is all-or-nothing, it would be much more useful to be able
to control which specific blocks of debugging code get
included. Try extending the syntax for debug blocks to allow
each to be identified. The contents of the "DEBUG" environ-
ment variable can then be used to control which blocks get
included.
Once you can identify individual blocks, try allowing them
to be nested. That isn't difficult either.
Here is an interesting idea that doesn't involve the Debug
filter. Currently Perl subroutines have fairly limited sup-
port for formal parameter lists. You can specify the number
of parameters and their type, but you still have to manually
take them out of the @_ array yourself. Write a source
filter that allows you to have a named parameter list. Such
a filter would turn this:
sub MySub ($first, $second, @rest) { ... }
into this:
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sub MySub($$@) {
my ($first) = shift;
my ($second) = shift;
my (@rest) = @_;
...
}
Finally, if you feel like a real challenge, have a go at
writing a full-blown Perl macro preprocessor as a source
filter. Borrow the useful features from the C preprocessor
and any other macro processors you know. The tricky bit will
be choosing how much knowledge of Perl's syntax you want
your filter to have.
THINGS TO LOOK OUT FOR
Some Filters Clobber the "DATA" Handle
Some source filters use the "DATA" handle to read the
calling program. When using these source filters you
cannot rely on this handle, nor expect any particular
kind of behavior when operating on it. Filters based
on Filter::Util::Call (and therefore Filter::Simple) do
not alter the "DATA" filehandle.
REQUIREMENTS
The Source Filters distribution is available on CPAN, in
CPAN/modules/by-module/Filter
Starting from Perl 5.8 Filter::Util::Call (the core part of
the Source Filters distribution) is part of the standard
Perl distribution. Also included is a friendlier interface
called Filter::Simple, by Damian Conway.
AUTHOR
Paul Marquess <Paul.Marquess@btinternet.com>
Copyrights
This article originally appeared in The Perl Journal #11,
and is copyright 1998 The Perl Journal. It appears courtesy
of Jon Orwant and The Perl Journal. This document may be
distributed under the same terms as Perl itself.
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