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PERLFILTER(1)
NAME
perlfilter - 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 intercepts and
modifies a source stream before it reaches the parser. A source filter
changes our diagram like this:
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 preprocessor 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 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 = $a\n";
The "cpp" filter does not apply to the text of the Fred module, only to the
text of the file that used it ("cpp_test"). Although the use statement on
line 3 will pass through the cpp filter, the module that gets included
("Fred") will not. The source streams look like this after line 3 has been
parsed and before line 4 is parsed:
cpp_test ---> cpp filter ---> parser (INACTIVE)
Fred.pm ----> parser
As you can see, a new stream has been created for reading the source from
"Fred.pm". This stream will remain active until all of "Fred.pm" has been
parsed. The source stream for "cpp_test" will still exist, but is inactive.
Once the parser has finished reading Fred.pm, the source stream associated
with it will be destroyed. The source stream for "cpp_test" then becomes
active again and the parser reads line 4 and subsequent lines from
"cpp_test".
You can use more than one source filter on a single file. Similarly, you
can reuse the same filter in as many files as you like.
For example, if you have a uuencoded and compressed source file, it is
possible to stack a uudecode filter and an uncompression filter like this:
use Filter::uudecode; use Filter::uncompress;
M'XL(".H<US4''V9I;F%L')Q;>7/;1I;_>_I3=&E=%:F*I"T?22Q/
M6]9*<IQCO*XFT"0[PL%%'Y+IG?WN^ZYN-$'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
All decryption filters work on the principle of "security through
obscurity." Regardless of how well you write a decryption filter and
how strong your encryption 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 coprocess to control the flow of data into and out
of the external 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 = $a\n";
The output you'll get when the script is executed:
PQR a = 1
Writing a source filter as a separate executable works fine, but a small
performance penalty is incurred. For example, if you execute the small
example above, a separate subprocess will be created to run the Unix "tr"
command. Each use of the filter requires its own subprocess. If creating
subprocesses is expensive on your system, you might want to consider one of
the other options for creating source filters.
WRITING A SOURCE FILTER IN PERL
The easiest and most portable option available for creating your own source
filter is to write it completely in Perl. To distinguish this from the
previous two techniques, I'll call it a Perl source filter.
To help understand how to write a Perl source filter we need an example to
study. Here is a complete source filter that performs rot13 decoding.
(Rot13 is a very simple encryption scheme used in Usenet postings to hide
the contents of offensive posts. It moves every letter forward thirteen
places, so that A becomes N, B becomes O, and Z becomes M.)
package Rot13;
use Filter::Util::Call;
sub import {
my ($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;
$status;
}
1;
All Perl source filters are implemented as Perl classes and have the same
basic structure as the example above.
First, we include the "Filter::Util::Call" module, which exports a number
of functions into your filter's namespace. The filter shown above uses two
of these functions, "filter_add()" and "filter_read()".
Next, we create the filter object and associate it with the source stream
by defining the "import" function. If you know Perl well enough, you know
that "import" is called automatically every time a module is included with
a use statement. This makes "import" the ideal place to both create and
install a filter object.
In the example filter, the object ($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 section 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()" function.
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
function 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";
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, $in;
If we encrypt this with "mkrot13":
print " hello fred \n";
the result will be this:
use Rot13;
cevag "uryyb serq\a";
Running it produces this output:
hello fred
USING CONTEXT: THE DEBUG FILTER
The rot13 example was a trivial example. Here's another demonstration that
shows off a few more features.
Say you wanted to include a lot of debugging code in your Perl script
during development, but you didn't want it available in the released
product. Source filters offer a solution. In order to keep the example
simple, let's say you wanted the debugging output to be controlled by an
environment variable, "DEBUG". Debugging code is enabled if the variable
exists, otherwise it is disabled.
Two special marker lines will bracket debugging code, like this:
## DEBUG_BEGIN
if ($year > 1999) {
warn "Debug: millennium bug in year $year\n";
}
## DEBUG_END
When the "DEBUG" environment variable exists, the filter ensures that Perl
parses only the code between the "DEBUG_BEGIN" and "DEBUG_END" markers.
That means that when "DEBUG" does exist, the code above should be passed
through the filter unchanged. The marker lines can also be passed through
as-is, because the Perl parser will see them as comment lines. When "DEBUG"
isn't set, we need a way to disable the debug code. A simple way to achieve
that is to convert the lines between the two markers into comments:
## DEBUG_BEGIN
#if ($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 $status;
}
if ($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 $status;
}
1;
The big difference between this filter and the previous example is the use
of context data in the filter object. The filter object is based on a hash
reference, and is used to keep various pieces of context information
between calls to the filter function. All but two of the hash fields are
used for error reporting. The first of those two, Enabled, is used by the
filter to determine whether the debugging code should be given to the Perl
parser. The second, InTraceBlock, is true when the filter has encountered a
"DEBUG_BEGIN" line, but has not yet encountered the following "DEBUG_END"
line.
If you ignore all the error checking that most of the code does, the
essence of the filter is as follows:
sub filter {
my ($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
}
# comment out debug lines when the filter is disabled
s/^/#/ if ! $self->{Enabled};
} elsif ( /^\s*##\s*DEBUG_BEGIN/ ) {
$self->{InTraceBlock} = TRUE;
}
return $status;
}
Be warned: just as the C-preprocessor doesn't know C, the Debug filter
doesn't know Perl. It can be fooled quite easily:
print <<EOM;
##DEBUG_BEGIN
EOM
Such things aside, you can see that a lot can be achieved with a modest
amount of code.
CONCLUSION
You now have better understanding of what a source filter is, and you might
even have a possible use for them. If you feel like playing with source
filters but need a bit of inspiration, here are some extra features you
could add to the Debug filter.
First, an easy one. Rather than having debugging code that is
all-or-nothing, it would be much more useful to be able to control which
specific blocks of debugging code get included. Try extending the syntax
for debug blocks to allow each to be identified. The contents of the
"DEBUG" environment variable can then be used to control which blocks get
included.
Once you can identify individual blocks, try allowing them to be nested.
That isn't difficult either.
Here is an interesting idea that doesn't involve the Debug filter.
Currently Perl subroutines have fairly limited support for formal parameter
lists. You can specify the number of parameters and their type, but you
still have to manually take them out of the @_ array yourself. Write a
source filter that allows you to have a named parameter list. Such a filter
would turn this:
sub MySub ($first, $second, @rest) { ... }
into this:
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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