I ran into an "interesting" bug in CGI.pm today, and figured I'd post here as this is a potential problem in Apache 2/modperl 2 setups.

I have a form of application server that runs as a CGI in a modperl 2 system, where the client submits a request in XML format, using a POST request.

Last night an error was reported - looking in the log I saw that the data from the POST had been truncated. After a bit of searching, including using tcpdump to verify that the data was really being sent correctly, I found a post on the modperl mailing list which pointed in the right direction.

It appears that in some circumstances apache 2/modperl 2 don't read the full POST in a single call.

CGI.pm has the following call to read the POST: 

sub read_from_client {
    my($self, $buff, $len, $offset) = @_;
    local $^W=0;                # prevent a warning
    return $MOD_PERL
        ? $self->r->read($$buff, $len, $offset)
        : read(\*STDIN, $$buff, $len, $offset);
}

[download]
The problem here is that $self->r->read() doesn't necessary read $len bytes, and CGI.pm doesn't check this.

My fix, based on a fix posted by Stas Bekman in the modperl list, is to simply call read_from_client() in a loop. The patch to CGI.pm (version 3.01) looks like this: 

*** CGI.pm~ 2003-12-12 15:09:45.000000000 -0800
--- CGI.pm 2003-12-12 15:12:29.000000000 -0800
***************
*** 551,558 ****
        }
   
        if ($meth eq 'POST') {
!    $self->read_from_client(\$query_string,$content_length,0)
!        if $content_length > 0;
     # Some people want to have their cake and eat it too!
     # Uncomment this line to have the contents of the query string
     # APPENDED to the POST data.
--- 551,565 ----
        }
   
        if ($meth eq 'POST') {
!  if($content_length > 0) {
!    my $len = $content_length;
!    while ($len > 0) {
!      my $data = '';
!      my $read = $self->read_from_client(\$data,$content_length,0);
!      $len -= $read;
!      $query_string .= $data if $read > 0;
!    }
!  }
     # Some people want to have their cake and eat it too!
     # Uncomment this line to have the contents of the query string
     # APPENDED to the POST data.

[download]
Michael

In reply to CGI.pm bug in Apache 2/mod-perl 2 setup by mpeppler

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