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   1  package filetest;
   2  
   3  our $VERSION = '1.02';
   4  
   5  =head1 NAME
   6  
   7  filetest - Perl pragma to control the filetest permission operators
   8  
   9  =head1 SYNOPSIS
  10  
  11      $can_perhaps_read = -r "file";    # use the mode bits
  12      {
  13          use filetest 'access';        # intuit harder
  14          $can_really_read = -r "file";
  15      }
  16      $can_perhaps_read = -r "file";    # use the mode bits again
  17  
  18  =head1 DESCRIPTION
  19  
  20  This pragma tells the compiler to change the behaviour of the filetest
  21  permission operators, C<-r> C<-w> C<-x> C<-R> C<-W> C<-X>
  22  (see L<perlfunc>).
  23  
  24  The default behaviour of file test operators is to use the simple
  25  mode bits as returned by the stat() family of system calls.  However,
  26  many operating systems have additional features to define more complex
  27  access rights, for example ACLs (Access Control Lists).
  28  For such environments, C<use filetest> may help the permission
  29  operators to return results more consistent with other tools.
  30  
  31  The C<use filetest> or C<no filetest> statements affect file tests defined in
  32  their block, up to the end of the closest enclosing block (they are lexically
  33  block-scoped).
  34  
  35  Currently, only the C<access> sub-pragma is implemented.  It enables (or
  36  disables) the use of access() when available, that is, on most UNIX systems and
  37  other POSIX environments.  See details below.
  38  
  39  =head2 Consider this carefully
  40  
  41  The stat() mode bits are probably right for most of the files and
  42  directories found on your system, because few people want to use the
  43  additional features offered by access(). But you may encounter surprises
  44  if your program runs on a system that uses ACLs, since the stat()
  45  information won't reflect the actual permissions.
  46  
  47  There may be a slight performance decrease in the filetest operations
  48  when the filetest pragma is in effect, because checking bits is very
  49  cheap.
  50  
  51  Also, note that using the file tests for security purposes is a lost cause
  52  from the start: there is a window open for race conditions (who is to
  53  say that the permissions will not change between the test and the real
  54  operation?).  Therefore if you are serious about security, just try
  55  the real operation and test for its success - think in terms of atomic
  56  operations.  Filetests are more useful for filesystem administrative
  57  tasks, when you have no need for the content of the elements on disk.
  58  
  59  =head2 The "access" sub-pragma
  60  
  61  UNIX and POSIX systems provide an abstract access() operating system call,
  62  which should be used to query the read, write, and execute rights. This
  63  function hides various distinct approaches in additional operating system
  64  specific security features, like Access Control Lists (ACLs)
  65  
  66  The extended filetest functionality is used by Perl only when the argument
  67  of the operators is a filename, not when it is a filehandle.
  68  
  69  =head2 Limitation with regard to C<_>
  70  
  71  Because access() does not invoke stat() (at least not in a way visible
  72  to Perl), B<the stat result cache "_" is not set>.  This means that the
  73  outcome of the following two tests is different.  The first has the stat
  74  bits of C</etc/passwd> in C<_>, and in the second case this still
  75  contains the bits of C</etc>.
  76  
  77   { -d '/etc';
  78     -w '/etc/passwd';
  79     print -f _ ? 'Yes' : 'No';   # Yes
  80   }
  81  
  82   { use filetest 'access';
  83     -d '/etc';
  84     -w '/etc/passwd';
  85     print -f _ ? 'Yes' : 'No';   # No
  86   }
  87  
  88  Of course, unless your OS does not implement access(), in which case the
  89  pragma is simply ignored.  Best not to use C<_> at all in a file where
  90  the filetest pragma is active!
  91  
  92  As a side effect, as C<_> doesn't work, stacked filetest operators
  93  (C<-f -w $file>) won't work either.
  94  
  95  This limitation might be removed in a future version of perl.
  96  
  97  =cut
  98  
  99  $filetest::hint_bits = 0x00400000; # HINT_FILETEST_ACCESS
 100  
 101  sub import {
 102      if ( $_[1] eq 'access' ) {
 103      $^H |= $filetest::hint_bits;
 104      } else {
 105      die "filetest: the only implemented subpragma is 'access'.\n";
 106      }
 107  }
 108  
 109  sub unimport {
 110      if ( $_[1] eq 'access' ) {
 111      $^H &= ~$filetest::hint_bits;
 112      } else {
 113      die "filetest: the only implemented subpragma is 'access'.\n";
 114      }
 115  }
 116  
 117  1;