package filetest;
+our $VERSION = '1.03';
+
=head1 NAME
filetest - Perl pragma to control the filetest permission operators
=head1 SYNOPSIS
-
+
$can_perhaps_read = -r "file"; # use the mode bits
{
use filetest 'access'; # intuit harder
=head1 DESCRIPTION
This pragma tells the compiler to change the behaviour of the filetest
-permissions operators, the -r -w -x -R -W -X (see L<perlfunc>).
+permission operators, C<-r> C<-w> C<-x> C<-R> C<-W> C<-X>
+(see L<perlfunc>).
-The default behaviour to use the mode bits as returned by the stat()
-family of calls. This, however, may not be the right thing to do if
-for example various ACL (access control lists) schemes are in use.
+The default behaviour of file test operators is to use the simple
+mode bits as returned by the stat() family of system calls. However,
+many operating systems have additional features to define more complex
+access rights, for example ACLs (Access Control Lists).
For such environments, C<use filetest> may help the permission
operators to return results more consistent with other tools.
-Each "use filetest" or "no filetest" affects statements to the end of
-the enclosing block.
+The C<use filetest> or C<no filetest> statements affect file tests defined in
+their block, up to the end of the closest enclosing block (they are lexically
+block-scoped).
+
+Currently, only the C<access> sub-pragma is implemented. It enables (or
+disables) the use of access() when available, that is, on most UNIX systems and
+other POSIX environments. See details below.
+
+=head2 Consider this carefully
+
+The stat() mode bits are probably right for most of the files and
+directories found on your system, because few people want to use the
+additional features offered by access(). But you may encounter surprises
+if your program runs on a system that uses ACLs, since the stat()
+information won't reflect the actual permissions.
+
+There may be a slight performance decrease in the filetest operations
+when the filetest pragma is in effect, because checking bits is very
+cheap.
+
+Also, note that using the file tests for security purposes is a lost cause
+from the start: there is a window open for race conditions (who is to
+say that the permissions will not change between the test and the real
+operation?). Therefore if you are serious about security, just try
+the real operation and test for its success - think in terms of atomic
+operations. Filetests are more useful for filesystem administrative
+tasks, when you have no need for the content of the elements on disk.
-There may be a slight performance decrease in the filetests
-when C<use filetest> is in effect, because in some systems
-the extended functionality needs to be emulated.
+=head2 The "access" sub-pragma
-B<NOTE>: using the file tests is a lost case from the start: there is
-a window open for race conditions (who is to say that the permissions
-will not change between the test and the real operation?). Therefore
-if you are serious about security, just try the real operation and
-test for its success. Think atomicity.
+UNIX and POSIX systems provide an abstract access() operating system call,
+which should be used to query the read, write, and execute rights. This
+function hides various distinct approaches in additional operating system
+specific security features, like Access Control Lists (ACLs)
-=head2 subpragma access
+The extended filetest functionality is used by Perl only when the argument
+of the operators is a filename, not when it is a filehandle.
-Currently only one subpragma, C<access> is implemented. It enables
-(or disables) the use of access() or similar system calls. This
-extended filetest functionality is used only when the argument of the
-operators is a filename, not when it is a filehandle.
+=head2 Limitation with regard to C<_>
+
+Because access() does not invoke stat() (at least not in a way visible
+to Perl), B<the stat result cache "_" is not set>. This means that the
+outcome of the following two tests is different. The first has the stat
+bits of F</etc/passwd> in C<_>, and in the second case this still
+contains the bits of C</etc>.
+
+ { -d '/etc';
+ -w '/etc/passwd';
+ print -f _ ? 'Yes' : 'No'; # Yes
+ }
+
+ { use filetest 'access';
+ -d '/etc';
+ -w '/etc/passwd';
+ print -f _ ? 'Yes' : 'No'; # No
+ }
+
+Of course, unless your OS does not implement access(), in which case the
+pragma is simply ignored. Best not to use C<_> at all in a file where
+the filetest pragma is active!
+
+As a side effect, as C<_> doesn't work, stacked filetest operators
+(C<-f -w $file>) won't work either.
+
+This limitation might be removed in a future version of perl.
=cut
+$filetest::hint_bits = 0x00400000; # HINT_FILETEST_ACCESS
+
sub import {
if ( $_[1] eq 'access' ) {
- $^H |= 0x00400000;
+ $^H |= $filetest::hint_bits;
} else {
die "filetest: the only implemented subpragma is 'access'.\n";
}
sub unimport {
if ( $_[1] eq 'access' ) {
- $^H &= ~0x00400000;
+ $^H &= ~$filetest::hint_bits;
} else {
die "filetest: the only implemented subpragma is 'access'.\n";
}
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