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 C<-r> C<-w> C<-x> C<-R> C<-W> C<-X>
+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
-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.
+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.
-B<NOTE>: using the file tests for security purposes is a lost case
+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 atomicity.
+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.
+
+=head2 The "access" sub-pragma
+
+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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