PATCH: Taintedness and ternary conditional

Message-Id:  <20040827044447.GA5268@petdance.com>

add tests and documentation to the effect that ($tainted ? $a : $b)
doesn't necessarily return a tainted value. Also tidy the markup in
perldoc.pod

p4raw-id: //depot/perl@23253

1 files changed, 31 insertions, 15 deletions

diff --git a/pod/perlsec.pod b/pod/perlsec.pod
index 5a09e32d8e..2fb6877b0d 100644
--- a/pod/perlsec.pod
+++ b/pod/perlsec.pod
@@ -32,10 +32,10 @@ program more secure than the corresponding C program.
 You may not use data derived from outside your program to affect
 something else outside your program--at least, not by accident.  All
 command line arguments, environment variables, locale information (see
-L<perllocale>), results of certain system calls (readdir(),
-readlink(), the variable of shmread(), the messages returned by
-msgrcv(), the password, gcos and shell fields returned by the
-getpwxxx() calls), and all file input are marked as "tainted".
+L<perllocale>), results of certain system calls (C<readdir()>,
+C<readlink()>, the variable of C<shmread()>, the messages returned by
+C<msgrcv()>, the password, gcos and shell fields returned by the
+C<getpwxxx()> calls), and all file input are marked as "tainted".
 Tainted data may not be used directly or indirectly in any command
 that invokes a sub-shell, nor in any command that modifies files,
 directories, or processes, B<with the following exceptions>:
@@ -129,11 +129,27 @@ For example:
 If you try to do something insecure, you will get a fatal error saying
 something like "Insecure dependency" or "Insecure $ENV{PATH}".
 
+The exception to the principle of "one tainted value taints the whole
+expression" is with the ternary conditional operator C<?:>.  Since code
+with a ternary conditional
+
+    $result = $tainted_value ? "Untainted" : "Also untainted";
+
+is effectively
+
+    if ( $tainted_value ) {
+        $result = "Untainted";
+    } else {
+        $result = "Also untainted";
+    }
+
+it doesn't make sense for C<$result> to be tainted.
+
 =head2 Laundering and Detecting Tainted Data
 
 To test whether a variable contains tainted data, and whose use would
 thus trigger an "Insecure dependency" message, you can use the
-tainted() function of the Scalar::Util module, available in your
+C<tainted()> function of the Scalar::Util module, available in your
 nearby CPAN mirror, and included in Perl starting from the release 5.8.0.
 Or you may be able to use the following C<is_tainted()> function.
 
@@ -179,7 +195,7 @@ Laundering data using regular expression is the I<only> mechanism for
 untainting dirty data, unless you use the strategy detailed below to fork
 a child of lesser privilege.
 
-The example does not untaint $data if C<use locale> is in effect,
+The example does not untaint C<$data> if C<use locale> is in effect,
 because the characters matched by C<\w> are determined by the locale.
 Perl considers that locale definitions are untrustworthy because they
 contain data from outside the program.  If you are writing a
@@ -247,26 +263,26 @@ privileges. Perl doesn't prevent you from opening tainted filenames for reading,
 so be careful what you print out.  The tainting mechanism is intended to
 prevent stupid mistakes, not to remove the need for thought.
 
-Perl does not call the shell to expand wild cards when you pass B<system>
-and B<exec> explicit parameter lists instead of strings with possible shell
-wildcards in them.  Unfortunately, the B<open>, B<glob>, and
+Perl does not call the shell to expand wild cards when you pass C<system>
+and C<exec> explicit parameter lists instead of strings with possible shell
+wildcards in them.  Unfortunately, the C<open>, C<glob>, and
 backtick functions provide no such alternate calling convention, so more
 subterfuge will be required.
 
 Perl provides a reasonably safe way to open a file or pipe from a setuid
 or setgid program: just create a child process with reduced privilege who
 does the dirty work for you.  First, fork a child using the special
-B<open> syntax that connects the parent and child by a pipe.  Now the
+C<open> syntax that connects the parent and child by a pipe.  Now the
 child resets its ID set and any other per-process attributes, like
 environment variables, umasks, current working directories, back to the
 originals or known safe values.  Then the child process, which no longer
-has any special permissions, does the B<open> or other system call.
+has any special permissions, does the C<open> or other system call.
 Finally, the child passes the data it managed to access back to the
 parent.  Because the file or pipe was opened in the child while running
 under less privilege than the parent, it's not apt to be tricked into
 doing something it shouldn't.
 
-Here's a way to do backticks reasonably safely.  Notice how the B<exec> is
+Here's a way to do backticks reasonably safely.  Notice how the C<exec> is
 not called with a string that the shell could expand.  This is by far the
 best way to call something that might be subjected to shell escapes: just
 never call the shell at all.  
@@ -330,7 +346,7 @@ outlaw scripts with any set-id bit set, which doesn't help much.
 Alternately, it can simply ignore the set-id bits on scripts.  If the
 latter is true, Perl can emulate the setuid and setgid mechanism when it
 notices the otherwise useless setuid/gid bits on Perl scripts.  It does
-this via a special executable called B<suidperl> that is automatically
+this via a special executable called F<suidperl> that is automatically
 invoked for you if it's needed.
 
 However, if the kernel set-id script feature isn't disabled, Perl will
@@ -357,12 +373,12 @@ of the set-id script to open to the interpreter, rather than using a
 pathname subject to meddling, it instead passes I</dev/fd/3>.  This is a
 special file already opened on the script, so that there can be no race
 condition for evil scripts to exploit.  On these systems, Perl should be
-compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>.  The B<Configure>
+compiled with C<-DSETUID_SCRIPTS_ARE_SECURE_NOW>.  The F<Configure>
 program that builds Perl tries to figure this out for itself, so you
 should never have to specify this yourself.  Most modern releases of
 SysVr4 and BSD 4.4 use this approach to avoid the kernel race condition.
 
-Prior to release 5.6.1 of Perl, bugs in the code of B<suidperl> could
+Prior to release 5.6.1 of Perl, bugs in the code of F<suidperl> could
 introduce a security hole.
 
 =head2 Protecting Your Programs