From: Stas Bekman <stas@stason.org>
Date: Tue, 4 Nov 2003 14:33:09 +0000 (-0800)
Subject: [patch t/op/hash.t] extending the hash attack test
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=d0c795901506276e696dc446752ed0b09ca418e5;p=p5sagit%2Fp5-mst-13.2.git

[patch t/op/hash.t] extending the hash attack test
Message-ID: <3FA82925.7020703@stason.org>

p4raw-id: //depot/perl@21672
---

diff --git a/t/op/hash.t b/t/op/hash.t
index 3beae84..15a336e 100644
--- a/t/op/hash.t
+++ b/t/op/hash.t
@@ -8,7 +8,7 @@ BEGIN {
 
 use strict;
 
-plan tests => 3;
+plan tests => 5;
 
 my %h;
 
@@ -25,3 +25,87 @@ foreach (11..20) {
 }
 
 ok (Internals::HvREHASH(%h), "20 entries triggers rehash");
+
+
+
+
+# second part using an emulation of the PERL_HASH in perl, mounting an
+# attack on a prepopulated hash. This is also useful if you need normal
+# keys which don't contain \0 -- suitable for stashes
+
+use constant MASK_U32  => 2**32;
+use constant HASH_SEED => 0;
+use constant THRESHOLD => 14;
+use constant START     => "a";
+
+# some initial hash data
+my %h2 = map {$_ => 1} 'a'..'cc';
+
+ok (!Internals::HvREHASH(%h2), 
+    "starting with pre-populated non-pathalogical hash (rehash flag if off)");
+
+my @keys = get_keys(\%h2);
+$h2{$_}++ for @keys;
+ok (Internals::HvREHASH(%h2), 
+    scalar(@keys) . " colliding into the same bucket keys are triggerring rehash");
+
+sub get_keys {
+    my $hr = shift;
+
+    # the minimum of bits required to mount the attack on a hash
+    my $min_bits = log(THRESHOLD)/log(2);
+
+    # if the hash has already been populated with a significant amount
+    # of entries the number of mask bits can be higher
+    my $keys = scalar keys %$hr;
+    my $bits = $keys ? log($keys)/log(2) : 0;
+    $bits = $min_bits if $min_bits > $bits;
+
+    $bits = int($bits) < $bits ? int($bits) + 1 : int($bits);
+    # need to add 2 bits to cover the internal split cases
+    $bits += 2;
+    my $mask = 2**$bits-1;
+    print "# using mask: $mask ($bits)\n";
+
+    my @keys;
+    my $s = START;
+    my $c = 0;
+    # get 2 keys on top of the THRESHOLD
+    my $hash;
+    while (@keys < THRESHOLD+2) {
+        # next if exists $hash->{$s};
+        $hash = hash($s);
+        next unless ($hash & $mask) == 0;
+        $c++;
+        printf "# %2d: %5s, %10s\n", $c, $s, $hash;
+        push @keys, $s;
+    } continue {
+        $s++;
+    }
+
+    return @keys;
+}
+
+
+# trying to provide the fastest equivalent of C macro's PERL_HASH in
+# Perl - the main complication is that it uses U32 integer, which we
+# can't do it perl, without doing some tricks
+sub hash {
+    my $s = shift;
+    my @c = split //, $s;
+    my $u = HASH_SEED;
+    for (@c) {
+        # (A % M) + (B % M) == (A + B) % M
+        # This works because '+' produces a NV, which is big enough to hold
+        # the intermidiate result. We only need the % before any "^" and "&"
+        # to get the result in the range for an I32.
+        # and << doesn't work on NV, so using 1 << 10
+        $u += ord;
+        $u += $u * (1 << 10); $u %= MASK_U32;
+        $u ^= $u >> 6;
+    }
+    $u += $u << 3;  $u %= MASK_U32;
+    $u ^= $u >> 11; $u %= MASK_U32;
+    $u += $u << 15; $u %= MASK_U32;
+    $u;
+}