Add construction checksum to safe-browsing's BloomFilter.

chrome/browser/safe_browsing/bloom_filter.cc

Index: chrome/browser/safe_browsing/bloom_filter.cc
diff --git a/chrome/browser/safe_browsing/bloom_filter.cc b/chrome/browser/safe_browsing/bloom_filter.cc
index 859d32011dabfd1a94857fcb2faca3bb4a382808..747a86f62b1715d36b3e76c726a4bf2fe0a1faba 100644
--- a/chrome/browser/safe_browsing/bloom_filter.cc
+++ b/chrome/browser/safe_browsing/bloom_filter.cc
@@ -30,6 +30,24 @@ uint32 HashMix(BloomFilter::HashKey hash_key, uint32 c) {
   return c;
 }
 
+uint32 CalculateChecksum(const char* data, size_t size,
+                         const BloomFilter::HashKeys& keys) {
+  uint32 checksum = 0;
+  for (size_t i = 0; i < keys.size(); ++i) {
+    checksum ^= static_cast<uint32>(keys[i]);
+    checksum ^= static_cast<uint32>(keys[i]>>32);
+  }
+
+  // Watch out for sign-extension in the calculation.
+  // TODO(shess): Bit manipulation of signed chars is insane.
+  const unsigned char* udata = reinterpret_cast<const unsigned char*>(data);
+  for (size_t i = 0; i < size; ++i) {
+    checksum ^= udata[i] << ((i * 8) % 32);
+  }
+
+  return checksum;
+}
+
 }  // namespace
 
 // static
@@ -56,6 +74,7 @@ BloomFilter::BloomFilter(int bit_size) {
   DCHECK_LE(bit_size, bit_size_);  // strictly more bits.
   data_.reset(new char[byte_size_]);
   memset(data_.get(), 0, byte_size_);
+  checksum_ = CalculateChecksum(data_.get(), 0, hash_keys_);
 }
 
 BloomFilter::BloomFilter(char* data, int size, const HashKeys& keys)
@@ -63,6 +82,7 @@ BloomFilter::BloomFilter(char* data, int size, const HashKeys& keys)
   byte_size_ = size;
   bit_size_ = byte_size_ * 8;
   data_.reset(data);
+  checksum_ = CalculateChecksum(data_.get(), byte_size_, hash_keys_);
 }
 
 BloomFilter::~BloomFilter() {
@@ -72,7 +92,12 @@ void BloomFilter::Insert(SBPrefix hash) {
   uint32 hash_uint32 = static_cast<uint32>(hash);
   for (size_t i = 0; i < hash_keys_.size(); ++i) {
     uint32 index = HashMix(hash_keys_[i], hash_uint32) % bit_size_;
-    data_[index / 8] |= 1 << (index % 8);
+    // Checksum needs to track the transition.  Remove the if() when
+    // the checksum goes away.
+    if (!(data_[index / 8] & (1 << (index % 8)))) {
+      data_[index / 8] |= 1 << (index % 8);
+      checksum_ ^= 1u << (index % 32);
+    }
   }
 }
 
@@ -190,3 +215,9 @@ bool BloomFilter::WriteFile(const FilePath& filter_name) const {
 
   return true;
 }
+
+bool BloomFilter::CheckChecksum() const {
+  const uint32 checksum = CalculateChecksum(data_.get(), byte_size_,
+                                            hash_keys_);
+  return checksum_ == checksum;
+}