[flags] Improve flag to verify and create snapshot checksums

- Rename --skip-snapshot-checksum to --verify-snapshot-checksum to
  avoid double negations in most of the code
- Conditionally create and verify checksums in SerializedCodeData
- Remove unused Deserializer::GetChecksum

Bug: chromium:1270752
Change-Id: I8360e0dd5f25dac68bf68909155771b302184a4b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3284883
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77925}

src/flags/flag-definitions.h

@@ -1754,8 +1754,9 @@ DEFINE_BOOL(rcs_cpu_time, false,
 DEFINE_IMPLICATION(rcs_cpu_time, rcs)
 
 // snapshot-common.cc
-DEFINE_BOOL(skip_snapshot_checksum, false,
-            "Skip snapshot checksum calculation when deserializing an Isolate.")
+DEFINE_BOOL(verify_snapshot_checksum, true,
+            "Verify snapshot checksums when deserializing snapshots. Enable "
+            "checksum creation and verification for code caches.")
 DEFINE_BOOL(profile_deserialization, false,
             "Print the time it takes to deserialize the snapshot.")
 DEFINE_BOOL(serialization_statistics, false,

src/snapshot/code-serializer.cc

@@ -557,8 +557,9 @@ SerializedCodeData::SerializedCodeData(const std::vector<byte>* payload,
   // Copy serialized data.
   CopyBytes(data_ + kHeaderSize, payload->data(),
             static_cast<size_t>(payload->size()));
-
-  SetHeaderValue(kChecksumOffset, Checksum(ChecksummedContent()));
+  uint32_t checksum =
+      FLAG_verify_snapshot_checksum ? Checksum(ChecksummedContent()) : 0;
+  SetHeaderValue(kChecksumOffset, checksum);
 }
 
 SerializedCodeSanityCheckResult SerializedCodeData::SanityCheck(
@@ -587,21 +588,23 @@ SerializedCodeSanityCheckResult SerializedCodeData::SanityCheckWithoutSource()
     return SerializedCodeSanityCheckResult::kMagicNumberMismatch;
   }
   uint32_t version_hash = GetHeaderValue(kVersionHashOffset);
-  uint32_t flags_hash = GetHeaderValue(kFlagHashOffset);
-  uint32_t payload_length = GetHeaderValue(kPayloadLengthOffset);
-  uint32_t c = GetHeaderValue(kChecksumOffset);
   if (version_hash != Version::Hash()) {
     return SerializedCodeSanityCheckResult::kVersionMismatch;
   }
+  uint32_t flags_hash = GetHeaderValue(kFlagHashOffset);
   if (flags_hash != FlagList::Hash()) {
     return SerializedCodeSanityCheckResult::kFlagsMismatch;
   }
+  uint32_t payload_length = GetHeaderValue(kPayloadLengthOffset);
   uint32_t max_payload_length = this->size_ - kHeaderSize;
   if (payload_length > max_payload_length) {
     return SerializedCodeSanityCheckResult::kLengthMismatch;
   }
-  if (Checksum(ChecksummedContent()) != c) {
-    return SerializedCodeSanityCheckResult::kChecksumMismatch;
+  if (FLAG_verify_snapshot_checksum) {
+    uint32_t checksum = GetHeaderValue(kChecksumOffset);
+    if (Checksum(ChecksummedContent()) != checksum) {
+      return SerializedCodeSanityCheckResult::kChecksumMismatch;
+    }
   }
   return SerializedCodeSanityCheckResult::kSuccess;
 }

src/snapshot/deserializer.h

@@ -49,8 +49,6 @@ class Deserializer : public SerializerDeserializer {
   Deserializer(const Deserializer&) = delete;
   Deserializer& operator=(const Deserializer&) = delete;
 
-  uint32_t GetChecksum() const { return source_.GetChecksum(); }
-
  protected:
   // Create a deserializer from a snapshot byte source.
   Deserializer(IsolateT* isolate, base::Vector<const byte> payload,

src/snapshot/snapshot-source-sink.h

@@ -103,10 +103,6 @@ class SnapshotByteSource final {
   int position() { return position_; }
   void set_position(int position) { position_ = position; }
 
-  uint32_t GetChecksum() const {
-    return Checksum(base::Vector<const byte>(data_, length_));
-  }
-
  private:
   const byte* data_;
   int length_;

src/snapshot/snapshot.cc

@@ -168,9 +168,7 @@ bool Snapshot::Initialize(Isolate* isolate) {
 
   const v8::StartupData* blob = isolate->snapshot_blob();
   SnapshotImpl::CheckVersion(blob);
-  if (!FLAG_skip_snapshot_checksum) {
-    CHECK(VerifyChecksum(blob));
-  }
+  if (FLAG_verify_snapshot_checksum) CHECK(VerifyChecksum(blob));
   base::Vector<const byte> startup_data =
       SnapshotImpl::ExtractStartupData(blob);
   base::Vector<const byte> read_only_data =