Create a minimal testcase to reproduce silent verity corruption

b/186196758 is triggered by the following sequence of events:

1. update_engine finish writing all install ops, emits kEndOfInstall
label
2. update_engine opens cow in append mode, invokes
InitialiazeAppend(kEndOfInstall)
3. update_engine writes verity data, invokes SnapshotWriter::Finalize()
4. update_engine repeats step 2, but does not write any data after
opening SnapshotWriter. Instead, it reads verity and make sure the hash
matches what's specified in OTA payload.
5. Reboot device, verity data corrupted, device rollback to slot _a.

This is because, during step 4, when calling
InitializeAppend(kEndOfInstall), the SnapshotWriter only reads up to the
given label. But OpenReader() completely disregards the resume label and
reads all ops. Therefore, update_engine sees the verity data, and
determines that everything is fine. However, when calling
SnapshotWriter::Finalize(), data after resume label are discarded,
therefore verity data is gone.

Test: th
Bug: 186196758

Change-Id: I0166271b64eb7b574434d617ce730f345ca93ff1

1 files changed, 6 insertions, 1 deletions

diff --git a/payload_consumer/filesystem_verifier_action_unittest.cc b/payload_consumer/filesystem_verifier_action_unittest.cc
index d2a015d8..586662d9 100644
--- a/payload_consumer/filesystem_verifier_action_unittest.cc
+++ b/payload_consumer/filesystem_verifier_action_unittest.cc
@@ -488,7 +488,6 @@ TEST_F(FilesystemVerifierActionTest, RunWithVABCNoVerity) {
 }
 
 TEST_F(FilesystemVerifierActionTest, ReadAfterWrite) {
-  constexpr auto BLOCK_SIZE = 4096;
   ScopedTempFile cow_device_file("cow_device.XXXXXX", true);
   android::snapshot::CompressedSnapshotWriter snapshot_writer{
       {.block_size = BLOCK_SIZE}};
@@ -507,6 +506,12 @@ TEST_F(FilesystemVerifierActionTest, ReadAfterWrite) {
   ASSERT_TRUE(snapshot_writer.Finalize());
   cow_reader = snapshot_writer.OpenReader();
   ASSERT_NE(cow_reader, nullptr);
+  std::vector<unsigned char> read_back;
+  read_back.resize(buffer.size());
+  cow_reader->Seek(BLOCK_SIZE, SEEK_SET);
+  const auto bytes_read = cow_reader->Read(read_back.data(), read_back.size());
+  ASSERT_EQ((size_t)(bytes_read), BLOCK_SIZE);
+  ASSERT_EQ(read_back, buffer);
 }
 
 }  // namespace chromeos_update_engine