[res] Zero big buffer memory on backing up

When a memory chunk gets returned into BigBuffer, it may have
some data written already. BigBuffer is supposed to give out
zeroed memory, so BackUp() needs to zero it as well

Bug: 336758568
Bug: 342579978
Test: unit test + bundletool on the attached aab
Flag: EXEMPT bugfix
Change-Id: I8ecd60e84dbe16570a92d82370d1633af72599c8

1 files changed, 18 insertions, 1 deletions

diff --git a/libs/androidfw/BigBuffer.cpp b/libs/androidfw/BigBuffer.cpp
index bedfc49a1b0d..43b56c32fb79 100644
--- a/libs/androidfw/BigBuffer.cpp
+++ b/libs/androidfw/BigBuffer.cpp
@@ -17,8 +17,8 @@
 #include <androidfw/BigBuffer.h>
 
 #include <algorithm>
+#include <iterator>
 #include <memory>
-#include <vector>
 
 #include "android-base/logging.h"
 
@@ -78,10 +78,27 @@ void* BigBuffer::NextBlock(size_t* out_size) {
 
 std::string BigBuffer::to_string() const {
   std::string result;
+  result.reserve(size_);
   for (const Block& block : blocks_) {
     result.append(block.buffer.get(), block.buffer.get() + block.size);
   }
   return result;
 }
 
+void BigBuffer::AppendBuffer(BigBuffer&& buffer) {
+  std::move(buffer.blocks_.begin(), buffer.blocks_.end(), std::back_inserter(blocks_));
+  size_ += buffer.size_;
+  buffer.blocks_.clear();
+  buffer.size_ = 0;
+}
+
+void BigBuffer::BackUp(size_t count) {
+  Block& block = blocks_.back();
+  block.size -= count;
+  size_ -= count;
+  // BigBuffer is supposed to always give zeroed memory, but backing up usually means
+  // something has been already written into the block. Erase it.
+  std::fill_n(block.buffer.get() + block.size, count, 0);
+}
+
 }  // namespace android