androidfw: Add support for compact resource entries

Bug: 237583012

Given the large number of simple resources such as strings in
Android resources, their ResTable_entry and Res_value can be
encoded together in a compact way.  This allows a significant
saving in both storage and memory footprint.

The basic observations for simple resources are:

* ResTable_entry.size will always be sizeof(ResTable_entry)
  unless it's a complex entry

* ResTable_entry.key is unlikely to exceed 16-bit

* ResTable_entry.flags only uses 3 bits for now

* Res_value.size will always be sizeof(Res_value)

Given the above, we could well encode the information into
a compact/compatible structure.

  struct compact {
      uint16_t key;
      uint16_t flags;
      uint32_t data;
  };

The layout of this structure will allow maximum backward
compatibility. e.g. the flags will be at the same offset,
and a

  `dtohs((ResTable_entry *)entry->flags) & FLAG_COMPACT`

would tell if this entry is a compact one or not. For a
compact entry:

  struct compact *entry;

  entry_size  == sizeof(*entry)
  entry_key   == static_cast<uint32_t>(dtohs(entry->key))
  entry_flags == dtohs(entry->flags) & 0xff	// low 8-bit
  data_type   == dtohs(entry->flags) >> 8	// high 8-bit
  data_size   == sizeof(Res_value)
  data_value  == dtohl(entry->data)

To allow minimum code change and backward compatibility,
we change 'struct ResTable_entry' to 'union ResTable_entry',
with an anonymous structure inside that's fully backward
compatible. Thus, any existing reference such as:

  ResTable_entry *entry = ...
  if (dtohs(entry->flags) & ResTable_entry::FLAG_COMPLEX) ...

would still work.

However, special care needs to be taken after an entry is
obtained, and when value needs to be extracted.

A compact entry will not encode a complex value, and hence
complex entries/values are handled the same way.

Change-Id: I15d97a4f5e85fab28c075496f7f0cf6b1fcd73e3

1 files changed, 12 insertions, 26 deletions

diff --git a/libs/androidfw/AssetManager2.cpp b/libs/androidfw/AssetManager2.cpp
index 1381bdd6a50d..06ffb72d183b 100644
--- a/libs/androidfw/AssetManager2.cpp
+++ b/libs/androidfw/AssetManager2.cpp
@@ -43,28 +43,19 @@ namespace {
 
 using EntryValue = std::variant<Res_value, incfs::verified_map_ptr<ResTable_map_entry>>;
 
+/* NOTE: table_entry has been verified in LoadedPackage::GetEntryFromOffset(),
+ * and so access to ->value() and ->map_entry() are safe here
+ */
 base::expected<EntryValue, IOError> GetEntryValue(
     incfs::verified_map_ptr<ResTable_entry> table_entry) {
-  const uint16_t entry_size = dtohs(table_entry->size);
+  const uint16_t entry_size = table_entry->size();
 
   // Check if the entry represents a bag value.
-  if (entry_size >= sizeof(ResTable_map_entry) &&
-      (dtohs(table_entry->flags) & ResTable_entry::FLAG_COMPLEX)) {
-    const auto map_entry = table_entry.convert<ResTable_map_entry>();
-    if (!map_entry) {
-      return base::unexpected(IOError::PAGES_MISSING);
-    }
-    return map_entry.verified();
+  if (entry_size >= sizeof(ResTable_map_entry) && table_entry->is_complex()) {
+    return table_entry.convert<ResTable_map_entry>().verified();
   }
 
-  // The entry represents a non-bag value.
-  const auto entry_value = table_entry.offset(entry_size).convert<Res_value>();
-  if (!entry_value) {
-    return base::unexpected(IOError::PAGES_MISSING);
-  }
-  Res_value value;
-  value.copyFrom_dtoh(entry_value.value());
-  return value;
+  return table_entry->value();
 }
 
 } // namespace
@@ -814,17 +805,12 @@ base::expected<FindEntryResult, NullOrIOError> AssetManager2::FindEntryInternal(
     return base::unexpected(std::nullopt);
   }
 
-  auto best_entry_result = LoadedPackage::GetEntryFromOffset(best_type, best_offset);
-  if (!best_entry_result.has_value()) {
-    return base::unexpected(best_entry_result.error());
-  }
-
-  const incfs::map_ptr<ResTable_entry> best_entry = *best_entry_result;
-  if (!best_entry) {
-    return base::unexpected(IOError::PAGES_MISSING);
+  auto best_entry_verified = LoadedPackage::GetEntryFromOffset(best_type, best_offset);
+  if (!best_entry_verified.has_value()) {
+    return base::unexpected(best_entry_verified.error());
   }
 
-  const auto entry = GetEntryValue(best_entry.verified());
+  const auto entry = GetEntryValue(*best_entry_verified);
   if (!entry.has_value()) {
     return base::unexpected(entry.error());
   }
@@ -837,7 +823,7 @@ base::expected<FindEntryResult, NullOrIOError> AssetManager2::FindEntryInternal(
     .package_name = &best_package->GetPackageName(),
     .type_string_ref = StringPoolRef(best_package->GetTypeStringPool(), best_type->id - 1),
     .entry_string_ref = StringPoolRef(best_package->GetKeyStringPool(),
-                                      best_entry->key.index),
+                                      (*best_entry_verified)->key()),
     .dynamic_ref_table = package_group.dynamic_ref_table.get(),
   };
 }