Fix segfault with sparse encoding

The native code that underlies `Resources#getIdentifier()` did not take
sparse encoding into account when calculating offsets, resulting in
either garbage or SEGV_MAPERR whenever a resource is first encountered
in a sparsely encoded configuration.

Bug: 197976367
Test: atest libandroidfw_tests --host
Change-Id: Ib7550fe2e05005550f59129a06be5712b74bc9c8

libs/androidfw/LoadedArsc.cpp

@@ -384,7 +384,16 @@ base::expected<uint32_t, NullOrIOError> LoadedPackage::FindEntryByName(
         return base::unexpected(IOError::PAGES_MISSING);
       }
 
-      auto offset = dtohl(entry_offset_ptr.value());
+      uint32_t offset;
+      uint16_t res_idx;
+      if (type->flags & ResTable_type::FLAG_SPARSE) {
+        auto sparse_entry = entry_offset_ptr.convert<ResTable_sparseTypeEntry>();
+        offset = dtohs(sparse_entry->offset) * 4u;
+        res_idx  = dtohs(sparse_entry->idx);
+      } else {
+        offset = dtohl(entry_offset_ptr.value());
+        res_idx = entry_idx;
+      }
       if (offset != ResTable_type::NO_ENTRY) {
         auto entry = type.offset(dtohl(type->entriesStart) + offset).convert<ResTable_entry>();
         if (!entry) {
@@ -394,7 +403,7 @@ base::expected<uint32_t, NullOrIOError> LoadedPackage::FindEntryByName(
         if (dtohl(entry->key.index) == static_cast<uint32_t>(*key_idx)) {
           // The package ID will be overridden by the caller (due to runtime assignment of package
           // IDs for shared libraries).
-          return make_resid(0x00, *type_idx + type_id_offset_ + 1, entry_idx);
+          return make_resid(0x00, *type_idx + type_id_offset_ + 1, res_idx);
         }
       }
     }

libs/androidfw/tests/LoadedArsc_test.cpp

@@ -95,6 +95,38 @@ TEST(LoadedArscTest, LoadSparseEntryApp) {
   ASSERT_TRUE(LoadedPackage::GetEntry(type.type, entry_index).has_value());
 }
 
+TEST(LoadedArscTest, FindSparseEntryApp) {
+  std::string contents;
+  ASSERT_TRUE(ReadFileFromZipToString(GetTestDataPath() + "/sparse/sparse.apk", "resources.arsc",
+                                      &contents));
+
+  std::unique_ptr<const LoadedArsc> loaded_arsc = LoadedArsc::Load(contents.data(),
+                                                                   contents.length());
+  ASSERT_THAT(loaded_arsc, NotNull());
+
+  const LoadedPackage* package =
+      loaded_arsc->GetPackageById(get_package_id(sparse::R::string::only_v26));
+  ASSERT_THAT(package, NotNull());
+
+  const uint8_t type_index = get_type_id(sparse::R::string::only_v26) - 1;
+  const uint16_t entry_index = get_entry_id(sparse::R::string::only_v26);
+
+  const TypeSpec* type_spec = package->GetTypeSpecByTypeIndex(type_index);
+  ASSERT_THAT(type_spec, NotNull());
+  ASSERT_THAT(type_spec->type_entries.size(), Ge(1u));
+
+  // Ensure that AAPT2 sparsely encoded the v26 config as expected.
+  auto type_entry = std::find_if(
+    type_spec->type_entries.begin(), type_spec->type_entries.end(),
+    [](const TypeSpec::TypeEntry& x) { return x.config.sdkVersion == 26; });
+  ASSERT_NE(type_entry, type_spec->type_entries.end());
+  ASSERT_NE(type_entry->type->flags & ResTable_type::FLAG_SPARSE, 0);
+
+  // Test fetching a resource with only sparsely encoded configs by name.
+  auto id = package->FindEntryByName(u"string", u"only_v26");
+  ASSERT_EQ(id.value(), fix_package_id(sparse::R::string::only_v26, 0));
+}
+
 TEST(LoadedArscTest, LoadSharedLibrary) {
   std::string contents;
   ASSERT_TRUE(ReadFileFromZipToString(GetTestDataPath() + "/lib_one/lib_one.apk", "resources.arsc",

libs/androidfw/tests/data/sparse/R.h

@@ -27,21 +27,22 @@ struct R {
   struct integer {
     enum : uint32_t {
       foo_0 = 0x7f010000,
-      foo_1 = 0x7f010000,
-      foo_2 = 0x7f010000,
-      foo_3 = 0x7f010000,
-      foo_4 = 0x7f010000,
-      foo_5 = 0x7f010000,
-      foo_6 = 0x7f010000,
-      foo_7 = 0x7f010000,
-      foo_8 = 0x7f010000,
-      foo_9 = 0x7f010000,
+      foo_1 = 0x7f010001,
+      foo_2 = 0x7f010002,
+      foo_3 = 0x7f010003,
+      foo_4 = 0x7f010004,
+      foo_5 = 0x7f010005,
+      foo_6 = 0x7f010006,
+      foo_7 = 0x7f010007,
+      foo_8 = 0x7f010008,
+      foo_9 = 0x7f010009,
     };
   };
 
   struct string {
     enum : uint32_t {
       foo_999 = 0x7f0203e7,
+      only_v26 = 0x7f0203e8
     };
   };
 };

libs/androidfw/tests/data/sparse/gen_strings.sh

@@ -14,5 +14,7 @@ do
     fi
 done
 echo "</resources>" >> $OUTPUT_default
+
+echo "  <string name=\"only_v26\">only v26</string>" >> $OUTPUT_v26
 echo "</resources>" >> $OUTPUT_v26
 

libs/androidfw/tests/data/sparse/res/values-v26/strings.xml

@@ -333,4 +333,5 @@
   <string name="foo_993">9930</string>
   <string name="foo_996">9960</string>
   <string name="foo_999">9990</string>
+  <string name="only_v26">only v26</string>
 </resources>