Merge "Fix computation of 'entry_length' in AAPT2 container format"

tools/aapt2/format/Container.cpp

@@ -30,6 +30,7 @@ namespace aapt {
 
 constexpr const static uint32_t kContainerFormatMagic = 0x54504141u;
 constexpr const static uint32_t kContainerFormatVersion = 1u;
+constexpr const static size_t kPaddingAlignment = 4u;
 
 ContainerWriter::ContainerWriter(ZeroCopyOutputStream* out, size_t entry_count)
     : out_(out), total_entry_count_(entry_count), current_entry_count_(0u) {
@@ -49,11 +50,17 @@ ContainerWriter::ContainerWriter(ZeroCopyOutputStream* out, size_t entry_count)
   }
 }
 
-inline static void WritePadding(int padding, CodedOutputStream* out) {
-  if (padding < 4) {
-    const uint32_t zero = 0u;
-    out->WriteRaw(&zero, padding);
-  }
+inline static size_t CalculatePaddingForAlignment(size_t size) {
+  size_t overage = size % kPaddingAlignment;
+  return overage == 0 ? 0 : kPaddingAlignment - overage;
+}
+
+inline static void WritePadding(size_t padding, CodedOutputStream* out) {
+  CHECK(padding < kPaddingAlignment);
+  const uint32_t zero = 0u;
+  static_assert(sizeof(zero) >= kPaddingAlignment, "Not enough source bytes for padding");
+
+  out->WriteRaw(&zero, padding);
 }
 
 bool ContainerWriter::AddResTableEntry(const pb::ResourceTable& table) {
@@ -70,7 +77,7 @@ bool ContainerWriter::AddResTableEntry(const pb::ResourceTable& table) {
 
   // Write the aligned size.
   const ::google::protobuf::uint64 size = table.ByteSize();
-  const int padding = 4 - (size % 4);
+  const int padding = CalculatePaddingForAlignment(size);
   coded_out.WriteLittleEndian64(size);
 
   // Write the table.
@@ -103,9 +110,9 @@ bool ContainerWriter::AddResFileEntry(const pb::internal::CompiledFile& file,
 
   // Write the aligned size.
   const ::google::protobuf::uint32 header_size = file.ByteSize();
-  const int header_padding = 4 - (header_size % 4);
+  const int header_padding = CalculatePaddingForAlignment(header_size);
   const ::google::protobuf::uint64 data_size = in->TotalSize();
-  const int data_padding = 4 - (data_size % 4);
+  const int data_padding = CalculatePaddingForAlignment(data_size);
   coded_out.WriteLittleEndian64(kResFileEntryHeaderSize + header_size + header_padding + data_size +
                                 data_padding);
 

tools/aapt2/formats.md

@@ -23,7 +23,7 @@ boundary, so if a previous entry ends unaligned, padding must be inserted.
 | Size (in bytes) | Field          | Description                                                                                               |
 |:----------------|:---------------|:----------------------------------------------------------------------------------------------------------|
 | `4`             | `entry_type`   | The type of the entry. This can be one of two types: `RES_TABLE (0x00000000)` or `RES_FILE (0x00000001)`. |
-| `8`             | `entry_length` | The length of the data that follows.                                                                      |
+| `8`             | `entry_length` | The length of the data that follows.  Do not use if `entry_type` is `RES_FILE`; this value may be wrong.  |
 | `entry_length`  | `data`         | The payload. The contents of this varies based on the `entry_type`.                                       |
 
 If the `entry_type` is equal to `RES_TABLE (0x00000000)`, the `data` field contains a serialized
@@ -32,13 +32,14 @@ If the `entry_type` is equal to `RES_TABLE (0x00000000)`, the `data` field conta
 If the `entry_type` is equal to `RES_FILE (0x00000001)`, the `data` field contains the following:
 
 
-| Size (in bytes) | Field          | Description                                                                                               |
-|:----------------|:---------------|:----------------------------------------------------------------------------------------------------------|
-| `4`             | `header_size`  | The size of the `header` field.                                                                 |
-| `8`             | `data_size`    | The size of the `data` field.                                                                   |
-| `header_size`   | `header`       | The serialized Protobuf message [aapt.pb.internal.CompiledFile](ResourcesInternal.proto).       |
-| `x`             | `padding`      | Up to 4 bytes of zeros, if padding is necessary to align the `data` field on a 32-bit boundary. |
-| `data_size`     | `data`         | The payload, which is determined by the `type` field in the `aapt.pb.internal.CompiledFile`. This can be a PNG file, binary XML, or [aapt.pb.XmlNode](Resources.proto). |
+| Size (in bytes) | Field            | Description                                                                                               |
+|:----------------|:-----------------|:----------------------------------------------------------------------------------------------------------|
+| `4`             | `header_size`    | The size of the `header` field.                                                                           |
+| `8`             | `data_size`      | The size of the `data` field.                                                                             |
+| `header_size`   | `header`         | The serialized Protobuf message [aapt.pb.internal.CompiledFile](ResourcesInternal.proto).                 |
+| `x`             | `header_padding` | Up to 3 bytes of zeros, if padding is necessary to align the `data` field on a 32-bit boundary.           |
+| `data_size`     | `data`           | The payload, which is determined by the `type` field in the `aapt.pb.internal.CompiledFile`. This can be a PNG file, binary XML, or [aapt.pb.XmlNode](Resources.proto). |
+| `y`             | `data_padding`   | Up to 3 bytes of zeros, if `data_size` is not a multiple of 4.                                            |
 
 ## AAPT2 Static Library Format (extension `.sapk`)