Metal Library Archive

MetalLibraryArchive is a product of reverse-engineering Apple’s metallib file format.

You can use MetalLibraryArchive to get the library type, target platform, Metal functions, etc., from a metallib file.

The extracted information of a Metal function includes:

• Function name.
• Function type – vertex, fragment, kernel, extern, etc.
• Metal Shading Language version of the function.
• Bitcode of the function which can be converted into human-readable LLVM assembly language using llvm-dis.
• Source code of the function if the metallib is configured to include source code.

? Usage

Explorer App

An executable target called “Explorer” is included in the package. “Explorer” is a GUI app which can open, unpack and disassemble (with the help of llvm-dis) metallib files.

Note llvm-dis is not included, you can get a copy of the binary at https://github.com/llvm/llvm-project/releases

Use the “Disassembler” menu in the app to locate the llvm-dis executable file.

Library

You can also use MetalLibraryArchive as a library:

import MetalLibraryArchive

let archive = try Archive(data: Data(contentsOf: metallibURL))
let libraryType = archive.libraryType
let functions = archive.functions

? Metal Library Archive Binary Layout

Header

Function List

The number of tag groups equals to the number of functions.

Tag Group

Tag

Function Information Tags

Content of the TESS tag:

// Patch types:
//   - triangle: 1
//   - quad: 2

let content: UInt8 = controlPointCount << 2 | patchType

Public Metadata

Contains information about function constants, tessellation patches, return types, etc.

Tags: CNST, VATT, VATY, RETR, ARGR, etc.

Private Metadata

Contains pathes to the shader source (DEBI tag) and .air (DEPF tag) files.

Header Extension

Only exists if FunctionListOffset + FunctionListSize + 4 != PublicMetadataOffset

Header Extension Tags

Dynamic Header Section Tags

Variable List & Imported Symbol List

Variable list and imported symbol list have structures that are similar to that of the function list.

Embeded Source Code Section

Only exists if the metallib build process is configured to include source code.

Note “Working directory” only exists in HSRD.

Note SARC tag uses 4-bytes (UInt32) content size.

Content of the SARC tag:

❤️ Contributing

If you think there’s a mistake, please open an issue. You can also choose to open a pull request with the failure test included.

? The Story

This project would not have started without zhuowei’s research that revealed the basic binary layout of a metallib file, the function list as well as the bitcode section. Thanks, @zhuowei!

What the assembly can tell

I tried to continue the research to get a complete structure of the metallib file, but found it too hard to move forward based on guesswork alone. So I turned my attention to the Metal.framework hoping to find out how the framework loads a metallib file. Fortunately, it’s not too hard after dragging Metal.framework/Metal to Hopper Disassembler.

Metal.framework uses MTLLibraryDataWithArchive::parseArchiveSync(...) to load metallib files. There is a lot of information hidden in the assembly of MTLLibraryDataWithArchive. For example:

• The file starts with 0x424c544d(MTLB); The size of the file is recorded at offset 0x10.

  int __ZN25MTLLibraryDataWithArchive16parseArchiveSyncEPP7NSErrorb(void * * arg0, bool arg1) {
    r12 = rdx;
    r14 = arg1;
    r13 = arg0;
    (*(*arg0 + 0xb8))(arg0, 0x0); //LibraryWithFile::setPosition(...)
    r15 = r13 + 0x78;
    rbx = (*(*r13 + 0xc0))(r13, r15, 0x58);  //LibraryWithFile::readBytes(...)
    rax = *r13;
    rax = (*(rax + 0xc8))(r13); //LibraryWithFile::getFileSize(...)
    
    // 0x424c544d - MTLB
    // File size field offset: 0x88 - 0x78 = 0x10
    if (((rbx != 0x58) || (*(int32_t *)(r13 + 0x78) != 0x424c544d)) || (*(r13 + 0x88) != rax)) goto loc_6a65b;
    
    ...
    
    loc_6a65b:
    if (r14 == 0x0) goto loc_6a6c5;
  
    loc_6a660:
    rdx = @"Invalid library file";
     
    ...
  }

• A Int16 value at offset 0x4 is related to the target platform.

  loc_6a610:
  // 0x7c - 0x78 = 0x4
  rax = *(int16_t *)(r13 + 0x7c) & 0xffff;
  
  if ((rax >= 0x0) || (r12 == 0x0)) goto loc_6a6ea;
  
  loc_6a627:
  if (r14 == 0x0) goto loc_6a6c5;
  
  loc_6a630:
  rdx = @"This library format is not supported on this platform (or was built with an old version of the tools)";
  goto loc_6a689;

• There is a “Header Extension Section” that contains information about “Dynamic Header Section”, “Imported Symbol List” and “Variable List”:

  if (MTLLibraryDataWithArchive::parseHeaderExtension(r13, r13 + 0x100, r14) != 0x0) {
      if (MTLLibraryDataWithArchive::parseDynamicHeaderSection(r13) != 0x0) {
          if (MTLLibraryDataWithArchive::parseImportedSymbolListSection(r13) != 0x0) {
              rax = MTLLibraryDataWithArchive::parseVariableListSection(r13);
          } else {
              rax = 0x0;
          }
      } else {
          rax = 0x0;
      }
  } else {
      rax = 0x0;
  }

• The bitcode is validated using SHA256.

  int ____ZN25MTLLibraryDataWithArchive15validateBitCodeEmmPK6NSDataRK12MTLUINT256_t_block_invoke(int arg0) {
      ...
      CC_SHA256_Init(&var_B0);
      CC_SHA256_Update(&var_B0, r14, *(int32_t *)(r15 + 0x38));
      CC_SHA256_Final(&var_48, &var_B0);
      ...
  }

• A lot of FourCC codes:

  // 0x454e4454 - ENDT
  loc_6a8bc:
  if (rax == 0x454e4454) goto loc_6a871;
  ...
  // 0x54595045 - TYPE
  loc_6a984:
  if (rax == 0x54595045) goto loc_6a9dc;
  ...
  // 0x44594e4c - DYNL
  loc_6ae5b:
  if (rax != 0x44594e4c) goto loc_6b002;
  ...
  // 0x56455253 - VERS
  loc_6b731:
  if (rax == 0x56455253) goto loc_6b81c;

After some digging around I was able to get an overview of the metallib file’s structure:

• The file has a 88 bytes header that contains file version, target platform, library type, section indices, etc.

• There are 4 sections recorded in the file header:

  • Function list

  • Public metadata

  • Private metadata

  • Bitcode modules

  Each section is recorded with an offset and a size. This means sections can be non-contiguous, which allows Apple to introduce new sections in between without breaking the compatibility. And Apple did that exactly for the “header extension” section – it lies between the function list and the public metadata section.

• Most of the sections (except the bitcode section) resemble a “tag” based structure:

  • FourCharCode is used as the tag’s name/type.

  • An UInt16 (in most cases) value of size follows the tag’s name.

    The source archive data tag SARC unsurprisingly uses an UInt32 value for its size – a source archive can easily exceed 65KB.

  • Tags are grouped:

    • Each group represents a set of properties of an item.

    • Tag group ends with an ENDT tag.

TDG – “Test Driven Guessing”

Next, I need to figure out what information each tag/field holds. This can be hard to get from the assembly of the Metal.framework because:

• Some fields may be designed purely for tooling or debugging, so MTLLibraryDataWithArchive may just ignore them.

• The assembly is platform dependent. For example, the iOS version of MTLLibraryDataWithArchive may only check whether the metallib is built for iOS and cannot tell if the library is built for macOS.

• Some fields are just hard to analyze and follow. Examples:

  • There are 3 offsets in the OFFT tag of the function, where are they pointing to? and how are they finally used?

  • What are the possible values of the function type? What does each value mean?

It seems that the quickest way to get this information is through experiments.

I started by manually compiling metal files with different shaders, options and SDKs, then inspecting each field I was interested in. My desktop was quickly flooded with metallib files and HexFiend windows, but I didn’t find much useful information. I need something that can automatically build metallib and presents me only the field that I’m interested in.

I came up with the “Test Driven Guessing”:

1. Write a metallib parser based on the binary structure overview at hand.

2. In the parser, log the value of a field/tag (or some related fields) that is currently unknown.

3. Create tests that produce metallib files using different kinds of shaders and compile options that may affect the value of the field, and use the parser to parse the file data.

4. Run tests and analyze the log to make hypotheses.

5. Update the parser based on hypotheses.

6. Run tests again to verify.

After a few rounds, I was able to get the function type table, target OS table, and the meaning of 3 offsets in the OFFT tag.

I also found a few things interesting in this process:

• Metal does not support watchOS, however it is possible to build a metallib targeting watchOS. And Apple does include some metallibs in the watchOS SDK. (e.g. Xcode.app/Contents/Developer/Platforms/WatchOS.platform/Library/Developer/CoreSimulator/Profiles/Runtimes/watchOS.simruntime/Contents/Resources/RuntimeRoot/System/Library/Frameworks/CoreImage.framework/ci_filters.metallib)

• Empty metallibs targeting old versions of iOS are mistakenly marked as targeting macOS.

• I cannot build a metallib that has the target OS value 0x85. At first I thought it might be reserved for the concealed realityOS, but later found out it is more likely for the bridgeOS.

Updates

Mar 31, 2022

The air-lld (Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/metal/ios/bin/air-lld) also provides a lot of information about how the metallib file is built. Some section names and descriptions are updated.

int __ZN4llvm3air20MetalLibObjectWriter5writeEv() {
    r14 = rdi;
    rax = llvm::air::MetalLibObjectWriter::writeHeader();
    if (rax != 0x0) goto loc_1000351b9;

loc_100035135:
    rax = llvm::air::MetalLibObjectWriter::writeFunctionList();
    if (rax != 0x0) goto loc_1000351b9;

loc_100035141:
    rax = llvm::air::MetalLibObjectWriter::writeHeaderExtension();
    if (rax != 0x0) goto loc_1000351b9;

loc_10003514d:
    rax = llvm::air::MetalLibObjectWriter::writePublicMetadata();
    if (rax != 0x0) goto loc_1000351b9;

loc_100035159:
    rax = llvm::air::MetalLibObjectWriter::writePrivateMetadata();
    if (rax != 0x0) goto loc_1000351b9;

loc_100035165:
    rax = llvm::air::MetalLibObjectWriter::writeModuleList();
    if (rax != 0x0) goto loc_1000351b9;

loc_100035171:
    rax = llvm::air::MetalLibObjectWriter::writeSources();
    if (rax != 0x0) goto loc_1000351b9;

loc_10003517d:
    rax = llvm::air::MetalLibObjectWriter::writeDynamicHeader();
    if (rax != 0x0) goto loc_1000351b9;

loc_100035189:
    rax = llvm::air::MetalLibObjectWriter::writeVariableList();
    if (rax != 0x0) goto loc_1000351b9;

loc_100035195:
    rax = llvm::air::MetalLibObjectWriter::writeImportedSymbolList();
    if (rax != 0x0) goto loc_1000351b9;

loc_1000351a1:
    rax = llvm::air::MetalLibObjectWriter::computeUUID();
    if (rax != 0x0) goto loc_1000351b9;

loc_1000351ad:
    rax = llvm::air::MetalLibObjectWriter::backpatchAllLocations();
    if (rax == 0x0) goto loc_1000351c2;

loc_1000351b9:
    rbx = rax;
    goto loc_1000351bb;

loc_1000351bb:
    rax = rbx;
    return rax;

loc_1000351c2:
    rbx = 0x0;
    std::__1::system_category();
    goto loc_1000351bb;
}

GitHub

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