[wasm] Webcil-in-WebAssembly (#85932)

Define a WebAssembly module wrapper for Webcil assemblies.
Contributes to #80807 

### Why

In some settings serving `application/octet-stream` data, or files with weird extensions will trigger firewalls or AV tools.  But let's assume that if you're interested in deploying a .NET WebAssembly app, you're in an environment that can at least serve WebAssembly modules.

### How

Essentially we serve this WebAssembly module:

```wat
(module
  (data "\0f\00\00\00") ;; data segment 0: payload size
  (data "webcil Payload\cc")  ;; data segment 1: webcil payload
  (memory (import "webcil" "memory") 1)
  (global (export "webcilVersion") i32 (i32.const 0))
  (func (export "getWebcilSize") (param $destPtr i32) (result)
    local.get $destPtr
    i32.const 0
    i32.const 4
    memory.init 0)
  (func (export "getWebcilPayload") (param $d i32) (param $n i32) (result)
    local.get $d
    i32.const 0
    local.get $n
    memory.init 1))
```

The module exports two WebAssembly functions `getWebcilSize` and `getWebcilPayload` that write some bytes (being the size or payload of the webcil assembly) to the linear memory at a given offset.  The module also exports the constant `webcilVersion` to version the wrapper format.

So a runtime or tool that wants to consume the webcil module can do something like:

```js
const wasmModule = new WebAssembly.Module (...);
const wasmMemory = new WebAssembly.Memory ({initial: 1});
const wasmInstance =
      new WebAssembly.Instance(wasmModule, {webcil: {memory: wasmMemory}});
const { getWebcilPayload, webcilVersion, getWebcilSize } = wasmInstance.exports;
console.log (`Version ${webcilVersion.value}`);
getWebcilSize(0);
const size = new Int32Array (wasmMemory.buffer)[0]
console.log (`Size ${size}`);
console.log (new Uint8Array(wasmMemory.buffer).subarray(0, 20));
getWebcilPayload(4, size);
console.log (new Uint8Array(wasmMemory.buffer).subarray(0, 20));
```

### How (Part 2)

But actually, we will define the wrapper to consist of exactly 2 data segments in the WebAssembly data section: segment 0 is 4 bytes and encodes the webcil payload size; and segment 1 is of variable size and contains the webcil payload.

So to load a webcil-in-wasm module, the runtime gets the _raw bytes_ of the WebAssembly module (ie: without instantiating it), and parses it to find the data section, assert that there are 2 segments, ensure they're both passive, and get the data directly from segment 1.

---

* Add option to emit webcil inside a wasm module wrapper

* [mono][loader] implement a webcil-in-wasm reader

* reword WebcilWasmWrapper summary comment

* update the Webcil spec to include the WebAssembly wrapper module

* Adjust RVA map offsets to account for wasm prefix

   MonoImage:raw_data is used as a base when applying the RVA map to map virtual addresses to physical offsets in the assembly.  With webcil-in-wasm there's an extra wasm prefix before the webcil payload starts, so we need to account for this extra data when creating the mapping.

   An alternative is to compute the correct offsets as part of generating the webcil, but that would entangle the wasm module and the webcil payload.  The current (somewhat hacky approach) keeps them logically separate.

* Add a note about the rva mapping to the spec

* Serve webcil-in-wasm as .wasm

* remove old .webcil support from Sdk Pack Tasks

* Implement support for webcil in wasm in the managed WebcilReader

* align webcil payload to a 4-byte boundary within the wasm module

   Add padding to data segment 0 to ensure that data segment 1's payload (ie the webcil content itself) is 4-byte aligned

* assert that webcil raw data is 4-byte aligned

* add 4-byte alignment requirement to the webcil spec

* Don't modify MonoImageStorage:raw_data

   instead just keep track of the webcil offset in the MonoImageStorage.

   This introduces a situation where MonoImage:raw_data is different from MonoImageStorage:raw_data.  The one to use for accessing IL and metadata is MonoImage:raw_data.

   The storage pointer is just used by the image loading machinery

---------

Co-authored-by: Larry Ewing <[email protected]>

docs/design/mono/webcil.md

@@ -2,21 +2,83 @@
 
 ## Version
 
-This is version 0.0 of the Webcil format.
+This is version 0.0 of the Webcil payload format.
+This is version 0 of the WebAssembly module Webcil wrapper.
 
 ## Motivation
 
 When deploying the .NET runtime to the browser using WebAssembly, we have received some reports from
 customers that certain users are unable to use their apps because firewalls and anti-virus software
 may prevent browsers from downloading or caching assemblies with a .DLL extension and PE contents.
 
-This document defines a new container format for ECMA-335 assemblies
-that uses the `.webcil` extension and uses a new WebCIL container
-format.
+This document defines a new container format for ECMA-335 assemblies that uses the `.wasm` extension
+and uses a new WebCIL metadata payload format wrapped in a WebAssembly module.
 
 
 ## Specification
 
+### Webcil WebAssembly module
+
+Webcil consists of a standard [binary WebAssembly version 0 module](https://webassembly.github.io/spec/core/binary/index.html) containing the following WAT module:
+
+``` wat
+(module
+  (data "\0f\00\00\00") ;; data segment 0: payload size as a 4 byte LE uint32
+  (data "webcil Payload\cc")  ;; data segment 1: webcil payload
+  (memory (import "webcil" "memory") 1)
+  (global (export "webcilVersion") i32 (i32.const 0))
+  (func (export "getWebcilSize") (param $destPtr i32) (result)
+    local.get $destPtr
+    i32.const 0
+    i32.const 4
+    memory.init 0)
+  (func (export "getWebcilPayload") (param $d i32) (param $n i32) (result)
+    local.get $d
+    i32.const 0
+    local.get $n
+    memory.init 1))
+```
+
+That is, the module imports linear memory 0 and exports:
+* a global `i32` `webcilVersion` encoding the version of the WebAssembly wrapper (currently 0),
+* a function `getWebcilSize : i32 -> ()` that writes the size of the Webcil payload to the specified
+  address in linear memory as a `u32` (that is: 4 LE bytes).
+* a function `getWebcilPayload : i32 i32 -> ()` that writes `$n` bytes of the content of the Webcil
+  payload at the spcified address `$d` in linear memory.
+
+The Webcil payload size and payload content are stored in the data section of the WebAssembly module
+as passive data segments 0 and 1, respectively.  The module must not contain additional data
+segments. The module must store the payload size in data segment 0, and the payload content in data
+segment 1.
+
+The payload content in data segment 1 must be aligned on a 4-byte boundary within the web assembly
+module.  Additional trailing padding may be added to the data segment 0 content to correctly align
+data segment 1's content.
+
+(**Rationale**: With this wrapper it is possible to split the WebAssembly module into a *prefix*
+consisting of everything before the data section, the data section, and a *suffix* that consists of
+everything after the data section.  The prefix and suffix do not depend on the contents of the
+Webcil payload and a tool that generates Webcil files could simply emit the prefix and suffix from
+constant data.  The data section is the only variable content between different Webcil-encoded .NET
+assemblies)
+
+(**Rationale**: Encoding the payload in the data section in passive data segments with known indices
+allows a runtime that does not include a WebAssembly host or a runtime that does not wish to
+instantiate the WebAssembly module to extract the payload by traversing the WebAssembly module and
+locating the Webcil payload in the data section at segment 1.)
+
+(**Rationale**: The alignment requirement is due to ECMA-335 metadata requiring certain portions of
+the physical layout to be 4-byte aligned, for example ECMA-335 Section II.25.4 and II.25.4.5.
+Aligning the Webcil content within the wasm module allows tools that directly examine the wasm
+module without instantiating it to properly parse the ECMA-335 metadata in the Webcil payload.)
+
+(**Note**: the wrapper may be versioned independently of the payload.)
+
+
+### Webcil payload
+
+The webcil payload contains the ECMA-335 metadata, IL and resources comprising a .NET assembly.
+
 As our starting point we take section II.25.1 "Structure of the
 runtime file format" from ECMA-335 6th Edition.
 
@@ -40,12 +102,12 @@ A Webcil file follows a similar structure
 | CLI Data |
 | |
 
-## Webcil Headers
+### Webcil Headers
 
 The Webcil headers consist of a Webcil header followed by a sequence of section headers.
 (All multi-byte integers are in little endian format).
 
-### Webcil Header
+#### Webcil Header
 
 ``` c
 struct WebcilHeader {
@@ -75,11 +137,11 @@ The next pairs of integers are a subset of the PE Header data directory specifyi
 of the CLI header, as well as the directory entry for the PE debug directory.
 
 
-### Section header table
+#### Section header table
 
 Immediately following the Webcil header is a sequence (whose length is given by `coff_sections`
 above) of section headers giving their virtual address and virtual size, as well as the offset in
-the Webcil file and the size in the file.  This is a subset of the PE section header that includes
+the Webcil payload and the size in the file.  This is a subset of the PE section header that includes
 enough information to correctly interpret the RVAs from the webcil header and from the .NET
 metadata. Other information (such as the section names) are not included.
 
@@ -92,11 +154,13 @@ struct SectionHeader {
 };
 ```
 
-### Sections
+(**Note**: the `st_raw_data_ptr` member is an offset from the beginning of the Webcil payload, not from the beginning of the WebAssembly wrapper module.)
+
+#### Sections
 
 Immediately following the section table are the sections.  These are copied verbatim from the PE file.
 
-## Rationale
+### Rationale
 
 The intention is to include only the information necessary for the runtime to locate the metadata
 root, and to resolve the RVA references in the metadata (for locating data declarations and method IL).

src/libraries/Microsoft.NET.WebAssembly.Webcil/src/Webcil/WasmModuleReader.cs

@@ -0,0 +1,148 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Immutable;
+using System.IO;
+using System.Reflection;
+using System.Runtime.InteropServices;
+using System.Text;
+
+namespace Microsoft.NET.WebAssembly.Webcil;
+
+internal class WasmModuleReader : IDisposable
+{
+    public enum Section : byte
+    {
+        // order matters: enum values must match the WebAssembly spec
+        Custom,
+        Type,
+        Import,
+        Function,
+        Table,
+        Memory,
+        Global,
+        Export,
+        Start,
+        Element,
+        Code,
+        Data,
+        DataCount,
+    }
+
+    private readonly BinaryReader _reader;
+
+    private readonly Lazy<bool> _isWasmModule;
+
+    public bool IsWasmModule => _isWasmModule.Value;
+
+    public WasmModuleReader(Stream stream)
+    {
+        _reader = new BinaryReader(stream, Encoding.UTF8, leaveOpen: true);
+         _isWasmModule = new Lazy<bool>(this.GetIsWasmModule);
+    }
+
+
+    public void Dispose()
+    {
+        Dispose(true);
+    }
+
+
+    protected virtual void Dispose(bool disposing)
+    {
+        if (disposing)
+        {
+            _reader.Dispose();
+        }
+    }
+
+    protected virtual bool VisitSection (Section sec, out bool shouldStop)
+    {
+        shouldStop = false;
+        return true;
+    }
+
+    private const uint WASM_MAGIC = 0x6d736100u; // "\0asm"
+
+    private bool GetIsWasmModule()
+    {
+        _reader.BaseStream.Seek(0, SeekOrigin.Begin);
+        try
+        {
+            uint magic = _reader.ReadUInt32();
+            if (magic == WASM_MAGIC)
+                return true;
+        } catch (EndOfStreamException) {}
+        return false;
+    }
+
+    public bool Visit()
+    {
+        if (!IsWasmModule)
+            return false;
+        _reader.BaseStream.Seek(4L, SeekOrigin.Begin); // skip magic
+
+        uint version = _reader.ReadUInt32();
+        if (version != 1)
+            return false;
+
+        bool success = true;
+        while (success) {
+            success = DoVisitSection (out bool shouldStop);
+            if (shouldStop)
+                break;
+        }
+        return success;
+    }
+
+    private bool DoVisitSection(out bool shouldStop)
+    {
+        shouldStop = false;
+        byte code = _reader.ReadByte();
+        Section section = (Section)code;
+        if (!Enum.IsDefined(typeof(Section), section))
+            return false;
+        uint sectionSize = ReadULEB128();
+
+        long savedPos = _reader.BaseStream.Position;
+        try
+        {
+            return VisitSection(section, out shouldStop);
+        }
+        finally
+        {
+            _reader.BaseStream.Seek(savedPos + (long)sectionSize, SeekOrigin.Begin);
+        }
+    }
+
+    protected uint ReadULEB128()
+    {
+        uint val = 0;
+        int shift = 0;
+        while (true)
+        {
+            byte b = _reader.ReadByte();
+            val |= (b & 0x7fu) << shift;
+            if ((b & 0x80u) == 0) break;
+            shift += 7;
+            if (shift >= 35)
+                throw new OverflowException();
+        }
+        return val;
+    }
+
+    protected bool TryReadPassiveDataSegment (out long segmentLength, out long segmentStart)
+    {
+        segmentLength = 0;
+        segmentStart = 0;
+        byte code = _reader.ReadByte();
+        if (code != 1)
+            return false; // not passive
+        segmentLength = ReadULEB128();
+        segmentStart = _reader.BaseStream.Position;
+        // skip over the data
+        _reader.BaseStream.Seek (segmentLength, SeekOrigin.Current);
+        return true;
+    }
+}

src/libraries/Microsoft.NET.WebAssembly.Webcil/src/Webcil/WebcilConverter.cs

@@ -42,6 +42,8 @@ FilePosition SectionStart
 
     private string InputPath => _inputPath;
 
+    public bool WrapInWebAssembly { get; set; } = true;
+
     private WebcilConverter(string inputPath, string outputPath)
     {
         _inputPath = inputPath;
@@ -62,6 +64,26 @@ public void ConvertToWebcil()
         }
 
         using var outputStream = File.Open(_outputPath, FileMode.Create, FileAccess.Write);
+        if (!WrapInWebAssembly)
+        {
+            WriteConversionTo(outputStream, inputStream, peInfo, wcInfo);
+        }
+        else
+        {
+            // if wrapping in WASM, write the webcil payload to memory because we need to discover the length
+
+            // webcil is about the same size as the PE file
+            using var memoryStream = new MemoryStream(checked((int)inputStream.Length));
+            WriteConversionTo(memoryStream, inputStream, peInfo, wcInfo);
+            memoryStream.Flush();
+            var wrapper = new WebcilWasmWrapper(memoryStream);
+            memoryStream.Seek(0, SeekOrigin.Begin);
+            wrapper.WriteWasmWrappedWebcil(outputStream);
+        }
+    }
+
+    public void WriteConversionTo(Stream outputStream, FileStream inputStream, PEFileInfo peInfo, WCFileInfo wcInfo)
+    {
         WriteHeader(outputStream, wcInfo.Header);
         WriteSectionHeaders(outputStream, wcInfo.SectionHeaders);
         CopySections(outputStream, inputStream, peInfo.SectionHeaders);
@@ -210,7 +232,7 @@ private static void WriteStructure<T>(Stream s, T structure)
     }
 #endif
 
-    private static void CopySections(FileStream outStream, FileStream inputStream, ImmutableArray<SectionHeader> peSections)
+    private static void CopySections(Stream outStream, FileStream inputStream, ImmutableArray<SectionHeader> peSections)
     {
         // endianness: ok, we're just copying from one stream to another
         foreach (var peHeader in peSections)