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WebCIL assembly format

Version

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 .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 containing the following WAT module:

(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.
PE Headers
CLI Header
CLI Data
Native Image Sections

A Webcil file follows a similar structure
Webcil Headers
CLI Header
CLI Data

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

struct WebcilHeader {
	uint8_t id[4]; // 'W' 'b' 'I' 'L'
	// 4 bytes
	uint16_t version_major; // 0
	uint16_t version_minor; // 0
	// 8 bytes
	uint16_t coff_sections;
	uint16_t reserved0; // 0
	// 12 bytes

	uint32_t pe_cli_header_rva;
	uint32_t pe_cli_header_size;
	// 20 bytes

    uint32_t pe_debug_rva;
    uint32_t pe_debug_size;
    // 28 bytes
};

The Webcil header starts with the magic characters 'W' 'b' 'I' 'L' followed by the version in major minor format (must be 0 and 0). Then a count of the section headers and two reserved bytes.

The next pairs of integers are a subset of the PE Header data directory specifying the RVA and size of the CLI header, as well as the directory entry for the PE debug directory.

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 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.

struct SectionHeader {
    uint32_t st_virtual_size;
    uint32_t st_virtual_address;
    uint32_t st_raw_data_size;
    uint32_t st_raw_data_ptr;
};

(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

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).

A goal is for the files not to be executable by .NET Framework.

Unlike PE files, mixing native and managed code is not a goal.

Lossless conversion from Webcil back to PE is not intended to be supported. The format is being documented in order to support diagnostic tooling and utilities such as decompilers, disassemblers, file identification utilities, dependency analyzers, etc.