WebAssembly Dynamic Linking

This document describes the current WebAssembly dynamic linking ABI used by emscripten and by the llvm backend when targeting emscripten.

Note: This ABI is still a work in progress. There is no stable ABI yet.

Dynamic Libraries

A WebAssembly dynamic library is a WebAssembly binary with a special custom section that indicates this is a dynamic library and contains additional information needed by the loader.

The "dylink.0" Section

The "dylink.0" is a custom section, the existence of which signals that the module conforms the dynaminc linking ABI described in this document. This section is expected to be the very first section in the module.

A "dylink.0" consists of a series of sub-sections using the same format as found in the "names" section:

Field	Type	Description
subsections	subsection*	sequence of subsection

Each subsection is encoded as:

Field	Type	Description
type	uint8	code identifying type of subsection
payload_len	varuint32	size of this subsection in bytes
payload_data	bytes	content of this subsection, of length payload_len

The current list of valid type codes are:

• 1 / WASM_DYLINK_MEM_INFO - Specifies the memory and table space requirements of the module

• 2 / WASM_DYLINK_NEEDED - Specifies external modules that this one depends on.

• 3 / WASM_DYLINK_EXPORT_INFO - Specify additional metadata about exports.

• 4 / WASM_DYLINK_IMPORT_INFO - Specify additional metadata about imports.

For WASM_DYLINK_MEM_INFO the following fields are present in the subsection:

Field	Type	Description
memorysize	varuint32	Size of the memory area the loader should reserve for the module, which will begin at env.__memory_base
memoryalignment	varuint32	The required alignment of the memory area, in bytes, encoded as a power of 2.
tablesize	varuint32	Size of the table area the loader should reserve for the module, which will begin at env.__table_base
tablealignment	varuint32	The required alignment of the table area, in elements, encoded as a power of 2.

For WASM_DYLINK_NEEDED the following fields are present in the subsection:

Field	Type	Description
needed_dynlibs_count	varuint32	Number of needed shared libraries
needed_dynlibs_entries	string*	Repeated string names of dynamic libraries

The "string" type is defined as:

Field	Type	Description
string_len	varuint32	Length of string_payload in bytes
string_payload	bytes	valid UTF-8 byte sequence

env.__memory_base and env.__table_base are i32 imports that contain offsets into the linked memory and table, respectively. If the dynamic library has memorysize > 0 then the loader will reserve room in memory of that size and initialize it to zero (note: can be larger than the memory segments in the module, if the dynamic library wants additional space) at offset env.__memory_base, and similarly for the table (where initialization is to null, i.e., a trap will occur if it is called). The allocated regions of the table and memory are guaranteed to be at least as aligned as the library requests in the memoryalignment and tablealignment properties. The library can then place memory and table segments at the proper locations using those imports.

If needed_dynlibs_count > 0 then the loader, before loading the library, will first load needed libraries specified by needed_dynlibs_entries.

For WASM_DYLINK_EXPORT_INFO the following fields are present in the subsection:

Field	Type	Description
export_info_count	varuint32	Number of export info
export_info_entries	export_info*	Repeated export info

The "export_info" type is defined as:

Field	Type	Description
name	string	The name of the export
flags	varuint32	Symbol flags for the export

For WASM_DYLINK_IMPORT_INFO the following fields are present in the subsection:

Field	Type	Description
import_info_count	varuint32	Number of import info
import_info_entries	import_info*	Repeated import info

The "import_info" type is defined as:

Field	Type	Description
module	string	The module name of the import
field	string	The field name of the import
flags	varuint32	Symbol flags for the export

The set of possible symbol flags are the same as those specified in Linking.

The "dylink" section should be the very first section in the module; this allows detection of whether a binary is a dynamic library without having to scan the entire contents.

Interface and usage

A WebAssembly dynamic library must obey certain conventions. In addition to the dylink section described above a module may import the following globals that will be provided by the dynamic loader:

• env.memory - A wasm memory that is shared between all wasm modules that make up the program.
• env.__indirect_function_table - A wasm table that is shared between all wasm modules that make up the program.
• env.__stack_pointer - A mutable i32 global representing the explicit stack pointer as an offset into the above memory.
• env.__memory_base - An immutable i32 global representing the offset in the above memory which has been reserved and zero-initialized for this module, as described earlier. The module can use this global in the initializer of its data segments so that they loaded at the correct address.
• env.__table_base - An immutable i32 global representing the offset in the above table which has been reserved for this module, as described earlier. The module can use this global in the intializer of its table element segments so that they loaded at the correct offset.

Relocations

WebAssembly dynamic libraries do not require relocations in the code section. This allows for streaming compilation and better code sharing, and reduces the complexity of the dynamic linker. For external symbols this is achieved by referencing WebAssembly imports. For internal symbols we introduce two new relocation types for accessing data and functions address relative to __memory_base and __table_base global:

• 11 / R_WASM_MEMORY_ADDR_REL_SLEB, - a memory address relative to the __memory_base wasm global. Used in position independent code (-fPIC) where absolute memory addresses are not known at link time.
• 12 / R_WASM_TABLE_INDEX_REL_SLEB - a function address (table index) relative to the __table_base wasm global. Used in position indepenent code (-fPIC) where absolute function addresses are not known at link time.
• 17 / R_WASM_MEMORY_ADDR_REL_SLEB64 - the 64-bit counterpart of R_WASM_MEMORY_ADDR_REL_SLEB.
• 21 / R_WASM_MEMORY_ADDR_TLS_SLEB (in LLVM 12.0) - an offset from the __tls_base symbol encoded as a 5-byte [varint32]. Used for PIC case to avoid absolute relocation.
• 25 / R_WASM_MEMORY_ADDR_TLS_SLEB64 (in LLVM 13.0) - the 64-bit counterpart of R_WASM_MEMORY_ADDR_TLS_SLEB.

All code that gets linked into a WebAssembly dynamic library must be compiled as position independent. The corresponding absolute relocation types (R_WASM_MEMORY_ADDR_SLEB and R_WASM_TABLE_INDEX_SLEB) are not permitted in position independent code and will be rejected at link time.

For relocation within the data segments a runtime fixup may be required. For example, if the address of an external symbol is stored in global data. In this case the dynamic library must generate code to apply these relocations at startup. The module can export a function called __wasm_apply_data_relocs. If it is so exported, the loader will call it after the module is instantiated, and before any other function, including __wasm_call_ctors, is called.

Imports

Functions are directly imported from the env module (e.g. env.enternal_func). Data addresses and function addresses are imported as WebAssembly globals that store the memory offset or table offset of the symbol. Such address are imported from GOT.mem and GOT.func respectively. The GOT prefix is borrowed from the ELF linking world and stands for "Global Offset Table". In WebAssembly the GOT is modeled as a set of imported wasm globals.

For example, a dynamic library might import and use an external data symbol as follows:

(import "GOT.mem" "foo" (global $foo_addr (mut i32))
...
...
get_global $foo_addr
i32.load

And an external function symbol as follows:

(import "GOT.func" "bar" (global $bar_addr (mut i32)))
...
...
get_global $bar_addr
call_indirect

Note: This has no effect on exports, or the import of functions for direct call.

The imported global must be mutable as the dynamic linker might need to modify the value after instantiation.

Exports

Functions are directly exported as WebAssembly function exports. Exported addresses (i.e., exported memory locations or exported table locations) are exported as i32 WebAssembly globals. However since exports are static, modules connect export the final relocated addresses (i.e. they cannot add __memory_base before exporting). Thus, the exported address is before relocation; the loader, which knows __memory_base, can then calculate the final relocated address.

Implementation Status

LLVM Implementation

When llvm is run with --relocation-model=pic (a.k.a -fPIC) it will generate code that accesses non-DSO-local addresses via the GOT.mem and GOT.func entries. Such code must then be linked with either -shared to produce a shared library or -pie to produced a dynamically linked executable.

Emscripten

Emscripten can load WebAssembly dynamic libraries either at startup (using RUNTIME_LINKED_LIBS) or dynamically (using dlopen/dlsym/etc). See test_dylink_* and test_dlfcn_* in the test suite for examples.

Emscripten can create WebAssembly dynamic libraries with its SIDE_MODULE option, see the wiki.