Weasel

Weasel stands for WEbAssembly Specification Language, it can be used to annotate Webassembly text modules in custom annotation. Annotated modules can be instrumented to perform runtime assertion checking thanks to owi's code generator.

Commands and options related to Weasel:

• owi instrument to instrument an annotated text module.
• --rac for run, sym and conc to instrument an annotated text module and perform runtime assertion checking.
• --srac for sym and conc to instrument an annotated text module and perform runtime assertion checking symbolically.

The formal specification of Weasel can be found in src/annot/spec.ml.

Basic example

Suppose we have a function returning its parameter plus three:

(module
  (;...;)
  (func $plus_three (param $x i32) (result i32)
    local.get $x
    i32.const 3
    i32.add
  )
  (;...;)
)

With Weasel, we can annotate this function by specifying its postconditions:

(module
  (;...;)
  (@contract $plus_three
    (ensures (= result (+ $x 3)))
  )
  (func $plus_three (param $x i32) (result i32)
    local.get $x
    i32.const 3
    i32.add
  )
  (;...;)
)

owi instrument generates a new wrapper function checking this postcondition:

$ owi instrument plus_three.wat
$ cat plus_three.instrumented.wat
(module
  (import "symbolic" "assert" (func $assert  (param i32)))
  (type (sub final  (func (param $x i32) (result i32))))
  (type (sub final  (func)))
  (type (sub final  (func (param i32))))
  (type (sub final  (func (result i32))))
  (func $plus_three (param $x i32) (result i32)
    local.get 0
    i32.const 3
    i32.add
  )
  (func $start
    i32.const 42
    call 3
    drop
  )
  (func $__weasel_plus_three (param $x i32) (result i32) (local $__weasel_temp i32) (local $__weasel_res_0 i32)
    local.get 0
    call 1
    local.set 2
    local.get 2
    local.get 0
    i32.const 3
    i32.add
    i32.eq
    call 0
    local.get 2
  )
  (start 2)
)

We can perform runtime assertion checking either by owi sym plus_three.instrumented.wat or by owi sym --rac plus_three.wat.

$ owi sym plus_three.instrumented.wat
All OK!
$ owi sym --rac plus_three.wat
All OK!