The .wast Test Harness

A .wast file is used for testing purposes and simplifies the writing of tests for developers. We use .wast files to encode assertions that should pass when running an instrumented variation of a wasm module.

Writing .wast Tests

The high level structure looks like this:

<module_in_wat>

;; WHAMM --> <some_oneline_whamm_script>
<whamm0_assertion0> ;; The first assertion for the first whamm script
<whamm0_assertion1> ;; The second assertion for the first whamm script

;; WHAMM --> <some_oneline_whamm_script>
<whamm1_assertion0> ;; The first assertion for the second whamm script
<whamm1_assertion1> ;; The second assertion for the second whamm script
<whamm2_assertion1> ;; The third assertion for the second whamm script

The module encoded in the script above would be used for all the following whamm/assertion groups. To verify that all assertions fail before instrumenting, 5 new .wast files would be generated with the original module and run on the configured interpreters. To verify that all assertions pass after instrumenting, 2 new .wast files would be generated, one per specified whamm script, including the assertions under the respective whamm script.

Below is an example .wast test:

;; Test `wasm:opcode:call` event

;; @instrument
(module
    ;; Auxiliary definitions
    (func $other (param i32) (result i32) (local.get 1))
    (func $dummy (param i32) (result i32) (local.get 0))

    ;; Test case functions
    (func (export "instrument_me") (result i32)
        (call $dummy (i32.const 0))
    )
)

;; WHAMM --> wasm:opcode:call:before { arg0 = 1; }
(assert_return (invoke "instrument_me") (i32.const 1)) ;; will be run with the above WHAMM instrumentation

;; WHAMM --> wasm:opcode:call:alt { alt_call_by_name("other"); }
(assert_return (invoke "instrument_me") (i32.const 1)) ;; will be run with the above WHAMM instrumentation

Below is an example .wast test using imports:

(module
    (func (export "dummy") (param i32) (result i32)
        local.get 0
    )
)

(register "test")

;; @instrument
(module
    ;; Imports
    (type (;0;) (func (param i32) (result i32)))
    (import "test" "dummy" (func $dummy (type 0)))

    ;; Globals
    (global $var (mut i32) (i32.const 0))

    ;; Global getters
    (func $get_global_var (result i32)
        (global.get $var)
    )

    ;; Test case functions
    (func $foo
        (call $dummy (i32.const 0))
        global.set $var
    )

    (start $foo)
    (export "foo" (func $foo))
    (export "get_global_var" (func $get_global_var))
    (memory (;0;) 1)
 )
 
;; WHAMM --> var count: i32; wasm:opcode:call:alt / arg0 == 0 / { count = 5; return 1; }
(assert_return (invoke "get_global_var") (i32.const 1)) ;; alt, so global should be return value
(assert_return (invoke "get_count") (i32.const 5))

There are several conventions to follow when writing .wast test cases for whamm.

1. Only one module-to-instrument per .wast file.
   • The test setup goes at the top (which can include multiple modules when considering testing imports).
   • The module-to-instrument is the final part of the setup and is marked by ;; @instrument above the module.
2. Use comment to specify the Whamm script, syntax: ;; WHAMM --> <whamm_script>
   • The scripts are run on the module in the .wast file.
   • If there are multiple asserts under a Whamm comment, they are all run against the instrumented variation of the module that results from that Whamm script.
3. All asserts should fail if they were to run without instrumentation.

NOTE: For wei, don't do manipulations that change arg* (that requires the frame accessor). Instead change global state for now?

The Harness Code

The harness is located in tests/common/wast_harness.rs with the main function as the entrypoint. We invoke this harness through calling the main entrypoint in the run_wast_tests test case located in tests/integration_test.rs.

One can read the harness code and see that it performs the following logic:

1. Split out test components of each .wast file found under tests/wast_suite as an individual WastTestCase
   • a wasm module
   • a whamm script
   • a list of assertions that should be true post-instrumentation
2. Ensure all assertions fail before instrumenting with whamm. We do this to be able to claim that correctness of instrumentation was the sole purpose that some test passed. We ensure that this property holds by first creating new .wast files with one assertion per file and making sure that they fail when run on a supported interpreter.
   • We do this re-generation of the .wast files because the interpreters we use exit on the first failed assertion per .wast, but we want to guarantee this property for all assertions.
3. Ensure all assertions pass after instrumenting with whamm.
   • Run the specified whamm script on the module per set of assertions.
   • Output a new .wast file with the instrumented variation of the module with the respective assertions.

Supported Interpreters

The harness generates *.bin.wast files to run on a list of engines, e.g. wizeng and the spec interpreter.

See the repo's README.md for how to set up the interpreters to run with our test harness.

Some Ideas for Future Improvements

Report Variables

;; Use something like below to assert on the values of some report variable dynamically.
;; REPORT_TRACE(ID) --> 1, 3, 5, 6, 7

;; Use something like below to assert on report variable values!
;; WITH_WHAMM --> (assert_return (invoke "get_report_var" (i32.const 1)) (i32.const 7))