On Sun, Dec 02, 2018 at 10:23:14AM -0500, Bryan Steele wrote:
> In -current (and 6.4), chrome repeatedly attempts to mmap a large linear
> area of memory for WASM/WebAssembly and it always fails. WASM JIT avoids
> using the system malloc/free, opting instead for their own "Heartbleed
> accelerating custom allocator".
>
> Despite the horribleness, this feature is being increasingly used by
> default if the browser advertises this misfeature as supported, many
> sites will fallback if it is unavailable. Unfortunately, they *DO NOT*
> fallback if WASM fails to allocate memory. You simply get broken, half-
> working websites. :-(
>
> For example, the popular video streaming platform (twitch.tv) will show
> a black box where the stream should be. The error is not obvious unless
> you check the browsers Javascript console:
>
> Uncaught RangeError: WebAssembly.Memory(): could not allocate memory
> at wasmworker.min.js:51
> at blob:https://www.twitch.tv/2af8520d-25ad-4f42-bc94-497265c25c62:6
> (index):1 Script error
>
> After hours of searching, I found the magic knob to disable WASM in
> chrome: '--js-flags=--noexpose_wasm'
>
> There's no UI way that I can find to disable it, so you'll need to
> add a shell alias or put it in your window managers config.
>
> Note: Firefox is also not saved from this insanity, WASM will often
> fail there as well.. unless datasize is cranked to infinity and beyond.
For firefox, there's two things to frob if you want to debug this:
- www/mozilla-firefox/patches/patch-js_src_jit_ProcessExecutableMemory_h
clamps it to work with the default ulimits. Adapt the surrounding code
to depend on the limits at runtime ? I wouldnt do this before consulting
upstream first.
- (i guess) twiddle javascript.options.wasm in about:config to stop
advertising wasm as 'supported' ? If that improves things, i'll gladly
change the default.
Landry
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