[spec] Normative: Support [Serializable] for WebAssembly.Module #711

littledan · 2018-02-21T14:51:12Z

This patchset adds support to WebAssembly.Module to be used in HTML serialization, including from postMessage and IndexedDB. The change is based on @flagxor 's earlier PR, with the following changes:

Rather than actually including the Module in the serialization, this PR includes only the binary data, as the Module cannot be referenced across agents (which each have their own separate store).
The same-agent-cluster limitation is removed, as it is nonsensical in the context of forStorage serialization, for example IndexedDB, which is permitted by this specification.
The Memory aspects are omitted in this PR; those will be pursued in a separate PR in the threads repository.
The bytes of the module are explicit sub-serialized/deserialized

littledan · 2018-02-21T14:51:37Z

document/web-api/index.bs

@@ -2,7 +2,7 @@
 Title: WebAssembly Web API
 Shortname: wasm-web-api
 Group: wasm
-Status: FPWD
+Status: w3c/FPWD


This change was needed for me to be able to build locally

If you update your bikeshed version that should fix it (it will then know wasm is a w3c project).
The reason I changed these is that bikeshed rejects some of the normal stamping with the full string. I still owe Tab filing an issue on that.

How should I update it? I tried bikeshed update and it still gave this error.

I think you need to actually update your Bikeshed source code, so e.g. go to the director you checked out bikeshed and run git pull.

annevk · 2018-02-21T14:59:37Z

document/web-api/index.bs

@@ -125,6 +135,36 @@ Note: This algorithm accepts a {{Response}} object, or a
     1. Return |returnValue|.
 </div>

+<h2 id="serialization">Serialization</h2>
+
+On the Web, {{Module}} has the <code>[<a extended-attribute>Serializable</a>]</code> extended attribute.


Can't this be in its IDL? This seems somewhat unclear.

Well, the JS API specification is trying to avoid referencing anything which is Web-specific. [Serializable] is defined in HTML. I'm not sure how to put this in its IDL without violating that layering.

If these objects are not defined in IDL then you need to patch HTML directly as you did with BigNum.

(I said as much on the original PR.)

They are defined in IDL, it's just that that IDL is supposed to not reference Web-specific specifications.

Here's the IDL: https://webassembly.github.io/spec/js-api/index.html#module

It also has Exposed=(Window,Worker,Worklet) so I think you're failing somewhat?

@annevk Well, there's a plan for that: whatwg/webidl#468 . This layering is still a goal.

Okay. I think you need to make this something like "Web user agents must augment the Module interface with the [Serializable] extended attribute." so it's an actual requirement and not a statement of fact that isn't based on any established facts...

Thanks for the wording; I've switched to use it.

annevk · 2018-02-21T15:00:05Z

document/web-api/index.bs

+
+On the Web, {{Module}} has the <code>[<a extended-attribute>Serializable</a>]</code> extended attribute.
+
+The [=serialization steps=], given |value|, |serialized| and |forStorage|, are:


Oxford comma?

annevk · 2018-02-21T15:01:31Z

document/web-api/index.bs

+
+Given the above engine optimizations, structured serialization provides developers
+explicit control over both compiled-code caching and cross-window/worker code
+sharing.


This seems like a note?

annevk · 2018-02-21T15:01:47Z

document/web-api/index.bs

+    1. [=Compile a WebAssembly module=] from |bytes| and set |value|.\[[Module]] to the result.
+
+The semantics of a structured serialization is as-if the binary source, from which the
+{{Module}} was compiled, is serialized, then deserialized, and recompiled into the target realm.


This seems like a note as well.

jfbastien · 2018-02-21T17:24:05Z

This is a spec addition, right? I think you need to follow the same process to get this addition into WebAssembly. Should be very straightfoward, but we can't commit it until the working draft ships (unless we want to re-spin a new public working draft, and all that entails).

domenic · 2018-02-21T18:35:19Z

Hmm, I sure hope this group has the ability to publish new editor's draft without going through the W3C working draft process stuff...

domenic · 2018-02-21T18:36:15Z

This PR is very confusing BTW because it says "Memory" several times (e.g. in the PR title) when I believe the actual spec text is for "Module".

jfbastien · 2018-02-21T18:45:05Z

Hmm, I sure hope this group has the ability to publish new editor's draft without going through the W3C working draft process stuff...

Maybe I misunderstand, but IIUC this PR requires me to change code in WebKit. I agree with the change, but it's not editorial. It's a new capacity, and as such should go through the CG as documented in our process. Doesn't matter if it's small and obvious, we can't just commit non-editorial changes to the spec. All work would then iterate in a separate repo, which we'd merge into the spec when it's ready to be merged.

Separately, even if we go through the process, I'm worried that merging this change into the tip-of-tree of this repo does require going through the W3C working draft process. A way to avoid this is to work purely in a branch for each working draft. This change then wouldn't affect the draft.

flagxor · 2018-02-21T19:03:48Z

This was content that was present in the design repo material before it got converted to our current spec. It was lost in transcription as the old verbiage was very informal (and Dan had the impression it got moved to the HTML spec).
https://github.com/WebAssembly/design/blob/master/JS.md#structured-clone-of-a-webassemblymodule
The PR was my attempt to transform it into something more formal, but I paused that once Dan took over drafting our JS spec.

We could certainly churn through the proposal process, but I would argue that's unnecessarily bureaucratic.
This certainly exceeds the "editorial" bar and is a "substantive" change as it does have bearing on implementations, however, I don't think that requires the full process as this goes back to the seed material for the spec. I.e. this would be similar to if Andreas had managed to forget an opcode in the core spec. It is something we need to track and note when publishing the next working draft, but because of its origin, it seems like overkill to run the whole proposal process.

jfbastien · 2018-02-21T19:07:50Z

Brad's explanation makes sense. Agreed.

littledan · 2018-02-21T21:55:57Z

@domenic Oops re: Memory. Fixed up the cases I saw.

@jfbastien I think there are two discussions here and we're talking past each other. One question is whether this is a new feature. I don't think anyone is saying no to this. This definitely isn't an editorial change, it's a normative change and new feature (which might deserve to cut to the head of the line for @flagxor 's historical reasons). I'd be fine asking the Wasm CG for a repo here, but sounds you're not asking me to.

What @domenic is suggesting is that the WebAssembly CG may want to maintain two documents in parallel: A FWPD (what you've already sent out, leaving it frozen), and an "Editor's draft" (W3C speak for a document that you can just edit whenever you want). Basically, the standard that you put through the IP opt-out period and the other formal processes that follow it would be a fork of the editor's draft, which we'd occasionally backport editorial patches to or whatever else is urgent.

A nice thing about maintaining an Editor's Draft is that people working on new features don't have to block on W3C process to get their work done. At the same time, people who are focusing on W3C process have a stamp-ready, feature-filled document ready for them once the cycle opens up again--there's no need to "open the floodgates" and merge a bunch of things at a particular time.

flagxor

lgtm, but defer to folks that know the interop with the html spec better.

jfbastien · 2018-02-21T22:46:16Z

@jfbastien I think there are two discussions here and we're talking past each other. One question is whether this is a new feature. I don't think anyone is saying no to this. This definitely isn't an editorial change, it's a normative change and new feature (which might deserve to cut to the head of the line for @flagxor 's historical reasons). I'd be fine asking the Wasm CG for a repo here, but sounds you're not asking me to.

Correct, if this is just putting in something that was agree on, and that was dropped, then I don't think you need to go through the process. I encourage you to provide this type of context next time, so non-editorial changes are easy to categorize.

What @domenic is suggesting is that the WebAssembly CG may want to maintain two documents in parallel: A FWPD (what you've already sent out, leaving it frozen), and an "Editor's draft" (W3C speak for a document that you can just edit whenever you want). Basically, the standard that you put through the IP opt-out period and the other formal processes that follow it would be a fork of the editor's draft, which we'd occasionally backport editorial patches to or whatever else is urgent.

A nice thing about maintaining an Editor's Draft is that people working on new features don't have to block on W3C process to get their work done. At the same time, people who are focusing on W3C process have a stamp-ready, feature-filled document ready for them once the cycle opens up again--there's no need to "open the floodgates" and merge a bunch of things at a particular time.

Agreed, it's simply more maintenance to keep two branches going.

littledan · 2018-02-22T14:59:31Z

OK, thanks for the reviews everyone! I'll put this on the agenda for the next Wasm CG meeting so we can discuss next steps.

littledan · 2018-02-26T14:18:06Z

It seems like web-platform-tests already has good tests for module serialization, which verify that

postMessage works with Workers and iframes.
Modules can be serialized to IndexedDB
postMessage of a Module from a ServiceWorker doesn't work. This was previously specified by checking the agent cluster, but that check isn't in the current PR.

Two things from here:

Probably there should be a test that checks that IndexedDB is not allowed to serialize Modules from an insecure context.
Should we re-add the same agent cluster check? Does anyone remember why it was there in the first place? If we do add it back, I think we still want it disabled in the forStorage case (since it doesn't seem to make sense there). Otherwise, I can update the test to check that postMessage does actually work from a ServiceWorker.

flagxor · 2018-03-01T21:22:25Z

So my recollection was that there was disagreement around requiring a secure context for IDB serialization (from JSC + Firefox)? Chrome has taken the position that persisting code from a potentially insecure context is problematic.

I think the agent cluster check just fell out of the assumption that code sharing was most essential between threads. I'd be happy dropping it if others don't care?

littledan · 2018-03-01T22:12:29Z

Given the past disagreement on this security question about storing code from insecure contexts, how should we proceed? Should the specification permit the various positions taken by different browsers, or should we come to a common, single semantic?

jfbastien · 2018-03-01T22:14:18Z

The security question is specifically about caching compiled code in IDB from an insecure context, not serializing, correct?

What do other web caching things do?

domenic · 2018-03-01T22:42:36Z

We must become interoperable on this web-observable behavior.

Other modern caching things (e.g. the service worker cache API) are not accessible from insecure contexts.

littledan · 2018-03-01T22:52:17Z

@jfbastien "Serializing" is the name in the HTML specification that describes how things are stored in IndexedDB as well as postMessage. There is a forStorage mode switch which specifications can use to differentiate those two cases.

jfbastien · 2018-03-01T22:53:59Z

Other modern caching things (e.g. the service worker cache API) are not accessible from insecure contexts.

If other modern developer-controlled caches are spec'd to disallow caching from insecure contexts then it seems like we should follow the same rationale unless their motivation to do so doesn't apply to WebAssembly caching. Specifically, what's the bad stuff that an attacker can do if they can get a module cached? Can they permanently inject content into a site? If they decide on the IDB key then the answer is "yes", but if the key is specified as something they can't control then "no".

domenic · 2018-03-01T23:16:00Z

Right, it's about injecting content into the site. If I understand correctly the scenario, under insecure HTTP, an attacker can control everything, including the IDB key.

jfbastien · 2018-03-06T18:09:06Z

As we discussed in the CG call today, I'd like our decision to mention HTTP / HTTPS to be a principled one, not just "hey we could do it for IDB". It sounds like at least Chrome wouldn't allow IDB serialization under HTTP. I'd rather spec any secure context restrictions separately, following a WebAssembly overall guidance. Maybe this can be an offshoot out the secure context discussion?

littledan · 2018-03-06T22:06:21Z

We heard in the call that Chrome is unlikely to ship the ability to store Modules from insecure contexts in IndexedDB. If it's possible, I hope we can soon come to agreement among all the browsers that what we specify here will eventually be shipped.

To ensure compatibility, I'd prefer to aim to be only adding features over time, and not shipping additional restrictions on previously shipped features unless it is to correct a previous mistake. (If there isn't compatibility risk from a future restriction, it's because the feature isn't being used; in that case, why bother specifying and shipping it?)

I think additional restrictions are a decent conservative default given disagreement. In this case, I could see logic to the restriction--persistent storage of attacker code is worse than ephemerally running attacker code on a particular page load.

jfbastien · 2018-03-06T22:21:59Z

We heard in the call that Chrome is unlikely to ship the ability to store Modules from insecure contexts in IndexedDB. If it's possible, I hope we can soon come to agreement among all the browsers that what we specify here will eventually be shipped.

To ensure compatibility, I'd prefer to aim to be only adding features over time, and not shipping additional restrictions on previously shipped features unless it is to correct a previous mistake. (If there isn't compatibility risk from a future restriction, it's because the feature isn't being used; in that case, why bother specifying and shipping it?)

I don't really see Chrome not shipping this for HTTP as a big issue since all sites have to handle IDB storage failure already, and not all browsers currently support IDB module storage. The incompatibility is indistinguishable from the current state of the web, as well as from random failure.

If you want to aim for specifying HTTPS for module serialization, that's also fine with me. As I've outlined I think we should do so in a principled manner, not ad-hoc for this proposal. We can block serialization of modules on figuring out a precise policy for how WebAssembly should act under an insecure context. Should we discuss this with webappsec alongside CSP, or as a separate thing?

littledan · 2018-03-07T12:07:29Z

I don't really see Chrome not shipping this for HTTP as a big issue since all sites have to handle IDB storage failure already, and not all browsers currently support IDB module storage. The incompatibility is indistinguishable from the current state of the web, as well as from random failure.

If we're OK with unpredictable performance, I don't understand the motivation for this feature. Browsers could just transparently cache modules based on a hash of the bitcode. I thought the idea of serializing modules in IndexedDB was that you'd be able to fairly reliably control when they are cached and read out.

There's a bit of difference between a permanent state of differences between browsers and other cases where this feature won't be available: when comparing to the current state of the web, this is hopefully a one-time upgrade, and then all browsers should support IDB persistence of modules reliably. For eviction from storage, this can be controlled by the persistent storage API. For random failures, these should be pretty rare and analogous to a first page load; it should be pretty rare for a second load immediately after that to face the same issue.

jfbastien · 2018-03-10T05:01:20Z

@littledan I'm making a very narrow argument from the current state of which browser ships what. From that current state of things, there's little difference in a world where HTTP and HTTPS don't act the same, whether normative or not, because already different browsers don't act the same and already there's random eviction.

Longer term (say, later this year) we'll figure out how to handle HTTPS in this spec. I just don't think an ad-hoc solution for this specific feature is right for the long term, because I'm not sure we'll get the big picture right (we might, I'm simply not confident that we will). Otherwise we'd already have that big picture in the spec.

littledan · 2018-04-11T19:47:04Z

We discussed structured clone of modules in the April 4th WebAssembly CG call. To summarize the discussion:

The same agent cluster restriction for Module serialization through postMessage was justified for the following reasons:
- This restriction means that Modules are not sent cross-origin, which may be helpful as a security invariant
- There weren't clear cross-agent-cluster use cases.
- In some browsers, restricting to a single agent cluster makes for a simpler implementation
We did not address or come to a conclusion on the secure context restriction for storage of WebAssembly Modules. This will have to be discussed in the future.

I'll follow up with a patch which maintains the secure context restriction and follows the other restrictions listed above.

annevk · 2018-04-12T04:34:47Z

This restriction means that Modules are not sent cross-origin, which may be helpful as a security invariant

Not cross-site, but they can go cross-origin.

littledan · 2018-04-12T09:58:15Z

@annevk Are you talking about the postMessage case? Can you give an example/reference for more details about the cross-site/cross-origin differences and how it relates to agent clusters? We were having trouble in the last CG call.

annevk · 2018-04-12T10:04:04Z

@littledan if example.com embeds site.example.com through a frame they'll be in the same similar-origin window agent. That's because if they both set document.domain to example.com they can access each other directly.

Basically, an agent cluster by definition can consists of multiple origins, but those origins always have the same eTLD+1 (registrable domain).

littledan · 2018-04-12T10:05:53Z

Thanks for the explanation. Seems like we should discuss in the CG whether we want a same or similar origin restriction here.

annevk · 2018-04-12T10:07:25Z

@littledan that would require storing origin data on the Module object, which I would strongly recommend against (and have already done elsewhere). You want a capability-based model for objects, not tagging.

littledan · 2018-04-12T10:13:26Z

@annevk What do you think about this current patch, which stores the agent cluster if it's not forStorage (similar to SharedArrayBuffer, but for different reasons).

annevk · 2018-04-12T10:18:56Z

@littledan I think that's the correct approach. Though writing that down I do now recall that folks raised a concern in private with me about that approach combined with MessageChannel not working well in implementations for SharedArrayBuffer. I've asked them last year to report an issue for this, but that hasn't yet happened.

littledan · 2018-06-13T17:11:14Z

Apologies for my delay. Chrome is considering shipping same-agent-cluster postMessage but not IndexedDB serialization of WebAssembly modules. I've updated this PR to match that proposal. See 314099c for more information.

domenic · 2018-06-13T17:31:03Z

document/web-api/index.bs

+The [=serialization steps=], given |value|, |serialized|, and |forStorage|, are:
+
+    1. If |forStorage| is true, throw a
+        "<a exception>DataCloneError</a>" {{DOMException}}


Nit: "{{DataCloneError}}" is more conventional.

annevk

It would be good if there was a MessageChannel test where port2 was transferred cross-site. Then invoke port1's postMessage(). Then transfer port2 back same-site. Then invoke port1's start(). And some variations on that theme.

annevk · 2018-06-14T12:03:18Z

document/web-api/index.bs

+sharing.
+
+Note: The web platform does not permit cross-origin sharing of serialized Modules:
+Agent clusters in HTML do not span multiple origins.


This is incorrect. It does not allow sharing when two origins's hosts are not https://url.spec.whatwg.org/#host-same-site or do not have the same scheme. Otherwise sharing is possible.

(This is also why we call it the similar-origin window agent, not just window agent.)

Is there anything linkable that I could reference for "similar origin", or is this just something that's used in the air but not documents?

I've just removed the note.

…serialization This restriction was non-sensical because there is practical no way to serialize an agent cluster forStorage.

…odule deserialization" The same agent cluster restriction is present in the case of serializing within memory. Also add a note about how this restricts serialization to be within an origin.

Based on discussions in the WebAssembly CG, it sounds like IndexedDB serialization of WebAssembly Modules has the following problems: - On some engines, it's impractical to serialize more than just the binary module data - On some engines, upgrading from one version to another would cause the serialized module to be invalidated, with complicated implementation logic and a loss of reliability - Some engines plan to just return an error for IndexedDB module serialization as a design decision The purpose of IndexedDB Module serialization is for reliable performance, and these items provide risks to that. This patch proposes that, initially, IndexedDB serialization would not be provided for WebAssembly modules.

littledan · 2018-06-24T20:44:16Z

When do we want to consider landing this PR? Should it be in the v1 spec, or are we OK with waiting for v2? It's a significant new feature, so I'm not sure if it might reset some sort of clock. @lukewagner suggested that waiting for v2 might be fine, as most of the use cases for postMessage are motivated by threads, which are only coming in v2.

@Ms2ger Would you be interested in looking into the tests which @annevk suggested?

binji · 2018-06-25T19:33:51Z

Should it be in the v1 spec, or are we OK with waiting for v2?

We discussed having this for v1 already, and only decided recently not to move forward with serialization for IndexedDB part. I think it makes sense to keep for v1.

It's a significant new feature, so I'm not sure if it might reset some sort of clock.

It will, yes, but we're past the point where that matters. We need to put out a new version regardless which will necessarily reset the clock.

littledan · 2018-06-25T21:44:24Z

OK, let's discuss this at the next CG meeting tomorrow and merge it if it seems OK

binji · 2018-06-26T22:45:45Z

We discussed this in the June 26th CG meeting and decided to include this PR in v1 of the spec.

littledan · 2018-06-27T01:04:32Z

@Ms2ger Could you make sure we have good tests for this patch in web-platform-tests? In particular, we should have tests that serializing in IndexedDB throws, and that serialization in postMessage and MessageChannel works.

Ms2ger · 2018-10-22T09:43:01Z

Looks like web-platform-tests/wpt#12488 covers this neatly.

littledan requested a review from flagxor February 21, 2018 14:51

littledan commented Feb 21, 2018

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littledan mentioned this pull request Feb 21, 2018

Clarify structured clone behavior for WebAssembly objects. WebAssembly/design#1074

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annevk reviewed Feb 21, 2018

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littledan changed the title ~~[spec] Normative: Support [Serializable] for WebAssembly.Memory~~ [spec] Normative: Support [Serializable] for WebAssembly.Module Feb 21, 2018

flagxor reviewed Feb 21, 2018

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flagxor approved these changes Feb 21, 2018

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littledan added the wasm-web-api-1 label Feb 21, 2018

littledan mentioned this pull request Apr 11, 2018

HTML serialization of WebAssembly.Module objects WebAssembly/meetings#209

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littledan force-pushed the serialize branch from 6432d2a to 314099c Compare June 13, 2018 17:10

domenic approved these changes Jun 13, 2018

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binji mentioned this pull request Jun 13, 2018

Don't allow IndexedDB serialization of WebAssembly.Module #821

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annevk reviewed Jun 14, 2018

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littledan added 7 commits June 24, 2018 22:40

[spec] Normative: Add serialization support for Modules

6ff9a76

[spec] Normative: Remove same-agent-cluster restriction for Module de…

47cd8e7

…serialization This restriction was non-sensical because there is practical no way to serialize an agent cluster forStorage.

[spec] Editorial: Explicitly do subserialization

b037d1c

[spec] Editorial: Changes suggested by @annevk

a405031

[spec] Editorial: Adopt @annevk's suggested wording

cd30fe3

Revert "[spec] Normative: Remove same-agent-cluster restriction for M…

1606c8e

…odule deserialization" The same agent cluster restriction is present in the case of serializing within memory. Also add a note about how this restricts serialization to be within an origin.

littledan force-pushed the serialize branch from 314099c to ab29f51 Compare June 24, 2018 20:41

littledan merged commit f85f6e6 into WebAssembly:master Jun 27, 2018

binji mentioned this pull request May 17, 2019

[WASM] CR transition for WebAssembly Web API w3c/transitions#138

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		On the Web, {{Module}} has the <code>[<a extended-attribute>Serializable</a>]</code> extended attribute.

		The [=serialization steps=], given \|value\|, \|serialized\| and \|forStorage\|, are:

[spec] Normative: Support [Serializable] for WebAssembly.Module #711

[spec] Normative: Support [Serializable] for WebAssembly.Module #711

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