Death Before Confusion (was: [whatwg] Handling out of memory issues with getImageData/createImageData)

Mark S. Miller erights at google.com
Sun Sep 27 15:46:28 UTC 2015


[-whatwg, +es-discuss]
Reposting to es-discuss, as Anne's more general question is best seen as a
JS issue rather than a browser specific one



On Sun, Sep 27, 2015 at 8:30 AM, Mark S. Miller <erights at google.com> wrote:

> On Sat, Sep 26, 2015 at 7:34 AM, Anne van Kesteren <annevk at annevk.nl>
> wrote:
>
>> On Fri, Sep 25, 2015 at 4:48 PM, Justin Novosad <junov at google.com> wrote:
>> > Currently there is no spec'ed behavior for handling out-of memory issues
>> > for the specific case of attempting to allocate a large buffer through
>> > image data APIs.
>>
>> Actually, there is no specified behavior for out-of-memory behavior,
>> period. This is a problem that starts with the ECMAScript standard and
>> everything that builds upon it.
>>
>> I have seen Mark Miller discuss some of the issues surrounding this
>> and perhaps even the necessity to eventually define it, but so far
>> this has not happened. Not sure if the full story is documented
>> somewhere. Mark?
>>
>>
>> https://esdiscuss.org/topic/using-max-stack-limit-to-determine-current-js-engine-and-revision#content-7
>> indicates there may be security issues with throwing out-of-memory
>> exceptions.
>
>
> Well, the full story is never documented ;). However, that post and the
> links from there:
>
> http://www.eros-os.org/pipermail/e-lang/2007-January/011817.html
> https://github.com/google/caja/issues/460
>
> are a good start. The security issue is serious and needs to be fixed. It
> cannot practically be fixed by libraries without additional help by the
> platform. The problem is that
>
>     * In a language that implicitly allocates everywhere, like JavaScript,
> Java, and many other oo languages, it is impossible to prevent a code from
> causing OOM
>     * If OOM is thrown (see the first link for Java/Joe-E issues), and the
> language has try/finally, it is impossible to prevent the OOM being masked.
>     * In such languages, it is impossible to program defensively against
> the pervasive possibility of OOM -- if execution simply resumes in that
> context as if nothing bad happened.
>
> In Joe-E we took the painful step of outlawing the Java try/finally from
> the Joe-E subset of Java for this reason. There was no other reason to
> outlaw try/finally as there's nothing else inherently unsafe about it. We
> really tried to find another solution but under our constraints -- no
> rewriting of the Java nor change to the JVM -- we could not.
>
> By preventing Joe-E code from catching VirtualMachineErrors and from doing
> a try/finally, the Joe-E code was preemptively terminated immediately on
> occurrence of a VirtualMachineError. Only the spawner of the Joe-E
> computation could react to this termination of the computation it spawned.
>
> This mirrors one of the many thing that Erlang gets right. When a program
> is confused, that program is the last one you want to ask to recover from
> the confusion, since it is already impaired by its own confusion. If you
> don't know what is still true, you are unlikely to engage in repair actions
> correctly. Better to preemptively terminate some large unit containing the
> confusion and recover by
>     * restarting from earlier known good state, or
>     * if this is not yet feasible, propagating the termination to a yet
> larger granularity of computation.
>
> This is the "fail stop" philosophy of "Death Before Confusion". The
> contrasting philosophy appropriate for some computation is "best efforts".
> Some JavaScript code is best served by one and some by the other. Security
> enforcing code must maintain its own integrity at the price of termination
> (and restart from some coarser grain). Web pages using JavaScript only to
> spice up the user experience are often best served by best efforts. Erlang
> itself is an interesting case study, as its original motivating problem --
> telephone switches -- places a higher priority on uptime than on integrity.
> Nevertheless, both Erlang and the Tandem non-stop architecture found that
> uptime in the large is best served by fail-stop in the small combined with
> coarser-grain recovery logic.
>
> Because JavaScript comes from such a long legacy of de facto best efforts
> architecture, I think a direct du jure shift to fail-stop is unlikely.
> Instead, what we need is a trap-handling mechanism (Erlang "supervisor".
> KeyKOS "keeper"), where different policies can be expressed by user-defined
> trap handlers. When multiple policies co-exist, the platform obeys the
> more severe policies. For concreteness, I'll make here a first sketch:
>
> On OOM, the platform first scans the stack to enumerate all realms
> represented by in-progress stack frames as of that moment. (In progress
> meaning that the stack frame still would have been there even if that
> platform had implemented proper-tail-call.) It gathers the trap handlers
> associated with each of those realms. Each trap handler is a pair of a
> string and an optional function.
>
> The string indicates the choice of trap handling strategy, where these
> strategies are ordered by severity. Among the gathered strategies, the most
> severe win and the rest are discarded. From least to most severe, they are
>
> "THROW"
> "ABORT_JOB"
> "REFRESH"
> "ABORT_EVENT_LOOP"
>
> Except for "THROW", all the rest cause the current turn/job to first be
> preemptively terminated without running catch or finally blocks. If during
> any one trap handling strategy we run out of reserve memory, then we
> automatically escalate to the next more severe strategy. Alternatively, if
> a trap handling function is itself associated with yet another
> otherwise-uninvolved realm with its own trap handler, then an OOM inside
> this trap handler might be handled by that handler's handler.
>
> This is just a first sketch. It is probably too complicated in some ways
> and insufficiently general in others. I post it mostly to get the
> discussion started.
>


-- 
    Cheers,
    --MarkM
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