[swift-evolution] Swift null safety questions
Gwynne Raskind
gwynne at darkrainfall.org
Sat Mar 25 20:56:06 CDT 2017
> On Mar 25, 2017, at 19:27, Elijah Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>
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> On March 25, 2017 at 7:15:59 PM, Elijah Johnson (ejrx7753 at gmail.com <mailto:ejrx7753 at gmail.com>) wrote:
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>>
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>> On March 25, 2017 at 10:24:23 AM, Elijah Johnson (ejrx7753 at gmail.com <mailto:ejrx7753 at gmail.com>) wrote:
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>>>
>>>
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>>> On March 23, 2017 at 12:01:48 PM, Joe Groff (jgroff at apple.com <mailto:jgroff at apple.com>) wrote:
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>>>>>
>>>>> On Mar 22, 2017, at 7:25 PM, Elijah Johnson <ejrx7753 at gmail.com <mailto:ejrx7753 at gmail.com>> wrote:
>>>>>
>>>>> On March 22, 2017 at 8:41:25 PM, Joe Groff (jgroff at apple.com <mailto:jgroff at apple.com>) wrote:
>>>>>>
>>>>>> > On Mar 22, 2017, at 4:07 PM, Elijah Johnson <ejrx7753 at gmail.com <mailto:ejrx7753 at gmail.com>> wrote:
>>>>>> >
>>>>>> > Hi,
>>>>>> >
>>>>>> > Note that this thread has branched off a bit, so my latest proposal and the “head” of this thread can be found at https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170320/034189.html <https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170320/034189.html>
>>>>>> >
>>>>>> >
>>>>>> >
>>>>>> > Can you give an example of this corruption which would be exploitable in a web server context? An example where, having caught the force-unwrap or other precondition fatalError, that the web server would face more danger from continued execution that it would loose by crashing?
>>>>>> >
>>>>>> > The optional has been triggered, the request has been stopped, and incoming requests are going to a new process, but there are a few threads open still finishing what they were doing - I’d like to see a good example of the danger caused by a real “inconsistency”. Lets assume also that all objects shared between threads are Swift objects and not “UnsafePointers”, which is a very fair assumtion for a web server. Java even allows native access and they don’t even crash there.
>>>>>>
>>>>>> When considering security threats, we have to take the presence of an exploit as given until we prove otherwise. A precondition failure indicates a logic error somewhere in the program, and while safe Swift abstractions are intended not to lead to memory unsafety in the face of precondition failures, there's plenty of relatively young, unsafe code below the hood in the implementations of those abstractions, written by humans who make mistakes. Whatever lurking security bugs there may be, they're more likely to be exploitable if you're already in a situation where assumptions have been violated.
>>>>>>
>>>>>> > I’m sorry, but I completely fail to grasp the danger as being > crashing.
>>>>>>
>>>>>> Crashing is inconvenient. The user may have to retry their request or relaunch their app. Security breaches or data corruption are disasters that can have unbounded impact and cost—you can't undisclose sensitive data.
>>>>>>
>>>>>> > I’d be interested to know who these web server developers (ex. Kitura, Vapor) are targeting without any form of crash handling and if they read these threads. It really limits the pool of developers more so than on apps because a single crash on the app ie equivelant to a failed request, not a crashed web server. I realize you are not competing with Java, but I do not see yet the compelling reason for no crash handling.
>>>>>> >
>>>>>> > The “actors” with thread pool and proccesses is one idea, but I think these servers are aiming also for speed and scaleability - don’t know if they would accept this model. I know that it does get used, and do work for a company that uses it on their backend (just plain Java, not sep. processes). Their front-end probably runs like a pre-fork server, but Java is so stable that there is simply no need for that.
>>>>>>
>>>>>> You can use processes for fault isolation today, and that will have some cost. At Swift's current maturity level, that's really probably the only responsible isolation strategy. The goal of "actors" would be to get most of the benefits of processes through language-level isolation without paying the full cost of processes.
>>>>>>
>>>>>> -Joe
>>>>>
>>>>>
>>>>> I have no personal objection to actors or processes or anything else. I would just emphasize that the webservers in existence for Swift (and quite well funded) are right now on a “crash the whole thing” model AFAIK. I don’t know if they will be able to get by with catching all the precondition failures in testing - maybe they will - maybe not, but I see people either wisely turning down Swift for web development or getting their first crash as a “zero-day” system down event where some guy has to ssh in and actually start the server back up.
>>>>>
>>>> I haven't worked directly with server people that much personally, but on the opportunities when I have been able to talk to people working in this space, it sounds like they definitely want something better.
>>>>> But whatever the fix is, or nothing, they (developers of Perfect, Kitura, Vapor, etc) will have to like it and implement it. If they’re not interested, I’m not starting a development company to compete.
>>>>>
>>>> I don't see a problem with people developing frameworks to the best of the platform's ability today and refining their approach as new capabilities get developed.
>>>>> And also the development time is so short on my idea that you basically just have to approve it and the development time for it is hours not months or years. Just add a fatalError function override and any c developer could have it done in minutes like this guy https://github.com/ankurp/unsafe-swift <https://github.com/ankurp/unsafe-swift> (not endorsing the post, just showing evidence that setjmp and longjmp work fine with swift).
>>>>>
>>>> setjmp and longjmp do *not* work well with Swift since they need compiler support to implement their semantics, and since Swift does not provide this support, setjmp-ing from Swift is undefined behavior. Empirical evidence that small examples appear to work is not a good way of evaluating UB, since any changes to Swift or LLVM optimizations may break it. Ease of implementation is also not a good criterion for designing things. *Supporting* a trap hook is not easy; it still has serious language semantics and runtime design issues, and may limit our ability to do something better.
>>>>
>>>> -Joe
>>>
>>>
>>> Yeah, I suppose. Sounds like it is still about 2-3 years off.
>>>
>>
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>> But…. what happens when a shared or shared sub-process trips? If something about the process was shared, then it will end up tripping every other process except some master process which will re-generate them.
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>> Concerning longjmp, it is possible to call from Swift into C, and then from C into Swift (not in the example, but in general). Therefore, assuming that the execution state of the C function is still on the stack at the moment that the exception trips, the jump won’t necessarily involve the Swift runtime or compiler at all, so far as I can see, aside from leaked memory. Granted I don’t know that much about the runtime, but an example might be useful.
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>> The linked example is was done inline, so a bit different than I was thinking.
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>> BTW, not sure why no one mentioned it, but apparently the whole idea proposed is already possible without any proposal actually needed. A bit too complex for me and dangerous looking, but heres a nice post with it already done.
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>> https://www.cocoawithlove.com/blog/2016/02/02/partial-functions-part-two-catching-precondition-failures.html#setting-up-a-mach-exception-handler <https://www.cocoawithlove.com/blog/2016/02/02/partial-functions-part-two-catching-precondition-failures.html#setting-up-a-mach-exception-handler>
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> How about going half way on a proposal for a “fatalError” callback, which would take a “c” function (or be accessible only from c) to prevent recursion. ie. if the handler was in Swift the callback would then be liable to induce another fatalError.
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> The purpose of which would be to log the fatal error string to a file.
>
> int fatalErrorCallback(char* message, char* stacktrace);
> Any other usages would be unsupported and something to discuss in the dev channel (no bearing on the ABI or compiler design, only the reverse). If it returns “0” then Swift omits its own std output, but in either case it exits.
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> One can see easily from the error printout that Swift is running its own callback, even in Swift:
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> fatal error: unexpectedly found nil while unwrapping an Optional value
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> Current stack trace:
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> 0 TestJmp1 0x0000000104b11410 swift_reportError + 132
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> 1 TestJmp1 0x0000000104b2e7c0 _swift_stdlib_reportFatalError + 61
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> 2 TestJmp1 0x000000010489c550 specialized specialized StaticString.withUTF8Buffer<A> ((UnsafeBufferPointer<UInt8>) -> A) -> A + 355
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> 3 TestJmp1 0x0000000104a9e7f0 partial apply for (_fatalErrorMessage(StaticString, StaticString, StaticString, UInt, flags : UInt32) -> Never).(closure #2) + 109
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> 4 TestJmp1 0x000000010489c550 specialized specialized StaticString.withUTF8Buffer<A> ((UnsafeBufferPointer<UInt8>) -> A) -> A + 355
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> 5 TestJmp1 0x0000000104a59170 specialized _fatalErrorMessage(StaticString, StaticString, StaticString, UInt, flags : UInt32) -> Never + 96
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> 6 TestJmp1 0x00000001047f87f0 (closure #1) + 221
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> 7 TestJmp1 0x00000001047f8a30 @objc (closure #1) + 9
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> 8 TestJmp1 0x00000001047f8ae0 runCode + 49
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> 9 TestJmp1 0x00000001047f8700 main + 87
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> 10 libdyld.dylib 0x00007fffe1deb254 start + 1
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> Illegal instruction: 4
>
If all you can do is log an alternative to what Swift would output anyway, what's the use case? And certainly permitting more things to happen in that callback falls under the same caveats as an in-process crash reporter - compromised context, uncertain data state, very few safe operations.
-- Gwynne Raskind
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