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diff --git a/user/jkoenig/java/discussion.mdwn b/user/jkoenig/java/discussion.mdwn index 266a7bcc..f16d7678 100644 --- a/user/jkoenig/java/discussion.mdwn +++ b/user/jkoenig/java/discussion.mdwn @@ -8,6 +8,11 @@ Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled [[GNU Free Documentation License|/fdl]]."]]"""]] +[[!toc]] + + +# General + Some [[tschwinge]] comments regarding your proposal. Which is very good, if I may say so again! :-) @@ -70,3 +75,452 @@ either new ones or existing ones, as applicable. * We should probaby open up a *languages for Hurd* section on the web pages ([[!taglink open_issue_documentation]]). + + +# IRC, freenode, #hurd, 2011-07-13 + +[[!tag open_issue_documentation]] + + <jkoenig> Yes, I guess so. Maybe start investigating mig because it may + have repercussions on what the best approach would be for some aspects of + the Mach bindings. + <tschwinge> I still think that making MIG emit Java code is not too + difficult, once you have the required Java infrastructure (like what + you're writing at the moment). + <tschwinge> On the other hand, if there's another approach that you'd like + to use, I'm not trying to force using MIG. + <braunr> i still have a problem understanding your approach + <braunr> at which level are your bindings located ? + <jkoenig> I expect mig it will be the easiest route, but of course possibly + it won't. + <tschwinge> jkoenig: Yeah, be give some high-level to low-level overview? + <jkoenig> ok, so + <jkoenig> at the very core, low-level, we have a very thin amount of JNI + code to access (proper) system calls. + <jkoenig> by "proper" I mean things like mach_task_self, mach_msg and + mach_reply_port, which are actually system calls rather than RPCs to the + kernel. + <braunr> right + <jkoenig> at this level, we manipulate port names as integers, and the + message buffers for mach_msg are raw ByteBuffers (from the java.nio + package) + <jkoenig> actually, so-called /direct/ ByteBuffers, which are backed by + memory allocated outside of the Java heap, rather than as a byte[] array + <jkoenig> we can retreive the pointer from the JNI code and use the buffer + directly. + <jkoenig> (so, good for performance and it's also portable.) + <braunr> ok + <braunr> i'm more interested in the higher level bindings :) + <jkoenig> ok so, higher up. + <jkoenig> design goal from my proposal: "the memory safety of Java should + be maintained and extended to Mach primitives such as port names and + out-of-line memory regions" + <jkoenig> so integer port names are not "safe" in the sense that they can + be forged and misused in all kinds of way + <jkoenig> which is why I have a layer of Java code whose job is to wrap + this kind of low-level Mach stuff into safe abstractions + <jkoenig> and ideally the user should only use these safe abstractions. + <tschwinge> (Not to restrict the programmer, but to help him write correct + code.) + <jkoenig> right. + <braunr> so you can't use mach RPCs directly + <jkoenig> tschwinge, also to actually restrict them, in a Joe-E / + object-capability context, but that's not the primary concern right now + ;-) + <braunr> or you force your wrappers to have these abstractions as input + <jkoenig> braunr, well, actually at this level you still have Mach RPC + <jkoenig> but for instance, port names are encapsulated into "MachPort" + objects which ensure they are handled correcly + <tschwinge> As I understand it, you use these abstractions to prepare a + usual mach_msg message, and then invoke mach_msg. + <braunr> ok + <jkoenig> and message buffers are wrapped into "MachMsg" objects which both + help you write the messages into the ByteBuffer and prevent you from + doing funky stuff + <jkoenig> and ensure the ports which you send/receive/pseudo-receive after + an error/... are deallocated as required, etc. + <braunr> what's the interface to use IPC ? + <tschwinge> Is MIG doing that, too, I think? (And antrik once found some + error there, which is still to be reviewed...) + <jkoenig> braunr, so basically as a user you would be free to use either + one of these layers, or to use MIG-generated classes which would + construct and exchange messages for you using the second (safe) layer. + <braunr> ok, let's just finish with the low level layer before going + further please + <jkoenig> tschwinge, MIG does some type checking on the received message + and saves you the trouble of constructing/parsing them yourself, but I'm + not sure about how mach_msg errors are handled + <braunr> what are the main methods of MachMsg for example ? + <jkoenig> braunr, you may want to have a look at + http://jk.fr.eu.org/hurd-java/doc/html/classorg_1_1gnu_1_1mach_1_1MachMsg.html + <braunr> right, sorry + <braunr> grabbed the code at work and forgot here + <jkoenig> and also + https://github.com/jeremie-koenig/hurd-java/blob/master/HelloMach.java + which uses it + <jkoenig> but roughly, you'd use setRemotePort, setLocalPort, setId to + write your message's header + <jkoenig> then use one of the putFoo() methods to add data items to the + message + <braunr> ok, the mapping with the low level C interface is very clear + <braunr> that's good for me + <jkoenig> the putFoo() methods would write the appropriate type + descriptors, then the actual data. + <braunr> we can go on with the MiG part if you want :) + <jkoenig> right, + <jkoenig> so here you may want to look at the UML class diagram from + http://www.bddebian.com/~hurd-web/user/jkoenig/java/proposal/ + <jkoenig> so in the C case, mig generates 3 files + <jkoenig> a header file which has the prototypes of the mig-generated + stubs, + <jkoenig> a *User.c which has their actual implementation + <jkoenig> and a *Server.c which handles demultiplexing the incoming + messages and helps with implementing servers. + <jkoenig> so we would do something along these lines, more or less: + <jkoenig> mig would generate the code for a Java interface in lieu of the + *.h file. + <jkoenig> a generated FooUser class would implement this interface by doing + RPC + <jkoenig> (so basically you would pass a MachPort object to the + constructor, and then you could use the resulting object to do RPC with + whatever is on the other end) + <jkoenig> and the generated FooServer class would do the opposite, + <braunr> ok + <braunr> issues with threads ? + <jkoenig> you would pass an object implementing the Foo interface to the + constructor, + <braunr> i'm guessing the demux part may have to create threads, right ? + <jkoenig> and the resulting object would handle messages by using the + object you passed. + <jkoenig> braunr, right, so that would be more a libports kind of code, + <braunr> the libports-like library, i see + <jkoenig> to which you could pass Server objects (for instance the + FooServer above), and it would handle incoming messages. + <braunr> how is message content mapped to a java interface ? + <jkoenig> this would be determined from the .defs files and MIG would + generate the appropriate code, hopefully. + <braunr> so the demux part would handle rpc integer identifiers ? + <jkoenig> right. + <braunr> but hm + <jkoenig> also mapping .defs files to Java interfaces might prove to be + tricky. data types conversion and all + <antrik> tschwinge: my mamory is rather hazy. IIRC the issue was that the + MIG-generated stubs deallocate out-of-line port arrays after the + implementation returns, before returning to the dispatcher + <braunr> i'll just overlook this specific implementation detail + <jkoenig> but we could use some annotation-based system if we need to + provide more information to generate the java code. + <antrik> but the Hurd (or rather glibc) RPC handling also automatically + deallocates everything if an error occurs + <antrik> so I changed the MIG code to deallocate only when no error occurs + <braunr> jkoenig: ok, we'll talk about that when there is more progress and + you have a better view of the problem + <antrik> at that time I was pretty sure that this is a correctly working + solution, but it always seemed questionable conceptually... however, I + wasn't able to come up with a better one, and nobody else commented on it + <braunr> antrik: shouldn't the hurd be changed not to deallocate something + it didn't allocate in the first place ? + <antrik> braunr: no, the server has to deallocate stuff before returning to + the client. the request message is destroyed before returning the reply. + <tschwinge> jkoenig, braunr: That's what I had in mind where MIG might be a + bit awkward. Then we can indeed either add annotations to the .defs + files, or reproduce them in some other format. That's some work, but + it's mostly a one-time work. + <tschwinge> After all, the RPC interface is a binary one, and there may be + more than one API for creating these messages, etc. + <antrik> jkoenig: actually, at least in the Hurd, server-side and + client-side headers are separate -- so MIG actually creates four files + <jkoenig> tschwinge, wrt to annotations I was more thinking about Java + ones, such as: @MIGDefsFile("mach/task.defs") @MIGCType("task_t") public + interface Task { } + <jkoenig> antrik, oh, ok, it makes sense. + <braunr> jkoenig: anything else ? + <jkoenig> braunr, nothing that I can think of + <braunr> ok + <antrik> tschwinge: I think it would be a *very* bad idea to introduce + redundancy regarding RPC definitions + <braunr> thanks for the tour :) + <antrik> (the _request.defs/_reply.defs mess is bad enough...) + <jkoenig> did I speak about the "Unsafe" pseudo-exception? that's + interesting :-) + <tschwinge> jkoenig: Also, virtual memory abstractions? + <braunr> jkoenig: you didn't + <tschwinge> antrik: Well, then we could create some other super-format. + But that's just a detail IMO. + <jkoenig> ok, so wrt virtual memory, a page we received can be wrapped with + some JNI help into a (direct) ByteBuffer object. + <jkoenig> deallocating sent pages will be tricky, though. + <tschwinge> antrik: To put it this way: for me the .defs files are just one + way of expressing the RPC interfaces' contracts. (At the same time, they + happen to be the actual reference for these, too. But the specification + itself could just as well be a textual one.) + <jkoenig> on approach I've been thinking about would be to "wrap" the + ByteBuffer object into an object which has the sole reference to it, so + that when it's deallocated the reference can be replaced with "null", and + further attempts to access the buffer would throw exceptions. + <braunr> sounds reasonable + <jkoenig> but that's still in flux in my head, we may end up needing our + own implementation of ByteBuffer-like objects. + <tschwinge> The problem being that there is no mechanism to ``revoke'' an + object once a reference to it has been shared. + <jkoenig> right. + <tschwinge> A wrapper is one possibility indeed. + <antrik> tschwinge: they are called interface *definitions* for a reason + :-) + <tschwinge> This is a very similar problem as with capabilities when there + is no revoke operation for these, too. + <tschwinge> antrik: Yes, because they define MIG's input. :-P + <tschwinge> Isn't that what is called a membrane in the capability world? + <antrik> I do not say that we have to consider the format of the .defs to + be set in stone; but I do insist on using a canonical machine-parsable + source for all language bindings + <tschwinge> attenuation + <jkoenig> tschwinge, you mean the revokable proxy contruct ? (It's the same + principle indeed) + <tschwinge> A common design pattern in object-capability systems: given + one reference of an object, create another reference for a proxy object + with certain security restrictions, such as only permitting read-only + access or allowing revocation. The proxy object performs security checks + on messages that it receives and passes on any that are allowed. Deep + attenuation refers to the case where the same attenuation is applied + transitively to any + <tschwinge> objects obtained via the original attenuated object, + typically by use of a "membrane". + <tschwinge> http://en.wikipedia.org/wiki/Object-capability_model + <tschwinge> Yes. + <tschwinge> Good. I understood something. ;-) + <tschwinge> antrik: OKAY! :-P + <tschwinge> jkoenig: And hopefully the JVM will optimize away all the + additional indirection... :-D + <tschwinge> jkoenig: Is there anything more to say about the VM layer? + <jkoenig> tschwinge, "hopefully", yes :-) + <tschwinge> Like, the data that I'm sharing -- is it untyped, isn't it? + <jkoenig> tschwinge, you mean that within the received/sent pages ? + <tschwinge> Yes. + <tschwinge> But that'S how it is, indeed. + <jkoenig> well actually the type descriptor should indicate what they + contain. + <tschwinge> I cannot trust anything I receive from externally. + <jkoenig> it's most often used for MACH_MSG_TYPE_CHAR items I guess, and it + will be type checked when retreive + <tschwinge> Yeah, and that then just *is* arbitrary data, like a block read + from a disk file. + <jkoenig> you would have something like: ByteBuffer + MachMsg.getBuffer(MachMsg.Type expected), and MachMsg would check the + type descriptor against that which you specified + <tschwinge> Or a packet transmitted over the network. + <tschwinge> OK, yes. + <antrik> jkoenig: in theory ints should be used quite often too. the whole + purpose of the type descriptors is to allow byte order swapping when + messages are passed between hosts with different architecture... + <jkoenig> tschwinge, right, except for out-of-line port arrays, which need + to be handled differently obviously. + <antrik> (which is totally irrelevat for our purposes -- especially since + the actual network IPC code doesn't exist anymore ;-) ) + <jkoenig> antrik, oh, interesting + <tschwinge> Yes, that was one original idea. + <jkoenig> actually my litmus test for what the bindings should be, is you + should be able to implement such a proxy in Java :-) + <tschwinge> antrik: And hey, you now have processors that can switch + between different modes during runtime... :-) + <jkoenig> (although arguably that's a little bit ambitious) + <braunr> tschwinge: there should be bits in page tables to indicate the + endianness to use on a page .. :) + <tschwinge> Hehe! + <tschwinge> jkoenig: Don't worry -- you're already known for ambitious + projects. One more can't hurt. + <jkoenig> Also, actually the word size is not something that I've been able + to abstract so far, so I'll be hardcoding little-endian 32 bits for now. + <braunr> why is that ? + <antrik> some of the Hurd RPC break the idea anyways BTW + <jkoenig> the org.vmmagic package (from Jikes RVM and JNode) could help + with that, but GCJ does not support it unfortunately (not sure about + OpenJDK) + <jkoenig> braunr, Java does not allow us to define new unboxed types + <braunr> jkoenig: does it have its own definition of the word size ? + <jkoenig> braunr, nope. + <jkoenig> (although, maybe, and also we could use JNI to query it) + <braunr> even if virtual, i'd expect a machine to have such a defnition + <jkoenig> braunr, maybe it has, but basically in Java nothing depends on + the word size + <jkoenig> 'int' is 32 bits, 'long' is 64 and that's it. + <braunr> oh right, i remember most types are fixed size, right ? + <jkoenig> right. + <braunr> if not all + <jkoenig> now Jikes RVM's "org.vmmagic" provides an interface to defined + new unboxed types which can depend on the actual word size, but Jikes RVM + is its own JVM so obviously they can use and provide whatever extensions + they need :-) + <jkoenig> (but maybe they've implemented them in OpenJDK for bootstrap + purposes, I'm not sure) + <tschwinge> I'm missing this detail: where does the word size come into + play here? + <jkoenig> anyway, I _could_ indiscriminately use 'long' for port names, and + sparkle the code with word size tests but that would be very clumsy + <braunr> jkoenig: port names are actually ints :/ + <jkoenig> tschwinge, the actual format of the message header and type + descriptors, for instance. + <braunr> jkoenig: ok, got your point + <jkoenig> braunr, by 'long' I mean 64-bits integers (which they are on + 64-bits machines I think?) + <braunr> :) + <braunr> jkoenig: port names are as large as the word size + <braunr> but in C at least, they're int, not long + <braunr> it doesn't change many things, but you get lots of warnings if you + try with a long :) + <tschwinge> What is the reason that port names are an + architecture-dependent word size's width, and not simply 32 bit? + <jkoenig> "4 billions of port names should be enough for everyone" :-) + <braunr> tschwinge: an optimization is to use them as pointers in the + kernel + <antrik> tschwinge: the machine's native word size is what it can process + most efficiently, and what should be used for most normal + operations... it makes sense to define stuff as int, except for network + communication + <tschwinge> jkoenig: Well, yeah, but if you want to communicate with a + peer, you have to agree on the maximum number anyway (not for port names, + though, which are local). + <braunr> antrik: int isn't the word size everywhere + <braunr> antrik: the most common type matching the word size is long, at + least on ILP32/LP64 data models + <antrik> braunr: that's just because some idiots assumed int would always + be 32 bits, and consequently when 64 architectures came up the compiler + guys chickened out ;-) + <braunr> without int, you wouldn't have a 32 bits type + <antrik> that's not true for all architectures and/or operating systems + though AFAIK + <braunr> or a 16 bits one + <braunr> antrik: windows guys got even more scared, so windows 64 is LLP64 + <antrik> BTW, I haven't checked, but it's quite possible that 32 bit + numbers are actually preferable even on AMD64... + <tschwinge> jkoenig: So, back on track. :-) + <tschwinge> jkoenig: You didn't find anything yet in Mach's VM interfaces + as well a MemoryObject, etc., that can't be used/implemented in the Java + world? + <braunr> antrik: they consume less memory, but don't have much effect on + performance + <jkoenig> tschwinge, once we have the basic system calls and the + corresponding abstractions in place, I don't think anything else + fundamentally problematic could possibly show up + <antrik> braunr: if you really *need* a type of a certain bit size, you + should use stdint types. so not having a 16 or 32 bit type in the + short/int/long canon is *not* an excuse + <tschwinge> jkoenig: That speaks for the Mach designers! + <braunr> antrik: right + <jkoenig> tschwinge, on trick is that for instance, mach_task_self would + still be unsafe even if it returned a nicely wrapped Task object, because + you could still wreck your own address space and threads with it. So we + would need the "attenuation" pattern mentionned above to provide a safe + one. + <jkoenig> (which would disallow thinks such as the port/thread/vm calls) + <braunr> jkoenig: you mentioned the unsafe pseudo exception earlier + <jkoenig> braunr, right, so the issue is with distinguishing safe from + unsafe methods + <antrik> braunr: BTW, the Windows guys actually broke a lot of stuff by + fixing long at 32 bits -- this way long doesn't match size_t and pointer + types anymore, which was an assumption that was true for pretty much any + system so far... + <tschwinge> jkoenig: Yes. (And again hope for the JVM to optim...) + <braunr> antrik: that's right :) + <braunr> antrik: that's LLP64 + <braunr> antrik: long long and pointers + <jkoenig> braunr, so basically the idea is that unsafe methods are declared + as "throws Unsafe" + <jkoenig> the effect is that if you use such a method you must either + "throw Unsafe" yourself, + <jkoenig> or if you're building a safe abstraction on top of Unsafe + methods, you'll "catch" the "exception" in question to tell the compiler + that it's okay. + <jkoenig> it's more or less inspired from the "semantic regimes" idea from + the org.vmmagic paper which is referenced in my original proposal, + <jkoenig> only implementing by hijacking the exception checking machinery, + which has a behaviour similar to what we want. + <braunr> ok + <braunr> but hmm this seems pretty normal, what's the tricky part ? :) + <tschwinge> braunr: The idea is that the programmer explicitly has to + acknowledge if he'S using an unsafe interface. + <braunr> tschwinge: sounds pretty normal too + <jkoenig> braunr, the trick is that you would not usually declare + exceptions which are never actually thrown (and actually since the + compiler does not know it's never thrown, I need to work around it in a + few places) + <braunr> oh, ok + <braunr> jkoenig: that's interesting indeed + <jkoenig> braunr, the org.vmmagic paper provides an example which uses some + annotations called @UncheckedMemoryAccess and @AssertSafe to the same + effect (which is kind of cleaner), but it would be a headache to + implement without help from the compiler I think (as far as I can tell + the annotation processor would have to inspect the bytecode) + <braunr> but hm + <braunr> what's the true problem about this ? + <jkoenig> (the paper advocates "high-level low-level programming" and is a + very interesting read I think, + http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.151.5253&rep=rep1&type=pdf, + for what it's worth) + <braunr> what's wrong if you just declare your methods unsafe and don't + alter anything else ? + <tschwinge> Yes, I read it and it is interesting. Unfortunately, it seems + I forgot most of it again... + <jkoenig> braunr, declare? alter? + <jkoenig> you mean just tag them with an annotation? + <braunr> just stating a method "throws Unsafe" + <jkoenig> braunr, well some compiler will output a warning because they can + tell there's no way the method is going to throw such an exception. + <jkoenig> and then some other compiler will complain that my + @SuppressWarnings("unused") does not serve any purpose to them :-) + <jkoenig> also, when initializing final fields, I need to work around the + fact that the compiler thinks "Unsafe" might be thrown. + <jkoenig> see for instance MachPort.DEAD + <braunr> jkoenig: ok + <jkoenig> braunr, but I'm more than willing to accept this in exchange for + a clear, compiler-enforced materialization of the border between safe an + unsafe code. + <jkoenig> actually another question I have is the amount of static typing I + should add to the safe version, for instance should I subclass MachPort + into MachSendRight, MachReceiveRight and so on. I don't want to depart + from the C inteface too much but it could be useful. + <braunr> jkoenig: can't answer that :) + <braunr> jkoenig: keep them in mind for later i think + <tschwinge> jkoenig: What's the safety concern w.r.t. having MachPort (not) + final? + <jkoenig> tschwinge, actually I'm partly wrong in that we only need name() + and a couple other methods to be final + <tschwinge> jkoenig: That's what I was thinking. :-) + <tschwinge> I though I'm missing something here. + <jkoenig> tschwinge, the idea is that the user (ie., the adversary :-) + could extend MachPort and inject their own fake port name into messages + <jkoenig> by overriding name() or clear() + <tschwinge> Yeah, but if these are final, that's not possible. + <jkoenig> right. + <tschwinge> And that *should* be enough, I think. + <tschwinge> Unless I'm missing something. + <jkoenig> I don't think so. Also I hope it is, because as mentionned above + there might be some value in subclassing MachPort. + <tschwinge> Yep. + <jkoenig> incidentally, declaring the class or the method final will allow + the JVM to inline them I think. + <tschwinge> It will help the JVM, yes. It can also figure that out without + final, though. (And may have to de-optimize the code again in case there + are additional classes loaded during run-time.) + <tschwinge> jkoenig: The reference counting in MachPort. I think I'm + beginning to understand this. + <jkoenig> oh ok + <jkoenig> tschwinge, yes the javadoc is maybe a bit obscure so far. + <jkoenig> but basically you don't want the port name you acquire to become + invalid before you're done using it. + <tschwinge> But how is this different from the C world? + <jkoenig> here my goal is to provide some guarantees if you use only safe + methods + <jkoenig> like, you can't forge a port name and things like that + <jkoenig> so basically it should never be possible to include an invalid + port name in a message if you use only safe methods. + <tschwinge> Ah, I see! + <tschwinge> Now that does make sense. + <jkoenig> but the mechanism in itself is similar to the Hurd port cells and + user_link structures + <tschwinge> It's again ``only'' helping the programmer. + <jkoenig> right, no object-capability ulterior motives :-) + <jkoenig> another assumption which the javadoc does not state yet it that + basically there should be exactly one MachPort object for each mach-level + port name reference (in the sense of mach_port_mod_refs) + <tschwinge> Yes, I figured out that bit. |