Magnus, thanks for the response. Please see comments inlined below. On Fri, Apr 12, 2024 at 4:52 AM Magnus Ihse Bursie <magnus.ihse.bur...@oracle.com> wrote: > > On 2024-04-02 21:16, Jiangli Zhou wrote: > > Hi Magnus, > > In today's zoom meeting with Alan, Ron, Liam and Chuck, we (briefly) > discussed how to move forward contributing the static Java related changes > (additional runtime fixes/enhancements on top of the existing static support > in JDK) from https://github.com/openjdk/leyden/tree/hermetic-java-runtime to > JDK mainline. > > Just a bit more details/context below, which may be useful for others reading > this thread. > > The https://github.com/openjdk/leyden/tree/hermetic-java-runtime branch > currently contains following for supporting hermetic Java (without the > launcher work for runtime support): > > 1. Build change for linking the Java launcher (as bin/javastatic) with > JDK/hotspot static libraries (.a), mainly in > https://github.com/openjdk/leyden/blob/hermetic-java-runtime/make/StaticLink.gmk. > The part for creating the complete sets of static libraries (including > libjvm.a) has already been included in the mainline since last year. > https://github.com/openjdk/leyden/blob/hermetic-java-runtime/make/StaticLink.gmk > is in a very raw state and is intended to demonstrate the capability of > building a static Java launcher. > > Indeed. It is nowhere near being able to be integrated. >
The main purpose of StaticLink.gmk is to support the static-java-image make target, which can be used to perform the actual static linking step using libjvm.a and JDK static libraries. That currently doesn't exist in the JDK mainline. Creating a "fully" statically linked Java launcher is the first step (out of many) towards supporting static/hermetic Java. As part of cleaning/refactoring/integrating for the static linking step, we want to agree and decide/accept on the following: - Support the "fully" statically linked java launcher for testing and demoing the capability of static JDK support, e.g. - Support running jtreg testing using the "fully" statically linked Java launcher - Set up tests in github workflow to help detect any breaking changes for static support, e.g. new symbol issues introduced by any changes. There were some earlier discussions on this with Ron and Alan during the zoom meetings. - Which JDK native libraries to be statically linked with the new launcher target? E.g. StaticLink.gmk currently excludes libjsound.a, libawt_xawt.a, etc from statically linked with the launcher. - Do we want more than one statically linked launcher target, based on the set of linked native libraries? Based on the decisions of the above, the launcher static linking part would mostly be in a different shape when it's integrated into the mainline. That's why I referred to StaticLink.gmk as in a "very raw" state. Here is a high-level view of the state of things for static support: (I) What we already have in the JDK mainline: - Able to build a complete set of JDK/VM static libraries using `static-libs-image` make target (necessary for supporting static JDK) - Compilation for .o files are done separately for the static libraries and dynamic library (ok for now) (II) What missing: - Static linking step as mentioned above (III) What needs to be improved (require cleanups and refactoring, and you mentioned some of those in your response as well): - Support building both the static libraries and dynamic libraries using the same set of .o files, instead of separately compiled .o files. That helps improve build speed and reduce memory overhead for building JDK. Your current refactoring work aims to help that. - Clean up the usages of STATIC_BUILD macro. Most of the usages are in test code. - Other runtime fixes/enhancements in the leyden https://github.com/openjdk/leyden/tree/hermetic-java-runtime branch I think most work mentioned in III has dependencies on II. We need a workable base to be able to build the "fully" statically linked launcher for building and testing the work mentioned in III, when integrating any of those to the JDK mainline. The makefile refactoring work can be done in parallel but does not need to be completed before we add the static linking step in JDK mainline. > > 2. Additional runtime fixes/enhancements on top of the existing static > support in JDK, e.g. support further lookup dynamic native library if the > built-in native library cannot be found. > > 3. Some initial (prototype) work on supporting hermetic JDK resource files in > the jimage (JDK modules image). > > To move forward, one of the earliest items needed is to add the capability of > building the fully statically linked Java launcher in JDK mainline. The other > static Java runtime changes can be followed up after the launcher linking > part, so they can be built and tested as individual PRs created for the JDK > mainline. Magnus, you have expressed interest in helping get the launcher > linking part (refactor from > https://github.com/openjdk/leyden/blob/hermetic-java-runtime/make/StaticLink.gmk) > into JDK mainline. What's your thought on prioritizing the launcher static > linking part before other makefile clean ups for static libraries? > > Trust me, my absolute top priority now is working on getting the proper build > support needed for Hermetic Java. I can't prioritize it any higher. Thanks! > > I am not sure what you are asking for. We can't just merge StaticLink.gmk > from your prototype. And even if we did, what good will it do you? Please see my comments above. > > The problem you are running into is that the build system has not been > designed to properly support static linking. There are already 3-4 hacks in > place to get something sort-of useful out, but they are prone to breaking. I > assume that we agree that for Hermetic Java to become a success, we need to > have a stable foundation for static builds. > > The core problem of all static linking hacks is that they are not integrated > in the right place. They need to be a core part of what NativeCompilation > delivers, not something done in a separate file. To put it in other words, > StaticLink.gmk from your branch do not need cleanup -- it needs to go away, > and the functionality moved to the proper place. > > My approach is that NativeCompilation should support doing either only > dynamic linking (as today), or static linking (as today with STATIC_LIBS or > STATIC_BUILD), or both. The assumption is that the latter will be default, or > at least should be tested by default in GHA. For this to work, we need to > compile the source code to .o files only once, and then link these .o files > either into a dynamic or a static library (or both). As of today, the leyden https://github.com/openjdk/leyden/tree/hermetic-java-runtime branch can build a "fully" statically linked Java launcher. The issue of compiling the dynamic and static libraries .o files separately is not a blocker. It's good to have it resolved at some point of time. > > This, in turn, require several changes: > > 1) The linking code needs to be cleaned up, and all technical debt needs to > be resolved. This is what I have been doing since I started working on static > builds for Hermetic Java. JDK-8329704 (which was integrated yesterday) was > the first major milestone of this cleanup. Now, the path were to find a > library created by the JDK (static or dynamic) is encapsulated in > ResolveLibPath. This is currently a monster, but at least all knowledge is > collected in a single location, instead of spread over the code base. Getting > this simplified is the next step. > > 2) We need to stop passing the STATIC_BUILD define when compiling. This is > partially addressed in your PR, where you have replaced #ifdef STATIC_BUILD > with a dynamic lookup. But there is also the problem of JNI/JVMTI entry > points. I have been pondering how we can compile the code in a way so we > support both dynamic and static name resolution, and I think I have a > solution. > > This is unfortunately quite complex, and I have started a discussion with > Alan if it is possible to update the JNI spec so that both static and dynamic > entry points can have the form "JNI_OnLoad_<library-name>". Ideally, I'd like > to see us push for this with as much effort as possible. If we got this in > place, static builds would be much easier, and the changes required for > Hermetic Java even smaller. Thumbs up! That seems to be a good direction. Currently in the leyden branch, it first looks up the unique JNI_OnLoad<_lib_name>|Agent_OnLoad<_lib_name> etc for built-in libraries, then search for the dynamic libraries using the conventional naming when necessary. e.g.: https://github.com/openjdk/leyden/commit/a5c886d2e85a0ff0c3712a5488ae61d8c9d7ba1a https://github.com/openjdk/leyden/commit/1da8e3240e0bd27366d19f2e7dde386e46015135 When spec supports JNI_OnLoad_<library-name> and etc. for dynamic libraries, we may still need to support the conventional naming without the <_lib_name> part for existing libraries out there. > > And finally, on top of all of this, is the question of widening the platform > support. To support linux/gcc with objcopy is trivial, but the question about > Windows still remain. I have two possible ways forward, one is to check if > there is alternative tooling to use (the prime candidate is the clang-ldd), > and the other is to try to "fake" a partial linking by concatenating all > source code before compiling. This is not ideal, though, for many reasons, > and I am not keen on implementing it, not even for testing. And at this > point, I have not had time to investigate any of these options much further, > since I have been focusing on 1) above. > > A third option is of course to just say that due to toolchain limitations, > static linking is not available on Windows. Thank you for taking this on! Potentially we could consider taking the objcopy to localizing hotspot symbols on unix-like platforms, based on https://github.com/openjdk/jdk/pull/17456 discussions. Additional testing is still needed to verify the solution. > > My recommendation is that you keep on working to resolve the (much more > thorny) issues of resource access in Hermetic Java in your branch, where you > have a prototype static build that works for you. In the meantime, I will > make sure that there will be a functioning, stable and robust way of creating > static builds in the mainline, that can be regularly tested and not bit-rot, > like the static build hacks that has gone in before. Most of the JDK resources are now supported as hermetic jimage (lib/modules) bundled in the https://github.com/openjdk/leyden/tree/hermetic-java-runtime branch. The remaining sound.properties, ct.sym and .jfc files can be handled later. Overally, that part of the work has confirmed the hermetic jimage bundled solution is robust and helps resolve some of the difficult start-up sequence issues observed when the hermetic resource was implemented using JAR file based solution. It might be a good idea to follow up on the static linking discussion in tomorrow's zoom meeting (hope you'll be able to join tomorrow). Thanks! Jiangli > > /Magnus > > > > Thanks! > Jiangli > > On Thu, Feb 15, 2024 at 12:01 PM Jiangli Zhou <jiangliz...@google.com> wrote: >> >> On Wed, Feb 14, 2024 at 5:07 PM Jiangli Zhou <jiangliz...@google.com> wrote: >> > >> > Hi Magnus, >> > >> > Thanks for looking into this from the build perspective. >> > >> > On Wed, Feb 14, 2024 at 1:00 AM Magnus Ihse Bursie >> > <magnus.ihse.bur...@oracle.com> wrote: >> > > >> > > First some background for build-dev: I have spent some time looking at >> > > the build implications of the Hermetic Java effort, which is part of >> > > Project Leyden. A high-level overview is available here: >> > > https://cr.openjdk.org/~jiangli/hermetic_java.pdf and the current source >> > > code is here: >> > > https://github.com/openjdk/leyden/tree/hermetic-java-runtime. >> > >> > Some additional hermetic Java related references that are also useful: >> > >> > - https://bugs.openjdk.org/browse/JDK-8303796 is an umbrella bug that >> > links to the issues for resolving static linking issues so far >> > - https://github.com/openjdk/jdk21/pull/26 is the enhancement for >> > building the complete set of static libraries in JDK/VM, particularly >> > including libjvm.a >> > >> > > >> > > Hermetic Java faces several challenges, but the part that is relevant >> > > for the build system is the ability to create static libraries. We've >> > > had this functionality (in three different ways...) for some time, but >> > > it is rather badly implemented. >> > > >> > > As a result of my investigations, I have a bunch of questions. :-) I >> > > have gotten some answers in private discussion, but for the sake of >> > > transparency I will repeat them here, to foster an open dialogue. >> > > >> > > 1. Am I correct in understanding that the ultimate goal of this exercise >> > > is to be able to have jmods which include static libraries (*.a) of the >> > > native code which the module uses, and that the user can then run a >> > > special jlink command to have this linked into a single executable >> > > binary (which also bundles the *.class files and any additional >> > > resources needed)? >> > > >> > > 2. If so, is the idea to create special kinds of static jmods, like >> > > java.base-static.jmod, that contains *.a files instead of lib*.so files? >> > > Or is the idea that the normal jmod should contain both? >> > > >> > > 3. Linking .o and .a files into an executable is a formidable task. Is >> > > the intention to have jlink call a system-provided ld, or to bundle ld >> > > with jlink, or to reimplement this functionality in Java? >> > >> > I have a similar view as Alan responded in your other email thread. >> > Things are still in the early stage for the general solution. >> > >> > In the https://github.com/openjdk/leyden/tree/hermetic-java-runtime >> > branch, when configuring JDK with --with-static-java=yes, the JDK >> > binary contains the following extra artifacts: >> > >> > - static-libs/*.a: The complete set of JDK/VM static libraries >> > - jdk/bin/javastatic: A demo Java launcher fully statically linked >> > with the selected JDK .a libraries (e.g. it currently statically link >> > with the headless) and libjvm.a. It's the standard Java launcher >> > without additional work for hermetic Java. >> > >> > In our prototype for hermetic Java, we build the hermetic executable >> > image (a single image) from the following input (see description on >> > singlejar packaging tool in >> > https://cr.openjdk.org/~jiangli/hermetic_java.pdf): >> > >> > - A customized launcher (with additional work for hermetic) executable >> > fully statically linked with JDK/VM static libraries (.a files), >> > application natives and dependencies (e.g. in .a static libraries) >> > - JDK lib/modules, JDK resource files >> > - Application classes and resource files >> > >> > Including a JDK library .a into the corresponding .jmod would require >> > extracting the .a for linking with the executable. In some systems >> > that may cause memory overhead due to the extracted copy of the .a >> > files. I think we should consider the memory overhead issue. >> > >> > One possibility (as Alan described in his response) is for jlink to >> > invoke the ld on the build system. jlink could pass the needed JDK >> > static libraries and libjvm.a (provided as part of the JDK binary) to >> > ld based on the modules required for the application. >> > >> >> I gave a bit more thoughts on this one. For jlink to trigger ld, it >> would need to know the complete linker options and inputs. Those >> include options and inputs related to the application part as well. In >> some usages, it might be easier to handle native linking separately >> and pass the linker output, the executable to jlink directly. Maybe we >> could consider supporting different modes for various usages >> requirements, from static libraries and native linking point of view: >> >> Mode #1 >> Support .jmod packaged natives static libraries, for both JDK/VM .a >> and application natives and dependencies. If the inputs to jlink >> include .jmods, jlink can extract the .a libraries and pass the >> information to ld to link the executable. >> >> Mode #2 >> Support separate .a as jlink input. Jlink could pass the path >> information to the .a libraries and other linker options to ld to >> create the executable. >> >> For both mode #1 and #2, jlink would then use the linker output >> executable to create the final hermetic image. >> >> Mode #3 >> Support a fully linked executable as a jlink input. When a linked >> executable is given to jlink, it can process it directly with other >> JDK data/files to create the final image, without native linking step. >> >> Any other thoughts and considerations? >> >> Best, >> Jiangli >> >> > > >> > > 4. Is the intention is to allow users to create their own jmods with >> > > static libraries, and have these linked in as well? This seems to be the >> > > case. >> > >> > An alternative with less memory overhead could be using application >> > modular JAR and separate .a as the input for jlink. >> > >> > > If that is so, then there will always be the risk for name >> > > collisions, and we can only minimize the risk by making sure any global >> > > names are as unique as possible. >> > >> > Part of the current effort includes resolving the discovered symbol >> > collision issues with static linking. Will respond to your other email >> > on the symbol issue separately later. >> > >> > > >> > > 5. The original implementation of static builds in the JDK, created for >> > > the Mobile project, used a configure flag, --enable-static-builds, to >> > > change the entire behavior of the build system to only produce *.a files >> > > instead of lib*.so. In contrast, the current system is using a special >> > > target instead. >> > >> > I think we would need both configure flag and special target for the >> > static builds. >> > >> > > In my eyes, this is a much worse solution. Apart from >> > > the conceptual principle (if the build should generate static or dynamic >> > > libraries is definitely a property of what a "configuration" means), >> > > this makes it much harder to implement efficiently, since we cannot make >> > > changes in NativeCompilation.gmk, where they are needed. >> > >> > For the potential objcopy work to resolve symbol issues, we can add >> > that conditionally in NativeCompilation.gmk if STATIC_LIBS is true. We >> > have an internal prototype (not included in >> > https://github.com/openjdk/leyden/tree/hermetic-java-runtime yet) done >> > by one of colleagues for localizing symbols in libfreetype using >> > objcopy. >> > >> > > >> > > That was not as much a question as a statement. 🙂 But here is the >> > > question: Do you think it would be reasonable to restore the old >> > > behavior but with the new methods, so that we don't use special targets, >> > > but instead tells configure to generate static libraries? I'm thinking >> > > we should have a flag like "--with-library-type=" that can have values >> > > "dynamic" (which is default), "static" or "both". >> > >> > If we want to also build a fully statically linked launcher, maybe >> > --with-static-java? Being able to configure either dynamic, static or >> > both as you suggested also seems to be a good idea. >> > >> > > I am not sure if "both" are needed, but if we want to bundle both >> > > lib*.so and *.a files >> > > into a single jmod file (see question 2 above), then it definitely is. >> > > In general, the cost of producing two kinds of libraries are quite >> > > small, compared to the cost of compiling the source code to object files. >> > >> > Completely agree. It would be good to avoid recompiling the .o file >> > for static and dynamic builds. As proposed in >> > https://bugs.openjdk.org/browse/JDK-8303796: >> > >> > It's beneficial to be able to build both .so and .a from the same set >> > of .o files. That would involve some changes to handle the dynamic JDK >> > and static JDK difference at runtime, instead of relying on the >> > STATIC_BUILD macro. >> > >> > > >> > > Finally, I have looked at how to manipulate symbol visibility. There >> > > seems many ways forward, so I feel confident that we can find a good >> > > solution. >> > > >> > > One way forward is to use objcopy to manipulate symbol status >> > > (global/local). There is an option --localize-symbol in objcopy, that >> > > has been available in objcopy since at least 2.15, which was released >> > > 2004, so it should be safe to use. But ideally we should avoid using >> > > objcopy and do this as part of the linking process. This should be >> > > possible to do, given that we make changes in NativeCompilation.gmk -- >> > > see question 5 above. >> > > >> > > As a fallback, it is also possible to rename symbols, either piecewise >> > > or wholesale, using objcopy. There are many ways to do this, using >> > > --prefix-symbols, --redefine-sym or --redefine-syms (note the -s, this >> > > takes a file with a list of symbols). Thus we can always introduce a >> > > "post factum namespace" by renaming symbols. >> > >> > Renaming or redefining the symbol at build time could cause confusions >> > with debugging. That's a concern raised in >> > https://github.com/openjdk/jdk/pull/17456 discussions. >> > >> > Additionally, redefining symbols using tools like objcopy may not >> > handle member names referenced in string literals. For example, in >> > https://github.com/openjdk/jdk/pull/17456 additional changes are >> > needed in assembling and SA to reflect the symbol change. >> > >> > > >> > > So in the end, I think it will be fully possible to produce .a files >> > > that only has global symbols for the functions that are part of the API >> > > exposed by that library, and have all other symbols local, and make this >> > > is in a way that is consistent with the rest of the build system. >> > > >> > > Finally, a note on Hotspot. Due to debugging reasons, we export >> > > basically all symbols in hotspot as global. This is not reasonable to do >> > > for a static build. The effect of not exporting those symbols will be >> > > that SA will not function to 100%. On the other hand, I have no idea if >> > > SA works at all with a static build. Have you tested this? Is this part >> > > of the plan to support, or will it be officially dropped for Hermetic >> > > Java? >> > >> > We have done some testing with jtreg SA related tests for the fully >> > statically linked `javastatic`. >> > >> > If we use objcopy to localize symbols in hotspot, it's not yet clear >> > what's the impact on SA. We could do some tests. The other question >> > that I raised is the supported gcc versions (for partial linking) >> > related to the solution. >> > >> > Best, >> > Jiangli >> > >> > > >> > > /Magnus >> > >
