Questions about the Hermetic Java project
Magnus Ihse Bursie
magnus.ihse.bursie at oracle.com
Fri Apr 12 11:52:00 UTC 2024
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.
>
> 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.
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?
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).
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.
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.
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.
/Magnus
>
> Thanks!
> Jiangli
>
> On Thu, Feb 15, 2024 at 12:01 PM Jiangli Zhou <jianglizhou at google.com>
> wrote:
>
> On Wed, Feb 14, 2024 at 5:07 PM Jiangli Zhou
> <jianglizhou at 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.bursie at 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
> > >
>
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