RFR: 8239563 - Reduce public exports in dynamic libraries built from static JDK libraries

Bob Vandette bob.vandette at oracle.com
Wed Feb 26 14:56:59 UTC 2020

> On Feb 26, 2020, at 9:17 AM, Magnus Ihse Bursie <magnus.ihse.bursie at oracle.com> wrote:
> On 2020-02-26 14:31, Bob Vandette wrote:
>>> On Feb 26, 2020, at 7:31 AM, Magnus Ihse Bursie <magnus.ihse.bursie at oracle.com> wrote:
>>> On 2020-02-26 08:41, David Holmes wrote:
>>>> Hi Bob,
>>>> Adding build-dev.
>>> Thanks for noticing that we were missing, David!
>> Sorry, I should have included you folks.
>>>> On 26/02/2020 6:37 am, Bob Vandette wrote:
>>>>> Please review this RFE that alters the visibility of JNI entrypoints to hidden when a shared library
>>>>> is created using static JDK libraries.
>>>>> RFE:
>>>>> https://bugs.openjdk.java.net/browse/JDK-8239563
>>>>> WEBREV:
>>>>> http://cr.openjdk.java.net/~bobv/8239563/webrev.00/
>>>>> CSR:
>>>>> https://bugs.openjdk.java.net/browse/JDK-8239791
>>>>> All JNI entrypoints that exist in JDK static libraries are declared as exported or visible.
>>>>> If a dynamic library is built from these static libraries, we end up with many exported
>>>>> functions that we don't want to provide access to,
>>>>> This RFE will change the definition of JNIEXPORT for libraries built when JNI_STATIC_BUILD
>>>>> is defined.  When defined, functions declared with JNIEXPORT will not be exported when
>>>>> linked into shared or dynamic libraries.  This will still allow linking of these functions into
>>>>> dynamic libraries but will not export the JDK symbols outside of the shared library.
>>>>> A CSR has been filed (https://bugs.openjdk.java.net/browse/JDK-8239791) to add the JNI_STATIC_BUILD
>>>>> define support in jni.h.
>>>> I have reservations about turning this into something we have to expose and support, rather than being something totally handled within the OpenJDK build system.
>>> I fully agree. The JNI headers are an exported interface. Our internal build mechanisms have nothing to do there.
>>>> I'm tempted to suggest the header files be adjusted to have:
>>>>     #ifndef JNIEXPORT
>>>>     <JNIEXPORT basic definitions as they are now >
>>>>     #endif
>>>> and then we define the modified JNIEXPORT via the build system when doing a static build.
>>>> Is that feasible?
>>> It's definitely doable, and a far better solution.
>> Yes, I like this solution.
>>> A patch something akin to this would be needed:
>>> diff --git a/make/autoconf/flags-cflags.m4 b/make/autoconf/flags-cflags.m4
>>> --- a/make/autoconf/flags-cflags.m4
>>> +++ b/make/autoconf/flags-cflags.m4
>>> @@ -709,7 +709,10 @@
>>>    # JDK libraries.
>>>    if test "x$TOOLCHAIN_TYPE" = xgcc || test "x$TOOLCHAIN_TYPE" = xclang; then
>>> -    STATIC_LIBS_CFLAGS="$STATIC_LIBS_CFLAGS -ffunction-sections -fdata-sections"
>>> +    STATIC_LIBS_CFLAGS="$STATIC_LIBS_CFLAGS -ffunction-sections \
>>> +        -fdata-sections -DJNIEXPORT=__attribute__((visibility(\"hidden\")))"
>>> +  else
>>>    fi
>>>    if test "x$TOOLCHAIN_TYPE" = xgcc; then
>>>      # Disable relax-relocation to enable compatibility with older linkers
>>> (With the reservation that I just wrote this on the fly and have not tested it -- things like quoting might be off. Also, I'm not sure if the match of
>>> compilers is correct -- it might be the case that all compilers except Microsoft defines __GNUC__, so maybe the addition of this -D flag might need
>>> a separate if statement to cover all our compilers correctly.)
>> Most of the STATIC_BUILD support is done in jni_util.h.  We could define JNIEXPORT in that header file after allowing it to be overridden in jni.h.
> I'm not sure I understand you correctly here. Do you mean that you'd like to re-define JNIEXPORT inside jni_util.h instead of using compiler command line flags? I don't think that'd work -- all libraries using JNIEXPORT that does not include jni_util.h first would then export their symbols just the same. Even if you fixed those, the system would be very fragile.

I was just trying to keep all static library building options in one place.  The static libraries that we produce need to include jni_util.h
or the JNI_OnLoad_xxx functions will not be declared properly.  Why not expand that dependency to the JNIEXPORT?

Do we really have access to all of these compiler defines from within our Makefiles?

#if (defined(__GNUC__) && ((__GNUC__ > 4) || (__GNUC__ == 4) && (__GNUC_MINOR__ > 2))) || __has_attribute(visibility)


> /Magnus
>> Bob.
>>> /Magnus
>>>> Thanks,
>>>> David
>>>>> In JDK8 the JNI specification and JDK implementation was enhanced to support static JNI libraries
>>>>> but we didn’t consider the issue of exportibility of JNI entrypoint symbols.
>>>>>    https://bugs.openjdk.java.net/browse/JDK-8005716
>>>>> If developers use these static JDK libraries in order to produce a custom shared library, all of the
>>>>> JNIEXPORTS will be exposed by this library even if the developer did not choose to export these.
>>>>> This is a security issue and a potential problem if this library is mixed with other libraries containing
>>>>> these symbols.
>>>>> Bob.

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