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

Magnus Ihse Bursie magnus.ihse.bursie at oracle.com
Wed Feb 26 14:17:22 UTC 2020

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 

> 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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