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

[Bob Vandette]

> 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.
>    STATIC_LIBS_CFLAGS="-DSTATIC_BUILD=1"
>    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
> +    STATIC_LIBS_CFLAGS="$STATIC_LIBS_CFLAGS -DJNIEXPORT="
>    fi
>    if test "x$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.

Bob.

> 
> /Magnus
>> 
>> Thanks,
>> David
>> 
>>> BACKGROUND:
>>> 
>>> 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.
>>> 
>>> 
>>> 
>