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

Bob Vandette bob.vandette at oracle.com
Thu Feb 27 15:00:34 UTC 2020 


> On Feb 27, 2020, at 9:59 AM, Magnus Ihse Bursie <magnus.ihse.bursie at oracle.com> wrote:
> 
> On 2020-02-27 15:52, Bob Vandette wrote:
>> 
>>> On Feb 27, 2020, at 7:15 AM, Magnus Ihse Bursie <magnus.ihse.bursie at oracle.com>
>>>  wrote:
>>> 
>>> On 2020-02-26 22:01, Bob Vandette wrote:
>>> 
>>>> Here’s an updated webrev that implementes the suggestion that allows JNIEXPORT in jni.h to be overridden
>>>> and the build limits visibility for static libraries.
>>>> 
>>>> If this webrev is accepted, I’ll update the CSR solution to match this implementation.
>>>> 
>>>> 
>>>> http://cr.openjdk.java.net/~bobv/8239563/webrev.01
>>> This looks basically ok, but some remarks:
>>> 
>>> You have forgotten to update the copyright year in the header files. 
>>> 
>> Thanks, I’ll update them.
>> 
>> 
>>> Also, the quoting looks suspicious. I would have guessed, thinking more carefully about this than the post yesterday, that the proper syntax would be -DJNIEXPORT='__attribute__((visibility("hidden")))' since otherwise the ' will make the \ literal. But, I usually end up being wrong about 50% of the time regarding this kind of stuff. :-) Have you verified that you get the proper define?
>>> 
>> I did verify that the quoting works on Mac and Linux.  I needed to add \” before hidden or the quotes would be eliminated causing 
>> the compiler to complain that visibility was expecting a string but didn’t see one.
>> 
> Very good. Just goes to show how much all these years in the build team has helped me learn how to spot quoting errors without trying (viz., not much :)).
> 
> Looks good to me, then. (I can't say if this fix still needs a CSR, though.)

I’m going to proceed with the CSR since I’m altering the behavior of JNIEXPORT in jni.h, which is a public header file.

Bob.

> 
> /Magnus
> 
>> 
>> Bob.
>> 
>> 
>> 
>>> /Magnus
>>> 
>>> 
>>>> Bob.
>>>> 
>>>> 
>>>> 
>>>>> On Feb 26, 2020, at 10:35 AM, Magnus Ihse Bursie <magnus.ihse.bursie at oracle.com>
>>>>>  wrote:
>>>>> 
>>>>> On 2020-02-26 15:56, Bob Vandette wrote:
>>>>> 
>>>>>>> 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.
>>>>>>>>>    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.
>>>>>>>> 
>>>>>>> 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?
>>>>>> 
>>>>> Unless *all* libraries that include jni.h also include jni_util.h, then the current definition of JNIEXPORT in jni.h will be used -- meaning that the so decorated functions will be exported -- which was exactly what you wanted to prevent. So I fail to see how this can be a solution.
>>>>> 
>>>>>> 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)
>>>>>> 
>>>>> Well, yes and no. I'm not certain which compilers define __GNUC__ just to show compatibility with gcc, but otoh that does not really matter. All that matters is that we know how we want JNIEXPORT to be defined when creating a static build -- and that we know, since we can check which toolchain we're using. (This is btw a far better check than to look for __GNUC__).
>>>>> 
>>>>> /Magnus
>>>>> 
>>>>>> 
>>>>>> Bob.
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>>> /Magnus
>>>>>>> 
>>>>>>>> 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.
>>>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>>>> 
>>>>>>>>>>> 
>

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