Bug 1536762

Summary:	Misuse of JNI by libguestfs Java bindings
Product:	[Community] Virtualization Tools	Reporter:	Demi Marie Obenour <demiobenour>
Component:	libguestfs	Assignee:	Richard W.M. Jones <rjones>
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Version:	unspecified	CC:	ptoscano, stenavin
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Description Demi Marie Obenour 2018-01-20 22:03:09 UTC

Description of problem:


The libguestfs Java bindings are buggy: they do not check for Java exceptions after all JNI calls that might throw them, and they do not acquire all needed `jclass`, `jmethodid`, and `jfieldid` objects during `JNI_OnLoad`.  They also leak local references.  Furthermore, potentially large objects are copied between Java and native, instead of using direct `ByteBuffer`s.

Probably the easiest solution is to use `sun.misc.Unsafe`/`jdk.unsupported.misc.Unsafe` and friends to do all marshaling in Java.

Comment 1 Richard W.M. Jones 2018-01-22 16:48:26 UTC

I have to say I don't know very much about JNI.  Do you have suggested
patches or outline changes that need to be made?

Comment 2 Demi Marie Obenour 2018-02-01 23:49:10 UTC

* Firstly, any calls to `FindClass`, `Get*MethodID`, and other such methods (anything that returns a `jclass`, `jmethodid`, or a `jfieldid`) should be done in `JNI_OnLoad` and the `jclass` objects converted to global refs.

* Java exceptions need to be checked (using `ExceptionCheck`) after every operation that might throw them, unless the function return code guarantees that there is no pending exception, or unless one is simply returning.

* Local references must be released with `DeleteLocalRef` when no longer needed.  If the number of local references may exceed 16, it is necessary to call `EnsureLocalCapacity` and check its return value.

* Any data that is not of small and fixed size should be stored in direct `ByteBuffer`s on the Java side and accessed using `GetDirectBufferAddress` or (better yet) the corresponding call from `sun.misc.Unsafe`.  It should be accessed from Java using the corresponding functions of `ByteBuffer` or `Unsafe`.  This is the only way to avoid an O(n) copy on JVMs that do not support pinning (most of them) with native methods that might block (most of them).

* Calling into native from Java is much, much faster than the reverse.  Marshalling code should thus be in Java, rather than in C.