bug 6693236 fix
kelly.ohair at oracle.com
Thu Jul 8 09:11:13 PDT 2010
On Jul 8, 2010, at 6:13 AM, Rémi Forax wrote:
> Le 08/07/2010 14:07, Keith McGuigan a écrit :
>> David Holmes wrote:
>>> Rémi Forax said the following on 07/08/10 19:55:
>>>> Hi David,
>>>> I've no problem with the fact that the default configuration of
>>>> mandate to use of the split verifier.
>>>> But the patch for bug 6693236 also make FailOverOldVerifier
>>>> command line flag useless.
>>>> I think it's an error. FailOverOldVerifier should "fail over the
>>>> old verifier" and not be silently ignored.
>>> For Java 7 you're not permitted to failover to the old verifier so
>>> the choices are to:
>>> a) silently ignore it
>>> b) ignore it but issue a warning
>>> c) refuse to start the VM
>>> I presume you would prefer (b) or (c)?
>> At startup time, the JVM doesn't know what kind of classfiles it's
>> going to load. It doesn't get fed a steady diet of same-versioned
>> classfiles. Failover is still an option for classfiles with
>> classfile version 50.
>> So it's really not being ignored, it still enables failover for old
>> classfile versions. Just not the new ones.
>> - Keith
> Hi Keith,
> The problem is that bytecode transformations at runtime is a common
> especially since the VM has an agent API (java.lanf.instrumentation).
> Such agents modify the bytecode at runtime by rewritting it.
> During the rewrite, the stackmap information are often discarded,
> because the agent was written before the release of 1.6 or
> because generating stackmap frames takes more time than using
> the old verifier.
> With this fix, these agent libraries will stop working with 1.7
> and there is no workaround until the agent code is updated.
Agent libraries that re-write bytecodes of classfiles and do not also
StackMapTable attributes seem like very broken libraries to me.
The java_crw_demo bytecode instrumentation code in the jdk used by
the necessary transformation of the StackMapTable offsets. See:
starting around line 1710. It wasn't that difficult.
Bytecode transformation people should have known about this for a very
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