[8u] RFR(S): 8244548: JDK 8u: sun.misc.Version.jdkUpdateVersion() returns wrong result

[Severin Gehwolf]

Hi,

Please review this OpenJDK 8u fix for an issue where the update version
as configured via --with-update-version=XXX might overflow in internal
JDK structures and thus, will get reported wrong. In particular, only 1
byte is being reserved for the update versions internally. That is, it
works fine up to a configured update version of 255 (2^8 - 1). We've
passed that in OpenJDK 8u with the 8u262 cycle currently in EA. Hence,
we are now seeing this issue.

Bug: https://bugs.openjdk.java.net/browse/JDK-8244548
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8244548/02/
testing: Linux tier 1

The proposed fix is to extend the update_version field in
jdk_version_info from 8 to 16 bit and to use 16 bit of the jvm_version
integer in jvm_version_info. Thus, the new upper bound for update
version number is 2^16-1 => 65535 which should be sufficient. I don't
think OpenJDK 8u will live that long ;-)

jvm_version_info.jvm_version currently holds this quadruplet:

Most significant 8 bits => major version, followed by 8 bits => minor
version, followed by 8 bits => micro version, followed by 8 bits =>
build version. Note that JVM minor version represents the update
version as passed in via configure and the micro version is currently
not used (always 0). See vm_version.cpp lines 100-102 where only major,
minor and build number are ever been set. Knowing this, we can still
preserve the same behavior after patch by defining JVM_VERSION_MICRO to
0 for any version.

For jdk_version_info the fix is simpler, since the update_version is a
separate field for which I've extended it to 16 bit. Andrew Brygin
suggested to reduce the reserved1 field from currently 16 bit to 8 bit
since we are extending update_version by 8 bits, thus making the whole
structure grow. I'm not sure reducing reserved1 to 8 bits so as to not
grow the structure would be necessary, but I'd be happy to do so if
there is such consensus.

Thoughts?

Thanks,
Severin