RFR : 8038491: Improve synchronization in ZipFile.read()

Seán Coffey sean.coffey at oracle.com
Tue Apr 8 18:32:19 UTC 2014

On 08/04/2014 19:12, Pavel Rappo wrote:
> Hi Sean, Sherman
> as far as the client is using ZipFileInputStream in a multithreaded fashion (de facto), don't we have to rethink synchronization for the machinery of ZipFileInputStream.read? As far as I understand it's not enough to put this particular read of 'rem' into the synchronized block, as there are
> at least 2 more accesses to it (and only in the 'read' method). We're reading it in the synchronized block, fine, but all the corresponding reads/writes have to be done with respect to 'happens-before' relationship between them.
The ZipFile API itself is not thread safe and the example given would be 
buggy application code. However - we should cover this case. As per 
Sherman's last mail then, I'll make the change and check for zentry == 0 
instead of rem variable. zentry is read and set under synchronization. 
just need to run it through JPRT.

One other note on the ZipFileInputStream.read(..) method :
             if (len <= 0) {
                 return 0;
It looks like the above check is not necessary since a negative len 
check is make in the native ZIP_READ function which this ends up calling 
(and has same behaviour / return value)

> -Pavel
> On 8 Apr 2014, at 18:29, Seán Coffey <sean.coffey at oracle.com> wrote:
>> Sherman,
>> I see rem == 0 condition becoming true before the zentry variable is set to 0 (in close()) - in a multi threaded scenario like this one, we could have zero remaining bytes in zentry and yet have a zentry != 0 - your suggestion would prevent the SEGV (since we're in the sync block) but we would needlessly cycle into the ensureOpenOrZipException() and ZipFile.read code (line 713) if another thread tasked with the close() call was knocked off by the dispatcher during the various rem == 0 checks that we make : e.g
>> 722             if (rem == 0) {
>> 723                 close();
>> 724             }
>> Am I reading this correctly ?
>> regards,
>> Sean.
>> On 08/04/2014 16:59, Xueming Shen wrote:
>>> Hi Sean,
>>> It might be more explicit to check "if (zentry == 0)" here?
>>> -Sherman
>>> On 4/8/14 8:21 AM, Seán Coffey wrote:
>>>> A recently reported bug shows a race condition is possible on the rem == 0 check in ZipFile.read(byte b[], int off, int len). A bad check can result in referencing a jzentry structure that might already be freed and hence result in a SEGV. The fix is trivial and involves moving the rem == 0 check into a synchronized block. The ZipFile API itself is not thread safe so having mutiple threads operate on the same ZipFileInputStream is something that should never be performed.
>>>> webrev : http://cr.openjdk.java.net/~coffeys/webrev.8038491/webrev/
>>>> regards,
>>>> Sean.

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