[9] RFR(L) 8158168: SIGSEGV: CollectedHeap::fill_with_objects(HeapWord*, unsigned long, bool)+0xa8

[dean.long at oracle.com]

On 3/21/17 5:02 PM, David Holmes wrote:

> I haven't been looking at the details of this but have been watching 
> from afar. As per my comments in the bug report (now public) I'm quite 
> concerned about the thread-non-safety issue here ...
>
> On 22/03/2017 4:47 AM, dean.long at oracle.com wrote:
>> On 3/21/17 9:37 AM, Vladimir Ivanov wrote:
>>
>>>> and webrev.2 with it removed:
>>>>
>>>> http://cr.openjdk.java.net/~dlong/8158168/webrev.2/
>>>
>>> Thanks, Dean. I started with webrev.2 and tried to minimize the
>>> changes. I ended up with the following version:
>>>
>>> http://cr.openjdk.java.net/~vlivanov/dlong/8158168/webrev.00/
>>>
>>
>> Thanks.  The reason I didn't go with that approach from the beginning is
>> because I couldn't convince myself that I could find all the missing
>> bounds checks, and I wanted an interface to test against.  With the
>> bounds checks in AbstractStringBuilder, it is very hard to test all the
>> possible race conditions, because some of the race conditions only
>> happen when an ASB field changes half-way through the method.
>
> So are we convinced that the proposed changes will never lead to a 
> crash due to a missing or incorrect bounds check, due to a racy use of 
> an unsynchronized ASB instance e.g. StringBuilder?
>

If only we had a static analysis tool that could tell us if the code is 
safe.  Because we don't, in my initial changeset, we always take a 
snapshot of the ASB fields by passing those field values to StringUTF16 
before doing checks on them.  And I wrote a test to make sure that those 
StringUTF16 interfaces are catching all the underflows and overflows I 
could imagine, and I added verification code to detect when a check was 
missed.

However, all the reviewers have requested to minimize the amount of 
changes.  In Vladimir's version, if there is a missing check somewhere, 
then yes it could lead to a crash.


dl

> Thanks,
> David
> -----
>
>>> Some clarifications:
>>>
>>> ============
>>> src/java.base/share/classes/java/lang/String.java:
>>>
>>> The bounds check is needed only in String.nonSyncContentEquals when it
>>> extracts info from AbstractStringBuilder. I don't see how out of
>>> bounds access can happen in String.contentEquals:
>>>          if (n != length()) {
>>>              return false;
>>>          }
>>> ...
>>>              for (int i = 0; i < n; i++) {
>>>                  if (StringUTF16.getChar(val, i) != cs.charAt(i)) {
>>>
>>
>> OK.
>>
>>> ============
>>> src/java.base/share/classes/java/lang/StringConcatHelper.java:
>>>
>>> I think bounds checks in StringConcatHelper.prepend() are skipped
>>> intentionally, since java.lang.invoke.StringConcatFactory constructs
>>> method handle chains which already contain bounds checks: array length
>>> is precomputed based on argument values and all accesses are
>>> guaranteed to be in bounds.
>>>
>>
>> This is calling the trusted version of getChars() with no bounds
>> checks.  It was a little more obvious when I had the Trusted inner 
>> class.
>>
>>> ============
>>> src/java.base/share/classes/java/lang/StringUTF16.java:
>>>
>>> +    static void putChar(byte[] val, int index, int c) {
>>> +        assert index >= 0 && index < length(val) : "Trusted caller
>>> missed bounds check";
>>>
>>> Unfortunately, asserts can affect inlining decisions (since they
>>> increase bytecode size). In order to minimize possible performance
>>> impact, I suggest to remove them from the fix targeting 9.
>>>
>>
>> Sure.
>>
>>> ============
>>>      private static int indexOfSupplementary(byte[] value, int ch, int
>>> fromIndex, int max) {
>>>          if (Character.isValidCodePoint(ch)) {
>>>              final char hi = Character.highSurrogate(ch);
>>>              final char lo = Character.lowSurrogate(ch);
>>> +            checkBoundsBeginEnd(fromIndex, max, value);
>>>
>>> The check is redundant here. fromIndex & max are always inbounds by
>>> construction:
>>>
>>>     public static int indexOf(byte[] value, int ch, int fromIndex) {
>>>         int max = value.length >> 1;
>>>         if (fromIndex < 0) {
>>>             fromIndex = 0;
>>>         } else if (fromIndex >= max) {
>>>             // Note: fromIndex might be near -1>>>1.
>>>             return -1;
>>>         }
>>> ...
>>>             return indexOfSupplementary(value, ch, fromIndex, max);
>>>
>>
>> OK.
>>
>>> ============
>>> I moved bounds checks from StringUTF16.lastIndexOf/indexOf to
>>> ABS.indexOf/lastIndexOf. I think it's enough to do range check on
>>> ABS.value & ABS.count. After that, all accesses should be inbounds by
>>> construction (in String.indexOf/lastIndexOf):
>>>
>>> jdk/src/java.base/share/classes/java/lang/StringUTF16.java:
>>>     static int lastIndexOf(byte[] src, byte srcCoder, int srcCount,
>>>                            String tgtStr, int fromIndex) {
>>>
>>>         int rightIndex = srcCount - tgtCount;
>>>         if (fromIndex > rightIndex) {
>>>             fromIndex = rightIndex;
>>>         }
>>>         if (fromIndex < 0) {
>>>             return -1;
>>>         }
>>>
>>> jdk/src/java.base/share/classes/java/lang/StringUTF16.java:
>>>     public static int lastIndexOf(byte[] src, int srcCount,
>>>                                   byte[] tgt, int tgtCount, int
>>> fromIndex) {
>>>         int min = tgtCount - 1;
>>>         int i = min + fromIndex;
>>>         int strLastIndex = tgtCount - 1;
>>>         char strLastChar = getChar(tgt, strLastIndex);
>>>
>>>     startSearchForLastChar:
>>>         while (true) {
>>>             while (i >= min && getChar(src, i) != strLastChar) {
>>>
>>> There are 2 places:
>>>   * getChar(tgt, strLastIndex) => getChar(tgt, tgtCount-1) - inbound
>>>
>>>   * getChar(src, i); i in [ min; min+fromIndex ]
>>>     min = tgtCount - 1
>>>     rightIndex = srcCount - tgtCount
>>>     fromIndex <= rightIndex
>>>
>>>            0 <= min + fromIndex <= min + rightIndex == (tgtCount - 1)
>>> + (srcCount - tgtCount) == srcCount - 1
>>>
>>>     Hence, should be covered by the check on count & value:
>>>       public int lastIndexOf(String str, int fromIndex) {
>>> +         byte[] value = this.value;
>>> +         int count = this.count;
>>> +         byte coder = this.coder;
>>> +         checkIndex(count, value.length >> coder);
>>>           return String.lastIndexOf(value, coder, count, str, 
>>> fromIndex);
>>>       }
>>>
>>
>> OK, I will go with your version if it's OK with Sherman.
>>
>> dl
>>
>>> Best regards,
>>> Vladimir Ivanov
>>>
>>>> On 3/17/17 5:58 AM, Vladimir Ivanov wrote:
>>>>>
>>>>>>> I have the same concern. Can we fix the immediate problem in 9 and
>>>>>>> integrate verification logic in 10?
>>>>>>>
>>>>>>
>>>>>> OK, Tobias is suggesting having verification logic only inside the
>>>>>> intrinsics.  Are you suggesting removing that as well?
>>>>>
>>>>> Yes and put them back in 10.
>>>>>
>>>>>> I'm OK with removing all the verification, but that won't reduce the
>>>>>> library changes much.  I could undo the renaming to
>>>>>> Trusted.getChar, but
>>>>>> we would still have the bounds checks moved into StringUTF16.
>>>>>
>>>>> I suggest to go with a point fix for 9: just add missing range 
>>>>> checks.
>>>>
>>