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

[Vladimir Ivanov]

>>> 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.

I'd like to point out that asserts and verification code are disabled by 
default. They are invaluable during problem diagnosis, but don't help at 
all from defence-in-depth perspective.

But I agree that it's easier to reason about and test the initial 
version of the fix.

> I wonder if the reviewers have fully realized the potential impact here?
> This has exposed a flaw in the way intrinsics are used from core classes.

FTR here are the checks I omitted in the minimized version (modulo 
separation of indexOf/lastIndexOf for trusted/non-trusted callers):

   http://cr.openjdk.java.net/~vlivanov/dlong/8158168/redundant_checks/

Other than that, the difference is mainly about undoing refactorings and 
removing verification logic (asserts + checks in the JVM).

There are still unsafe accesses which are considered safe in both 
versions (see StringUTF16.Trusted usages in the initial version [1]).

We used to provide safe wrappers for unsafe intrinsics which makes it 
much easier to reason about code correctness. I'd like to see compact 
string code refactored that way and IMO the initial version by Dean is a 
big step in the right direction.

I still prefer to see a point fix in 9 and major refactoring happening 
in 10, but I'll leave the decision on how to proceed with the fix to 
core-libs folks. After finishing the exercise minimizing the fix, I'm 
much more comfortable with the initial fix [1] (though there are changes 
I consider excessive).

Best regards,
Vladimir Ivanov

[1] http://cr.openjdk.java.net/~dlong/8158168/webrev.0

>>>>> 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.
>>>>>>
>>>>
>>