ADDITIONAL SYSTEM INFORMATION :
openjdk version "25.0.1" 2025-10-21
OpenJDK Runtime Environment (build 25.0.1)
OpenJDK 64-Bit Server VM (build 25.0.1, mixed mode, sharing)

Arch Linux x86_64

A DESCRIPTION OF THE PROBLEM :
I've encountered a case where openjdk blows up with a spurious InternalError from in-bounds getAtIndex access on a mapped MemorySegment.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
A fairly small reproducer test case is provided below.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The test case does not touch out of bounds and should not fail
ACTUAL -
MappedMemorySegmentImpl
start: 0
end: 4294967094
13418024976 is where we blew up (at long index 1677253122)
34359736832 is the size of the segment
java.lang.InternalError: a fault occurred in an unsafe memory access operation
at nu.marginalia.btree.BTreeWriterTest.isSortedN(BTreeWriterTest.java:74)
at nu.marginalia.btree.BTreeWriterTest.wtf(BTreeWriterTest.java:56)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
at java.base/java.lang.reflect.Method.invoke(Method.java:565)
at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:725)
at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:149)

---------- BEGIN SOURCE ----------

    @Test
    void test() {
        Path file = Path.of("/tmp/btree.dat");

        try (var arena = Arena.ofConfined();
            var fc = (FileChannel) Files.newByteChannel(file, StandardOpenOption.READ, StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING, StandardOpenOption.WRITE)
        ) {
            MemorySegment ms = fc.map(FileChannel.MapMode.READ_WRITE, 0, 34359736832L, arena);

            isSortedN(ms, 2, 0, 4294967094L);

        } catch (IOException e) {
            throw new RuntimeException(e);
        }

    }

    boolean isSortedN(MemorySegment segment, int wordSize, long start, long end) {
        if (start == end) return true;

        System.out.println(segment.getClass().getSimpleName());
        System.out.println("start: " + start);
        System.out.println("end: " + end);

        if (end * 8L >= segment.byteSize())
            throw new IllegalArgumentException("End too big");

        long i = 0;
        try {
            long val = segment.getAtIndex(JAVA_LONG, 0);
            for (i = start + wordSize; i < end; i += wordSize) {
                long next;

                try {
                    next = segment.getAtIndex(JAVA_LONG, i);
                } catch (IndexOutOfBoundsException ex) {
                    throw new IndexOutOfBoundsException("@" + i + "(" + 0 + ":" + segment.byteSize() / 8 + ")");
                }

                if (next < val)
                    return false;
                val = next;
            }
        }
        catch (InternalError error) {
            System.err.println((8*i) + " is where we blew up (at long index " + i + ")");
            System.err.println((segment.byteSize()) + " is the size of the segment");
            throw error;
        }

        return true;
    }
---------- END SOURCE ----------