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Support signed offsets

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Both Structs and Lists support two's complement "Signed" offsets:

> B (30 bits) = Offset, in words, from the end
> of the pointer to the start of the struct's
> data section.  Signed.

> B (30 bits) = Offset, in words, from the end
> of the pointer to the start of the first
> element of the list.  Signed.

The prior code only supported positive offsets
because it used the Java >>> operator, which
is an unsigned shift operator. The Java >>
operator is the signed shift operator.

Audited the remaining uses of the shift
operator; they were correct and they are
documented as such.

Positive offsets are only guaranteed in a Canonical message.
This commit is contained in:
Jonah Beckford 2023-08-31 21:22:21 -07:00 committed by Semisol
parent c7d5354f23
commit 27ea6745e5
Signed by: Semisol
GPG key ID: 0949D3C25C7FD14F
5 changed files with 64 additions and 1 deletions
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4
runtime/src/main/java/org/capnproto/FarPointer.java
View file

@ -29,10 +29,14 @@ final class FarPointer {
}
public static int positionInSegment(long ref) {
/* The [ref] Offset (the "C" section) is "Unsigned",
so use unsigned >>> operator. */
return WirePointer.offsetAndKind(ref) >>> 3;
}
public static boolean isDoubleFar(long ref) {
/* The [ref] Offset (the "C" section) is "Unsigned",
so use unsigned >>> operator. */
return ((WirePointer.offsetAndKind(ref) >>> 2) & 1) != 0;
}

3
runtime/src/main/java/org/capnproto/ListPointer.java
View file

@ -29,6 +29,9 @@ final class ListPointer {
}
public static int elementCount(long ref) {
/* The [ref] List Size (the "D" section) is not specified
as Signed or Unsigned, but number of elements is inherently non-negative.
So use unsigned >>> operator. */
return WirePointer.upper32Bits(ref) >>> 3;
}

3
runtime/src/main/java/org/capnproto/StructPointer.java
View file

@ -30,6 +30,9 @@ final class StructPointer{
}
public static int ptrCount(long ref) {
/* The [ref] Pointer Section Size (the "D" section) is not specified
as Signed or Unsigned, but section size is inherently non-negative.
So use unsigned >>> operator. */
return WirePointer.upper32Bits(ref) >>> 16;
}

4
runtime/src/main/java/org/capnproto/WirePointer.java
View file

@ -42,7 +42,9 @@ final class WirePointer {
}
public static int target(int offset, long wirePointer) {
return offset + 1 + (offsetAndKind(wirePointer) >>> 2);
/* The [wirePointer] Offset (the "B" section) is "Signed",
so use signed >> operator. */
return offset + 1 + (offsetAndKind(wirePointer) >> 2);
}
public static void setKindAndTarget(ByteBuffer buffer, int offset, byte kind, int targetOffset) {

51
runtime/src/test/java/org/capnproto/SegmentReaderTest.java
View file

@ -1,9 +1,11 @@
package org.capnproto;
import org.capnproto.WireHelpers.FollowFarsResult;
import org.hamcrest.MatcherAssert;
import org.junit.Test;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import static org.hamcrest.CoreMatchers.is;
@ -29,4 +31,53 @@ public class SegmentReaderTest {
SegmentReader segmentReader = new SegmentReader(byteBuffer, null);
MatcherAssert.assertThat(segmentReader.isInBounds(7, 2), is(false));
}
@Test
public void validSegmentWithNegativeOffsetShouldBeInBounds() {
int refOffset;
long ref;
int refTarget;
int dataSizeWords;
int wordSize;
/*
Binary data:
echo -n
'FAAAAAEAAQDJqtK2cBpJhZ2LUEVMkYblyarStnAaSYWdi1BFTJGG4e3///8CAQAAAgAAAAAAAAD0////AAABAA=='
| base64 -d
Verify it is valid with:
capnp decode --flat dksdk_std_schema.capnp GenericReturn
where the .capnp comes from
https://gitlab.com/diskuv/dksdk-schema/-/blob/afbf9564a60f2670f6b9dfb3c423fc55dd4c3013/src/dksdk_std_schema.capnp
*/
ByteBuffer byteBuffer = ByteBuffer.wrap(new byte[] {
(byte)0x14, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x01, (byte)0x00, (byte)0x01, (byte)0x00,
(byte)0xC9, (byte)0xAA, (byte)0xD2, (byte)0xB6, (byte)0x70, (byte)0x1A, (byte)0x49, (byte)0x85,
(byte)0x9D, (byte)0x8B, (byte)0x50, (byte)0x45, (byte)0x4C, (byte)0x91, (byte)0x86, (byte)0xE5,
(byte)0xC9, (byte)0xAA, (byte)0xD2, (byte)0xB6, (byte)0x70, (byte)0x1A, (byte)0x49, (byte)0x85,
(byte)0x9D, (byte)0x8B, (byte)0x50, (byte)0x45, (byte)0x4C, (byte)0x91, (byte)0x86, (byte)0xE1,
(byte)0xED, (byte)0xFF, (byte)0xFF, (byte)0xFF, (byte)0x02, (byte)0x01, (byte)0x00, (byte)0x00,
(byte)0x02, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x00, (byte)0x00,
(byte)0xF4, (byte)0xFF, (byte)0xFF, (byte)0xFF, (byte)0x00, (byte)0x00, (byte)0x01, (byte)0x00
}).order(ByteOrder.LITTLE_ENDIAN);
SegmentReader segment = new SegmentReader(byteBuffer, null);
/* Read root Struct: GenericReturn. */
refOffset = 0; /* At the root STRUCT POINTER */
ref = segment.get(refOffset);
refTarget = WirePointer.target(refOffset, ref);
dataSizeWords = StructPointer.dataSize(ref);
wordSize = dataSizeWords + StructPointer.ptrCount(ref);
MatcherAssert.assertThat(segment.isInBounds(refTarget, wordSize), is(true));
/* Read inner Struct: ComObject. */
refOffset = refTarget + dataSizeWords; /* At the inner STRUCT POINTER */
ref = segment.get(refOffset);
refTarget = WirePointer.target(refOffset, ref);
dataSizeWords = StructPointer.dataSize(ref);
wordSize = dataSizeWords + StructPointer.ptrCount(ref);
MatcherAssert.assertThat(segment.isInBounds(refTarget, wordSize), is(true));
}
}