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IpUtils - make checksum work with empty/zero buffer

Browse source 
also:
- document buffer endianness assumption
- simplify fold logic - it cannot overflow

Test: TreeHugger, atest NetworkStaticLibTests
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I521a7377d1310c0e9c46a5dff95dca451e5f889e
This commit is contained in:
Maciej Żenczykowski 2023-04-17 09:10:14 +00:00
commit 5f95a67eef
2 changed files with 23 additions and 7 deletions
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16
staticlibs/framework/com/android/net/module/util/IpUtils.java
View file

@ -16,6 +16,8 @@
package com.android.net.module.util;
import com.android.internal.annotations.VisibleForTesting;
import static android.system.OsConstants.IPPROTO_ICMPV6;
import static android.system.OsConstants.IPPROTO_TCP;
import static android.system.OsConstants.IPPROTO_UDP;
@ -42,8 +44,9 @@ public class IpUtils {
* payload) or ICMP checksum on the specified portion of a ByteBuffer. The seed
* allows the checksum to commence with a specified value.
*/
private static int checksum(ByteBuffer buf, int seed, int start, int end) {
int sum = seed;
@VisibleForTesting
public static int checksum(ByteBuffer buf, int seed, int start, int end) {
int sum = seed + 0xFFFF; // to make things work with empty / zero-filled buffer
final int bufPosition = buf.position();
// set position of original ByteBuffer, so that the ShortBuffer
@ -69,13 +72,12 @@ public class IpUtils {
b += 256;
}
sum += b * 256;
sum += b * 256; // assumes bytebuffer is network order (ie. big endian)
}
sum = ((sum >> 16) & 0xFFFF) + (sum & 0xFFFF);
sum = ((sum + ((sum >> 16) & 0xFFFF)) & 0xFFFF);
int negated = ~sum;
return intAbs((short) negated);
sum = ((sum >> 16) & 0xFFFF) + (sum & 0xFFFF); // max sum is 0x1FFFE
sum = ((sum >> 16) & 0xFFFF) + (sum & 0xFFFF); // max sum is 0xFFFF
return sum ^ 0xFFFF; // u16 bitwise negation
}
private static int pseudoChecksumIPv4(

14
staticlibs/tests/unit/src/com/android/net/module/util/IpUtilsTest.java
View file

@ -73,6 +73,20 @@ public class IpUtilsTest {
// "hello")
// print JavaPacketDefinition(str(packet))
@Test
public void testEmptyAndZeroBufferChecksum() throws Exception {
ByteBuffer packet = ByteBuffer.wrap(new byte[] { (byte) 0x00, (byte) 0x00, });
// the following should *not* return 0xFFFF
assertEquals(0, IpUtils.checksum(packet, 0, 0, 0));
assertEquals(0, IpUtils.checksum(packet, 0, 0, 1));
assertEquals(0, IpUtils.checksum(packet, 0, 0, 2));
assertEquals(0, IpUtils.checksum(packet, 0, 1, 2));
assertEquals(0, IpUtils.checksum(packet, 0xFFFF, 0, 0));
assertEquals(0, IpUtils.checksum(packet, 0xFFFF, 0, 1));
assertEquals(0, IpUtils.checksum(packet, 0xFFFF, 0, 2));
assertEquals(0, IpUtils.checksum(packet, 0xFFFF, 1, 2));
}
@Test
public void testIpv6TcpChecksum() throws Exception {
// packet = (scapy.IPv6(src="2001:db8::1", dst="2001:db8::2", tc=0x80) /